NOTE! This is the text only version of the PDF version of this thesis. Since there are many formulas in this thesis, the HTML version is not optimal.
Rene' L. Krikhaar Softw are  Architecture Reconstruction

Rene' L. Krikhaar 1999

Software Architecture

Reconstruction

Afhankelijk van rugdikte

Software Architecture

Reconstruction

The work described in this thesis has been carried out at the Philips
Research Laboratories in Eindhoven, The Netherlands, as part of the
Philips Research programme.

CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAG Krikhaar, Ren'e Leo
Software Architecture Reconstruction / Ren'e Leo Krikhaar. - Amsterdam:
Universiteit van Amsterdam, Faculteit Wiskunde, Informatica, Natuur-
en Sterrenkunde, RICS Proefschrift Universiteit van Amsterdam. - Met
samenvatting in het Nederlands. ISBN 90-74445-44-6 Trefw.: Software
Architecture / Reverse Engineering / Software Architecture Reconstruction
/ Architecture Verification.

cflPhilips Electronics N.V. 1999

All rights are reserved. Reproduction in whole or in part is prohibited
without the written consent of the copyright owner.

Software Architecture

Reconstruction

ACADEMISCH PROEFSCHRIFT ter verkrijging van de graad van doctor

aan de Universiteit van Amsterdam

op gezag van de Rector Magnificus

	Prof. Dr. J.J.M. Franse ten overstaan van een door het college voor
promoties ingestelde commissie, in het openbaar

te verdedigen in de Aula der Universiteit

op dinsdag 29 juni 1999, te 15.00 uur

door Ren'e Leo Krikhaar

geboren te Nijmegen

promotor: Prof. Dr. J.A. Bergstra co-promotor: Dr. C. Verhoef faculteit:
Wiskunde, Informatica, Natuur- en Sterrenkunde

RICS

Preface

Motivation Over the past few years, software architecture has become a
major topic in embedded systems development. It is commonly agreed that
a good software architecture is indispensible for the development of
product families [BCK98] of software-intensive systems. Our company, the
Royal Philips Electronics, develops a large range of software intensive
systems from medical systems to television sets.

Originally, medical systems were hardware systems with a small amount
of software, but in recent years the software has acquired a much more
important place in the system, e.g. in the reconstruction of medical
images obtained with an X-ray camera. Similarly, at first, televisions
did not contain any software, but nowadays these systems are controlled
mainly by software, providing e.g. automatic tuning of TV channels.

From an industrial point of view, products containing similar
functionalities will have to be introduced on the market ever more rapidly
(short lead time). A high level of hardware and software reuse is hence a
prerequisite for survival in the competitive market. Different customers
want different products, each with their own characterics, which may even
be expressed in non-functional product means, for example the different
natural languages as applied in the user interface.

Quality is always of great importance for products. The product's quality
must be continuously monitored and where possible improved. Software is
becoming a major part of all these products and quality activities are
consequently shifting from hardware to software. Short lead times of
products and high quality are in fact conflicting requirements which
must be carefully managed.

ii Preface

The software of many Philips' products is already undergoing changes
as indicated above (increased functionality implemented in software,
increased product diversity, decreased lead time and improved quality).
At the time of these product's initial development, in some cases decades
ago, the software architecture did not play the important role it has
today. In those days, the architecture was often not handled explicitly
in the engineering phase. At present, the software architecture is
of major importance for product development to be able to manage the
changes listed above. The spectrum of possible solutions to fill the
gap between the absence of an explicit architecture and the need for
such an architecture lies between:

ffl rebuilding the system from scratch and taking care of software
architecture explicitly; ffl reconstructing an architecture from the
implicit architecture and improving this architecture by re-architecting
the system.

In this thesis, we focus on the latter side of this spectrum. Our ultimate
objective is to define a method for reconstructing software architecture
of existing systems. Reconstruction of software architectures requires
synergy between tools and domain experts [Cor89, Kri97, SWM97, KC98].
Therefore, we may conclude that there cannot be such a thing as a
full-flegded architecture reconstruction tool, though tools that support
reconstruction are indispensable.

We propose to make a clear separation between extraction of information
from a system and the presentation of the extracted results by means
of a separate abstraction activity. In current research an abstraction
activity is often not recognised as a separate activity. During software
architecture analysis one often wants to query an existing system,
e.g. which components use the functionality of component DB ? It is
not possible to capture all such queries in advance. A flexible set of
abstraction operators helps to formulate such queries in a expressive
way. In this thesis we will use Relation Partition Algebra to express,
amongst other things, such queries.

Research Contributions In this thesis we present a framework for our
software architecture reconstruction (SAR) method. This framework consists
of two dimensions: SAR levels and views upon software architecture.
We define ArchiSpects and InfoPacks as the components that fit in this
framework. For all SAR levels, each of the architectural views contain
a number of ArchiSpects and

Preface iii InfoPacks. The applicability of this framework is demonstrated
by the definition of a number of ArchiSpects and InfoPacks.

By many people in the software community, formal methods are often
considered as inapplicable especially for large real-world systems.
Nevertheless, we believed in a formal approach to reconstruct software
architectures. Therefore, we developed Relation Partition Algebra (RPA)
which is an extension of relational algebra. We showed the applicability
of RPA in different industrial settings, which resulted in several
ArchiSpects that are defined in terms of RPA.

Currently, a lot of research is performed on software architectures.
This research contributes to define better software architectures in
the different industrial settings. Besides defining a good architecture,
one must, in the various steps of software development, also take care
of the proper application of this architecture. A formal definition of
a software architecture makes it possible to automatically verify the
results of software development (e.g. design and source code) against
the defined software architecture. Although, currently, we are not able
to define the complete architecture in a formal fashion, we recommend
to introduce architecture verification, as much as possible, in any
development process.

History of the Project In the early nineties, a main research topic of our
department was to investigate, develop and adapt software development
methods to build embedded systems (e.g. televisions). During the
introduction of a new (formal) method [Jon88a, Jon88b] for developing
software for televisions the need for information extraction arose.
Small programs were written to retrieve design information from the
source code. In those days, visualisation of software information was
also needed. This has resulted in the proprietary tool Teddy-Classic
(discussed in Appendix C). In fact, Teddy-Classic was able to display a
graph consisting of nodes and edges; nodes represent modules and edges
represent module imports. In a later stage, so-called duppies (design
update proposals) were implemented using shell scripts to check the
consistency of the software structure (an early form of architecture
verification). But extraction and checking were still performed on an
ad-hoc basis. From this work arose the need for a mathematical foundation
for making abstractions upon software. This was the embryonic phase of
Relation Partition Algebra; see Chapter 3.

iv Preface

In 1993, research was started to analyse a public telephony switching
system (Tele) developed according to a dedicated method. The analysis
resulted in a description of the Building Block method. During this
research, again, a need for extraction and abstraction mechanisms for
software was recognized. This time, it was needed mainly to identify the
concepts behind the design method. Later, the Building Block method,
which we would nowadays call an architecture method, was partially
applied to another communication system. This meant that we first had
to analyse the architectural concepts of this system before we could
select and apply the most affecting concepts of the Building Block method.

In later projects, the focus shifted to the development of a uniform
approach or method (based on the extract-abstract-present paradigm)
for analysing the software architecture of existing systems.

Complexity of Systems In this section we list a number of system's
characteristics to give the reader an impression of the variety of
concerns a software architecture has to deal with. Therefore, in Table
1, we summarize some of the characteristics of three typical systems of
Philips: two professional systems Telecommunication (Switch) and Medical
Systems (Med) and a consumer electronics system (Cons)1. All of these
characteristics play their own role in almost any architectural decision,
or they are an outcome of such a decision (e.g. the number of subsystems).

The number of customers and the number of different products are given
in the Product View part of the table. In case of professional systems,
each customer gets his or her own dedicated system. But we have used
the definition that two products are different only when they differ
significantly either in hardware or in software.

The Evolutionary View part shows figures relating to the system's current
age and its expected total lifetime. The release cycle describes the
average time between two major releases. The release footprint indicates
the percentage of files that have been touched since the last release.

The number of code files is given in the Code View part of the table.

1It is hard to normalise the data of the different systems; we have
handled the figures in a non-scientific fashion. The table is therefore
meant mainly to illustrate the complexity of systems.

Preface v

Switch Med Cons Product View # customers 103 103 106 # products 101
101 102

Evolutionary View system's age/lifetime (years) 15/30 15/30 3/5 release
cycle (years) 0:7 0:5 N/A release footprint (% touched files) - 60% N/A

Code View # code files 4 \Theta  103 7 \Theta  103 0:6 \Theta  103
# lines of code 1:4 \Theta  106 2:4 \Theta  106 0:4 \Theta  106 #
programming languages 3 8 2

Module View # subsystems 8 11 5 # file imports 32 \Theta  103 70 \Theta
103 0:8 \Theta  103 # external components 0 5 0

Execution View # operating systems 1 4 1 # (main) computing devices 1
5 3 # software processes ? 1000 50 50

Table 1: Characteristics of Large Systems

One can argue about the way how the size of the source code should be
measured, but for our purpose the number of lines suffices. The number
of programming languages indicates problems that could arise in merging
software parts and e.g. the required educational background of developers.

The Module View part of the table shows the number of subsystems into
which the system is decomposed. The number of file includes gives an
indication of the interconnectivity between the various software parts.
In some cases parts of the software are built by external parties
(external components) with their typical integration difficulties.

The number of software processes listed in the Execution View deserves
some extra attention. For the Med system, these are software processes
with their own address space, but for the Cons system these are activities
that can be compared with threads (i.e. sharing an address space).
The Switch system has its own operating system supporting light-weight
processes. In the last two systems, the processes are in fact created
at initialisation time, whereas in the Switch system processes are
dynamically created during system operation. The number of computing
devices describes the processing units in the system in which software
runs.

vi Preface

Related Work and Tools In this thesis in appropriate sections, we will
relate our work to work of others. For the reader's convenience, in
advance, we will briefly discuss, some closely related work. This short
introduction is also meant for readers who are familiar with that work,
to put our work into perspective.

Rigi Rigi [SWM97] is a tool that supports the extraction of information
(e.g. rigiparse extracts information from C source code) and the
presentation of extracted information (e.g. showing coloured line-box
diagrams). After the initial information has been presented, one can
perform some simple abstractions, e.g. the creation of composites
and calculation of complexity quality measures. Rigi is an open tool,
which means that new functionality can be easily added using the Rigi
Command Language. The repository of Rigi consists of a resource-flow
graph, containing different types of vertices and edges, representing
software entities and relations between software entities.

Rigi can be very useful in the analysis of a system. The standard way of
presenting graphs (equally sized nodes and fixed points for connecting
edges to boxes) could, however, be a drawback. We think reconstructed
architectural information should be presented in a layout which is similar
to the software architecture information as initially documented in the
development group concerned.

The extraction and presentation functionalities of Rigi can be easily
combined with our ideas of a separate abstraction activity. For example,
the user interface of Rigi can be extended with a menu and abstraction
machinery to query software.

Reflexion Models Murphy et al. [MNS95, MN97] have described reflexion
models. A reflexion model shows the differences and agreements between
the engineer's high-level model and the model of the source code.
An engineer defines a high-level model and specifies how this model
maps to source code. A tool computes a reflexion model that shows where
the engineer's high-level model agrees with, and where it differs from
the source model. A forPreface vii mal model is used to calculate
convergences, divergences, and absences of relations in the high-level
model or the source model.

Reflexion models show differences between the engineer's mental model of
the system and the as-built model of the system. We will use architecture
verfication, which is a process that makes explicit distinctions between
the as-built architecture of the system and the intended architecture
of the system (as defined in advance by architects). A similar approach,
called design conformance, is discussed in [MNS95].

Relational Algebra Holt [Hol96, Hol98] suggests Tarski's Relation Algebra
as a theoretical basis for software manipulations (or in fact he considers
manipulation of visualisation).

There is a remarkable correspondence between Holt's work and our own
work on Relation Partition Algebra [FO94, FKO98].

Software Bookshelf The software bookshelf [FHK+97] is a framework
for capturing, organizing, and managing information on the system's
software. The bookshelf framework is an open architecture, which allows
a variety of tools to be integrated, e.g. the Rigi presentation tool.
Reverse engineering tools can populate the bookshelf repository from
which information can be retrieved by other tools. All information
transport within this framework is performed via Web protocols.

The open architecture makes this framework interesting for integration
with other approaches, e.g. with our approach as described in this thesis.
Web technology incorporates many presentation and navigation techniques
that are useful for software reconstruction. The software bookshelf
distinguishes three different roles: builder, patron and librarian.
We experienced that these three roles are useful in introducing
reconstruction technology in an organisation.

Dali Dali [KC98] is an architecture analysis framework containing e.g.
Rigi as a presentation tool. It is based on view extraction, extraction
of static and

viii Preface

dynamic elements from the system, and view fusion. View fusion consists
of combining views in order to achieve new views that are richer and/or
more abstract. Dali contains an SQL database containing the various views.
We consider SQL less accurate for expressing software manipulations.
We will therefore introduce Relation Partition Algebra, which has
more accuracy (e.g. by means of the operations transitive closure and
transitive reduction).

Outline of Thesis In Chapter 1 we discuss the term software architecture.
An overview of some keynote papers is given, including models describing
various views on software architecture. Business goals, objectives and
patterns for software architecture are presented. The relations between
these items are illustrated in a so-called GOP (Goals, Objectives,
Patterns) diagram.

In Chapter 2 we focus on the engineering aspects of software architecture.
We discuss aspects of reverse engineering in general and the aspects of
reverse engineering software architectures in particular. The global
design of our software architecture reconstruction (SAR) method is
discussed, including an introduction to the notions of InfoPacks and
ArchiSpects (modular pieces of our method).

In Chapter 3 we discuss the mathematical foundation of our method:
Relation Partition Algebra (RPA). RPA is an extension of relation algebra
fine-tuned for, but certainly not restricted to, software.

In Chapter 4 we focus on the comprehension of existing software
architectures. The baseline is a system, typically evolved over fifteen
years, which is not completely known by all of its current developers.
A number of InfoPacks and ArchiSpects are presented.

Chapter 5 addresses re-defining the software architecture of an existing
system. Before one can improve, one must clarify the current architecture
and one must define the required architecture. Our reverse architecting
method supports the development of an improvement plan by analysing the
impact of certain changes.

In Chapter 6 a way of managing software architectures is presented:
by verifying whether the design/implementation satisfies the software
architecture one achieves architecture conformance.

Chapter 7 gives recommendations for the application of software architec
Preface ix ture reconstruction. The appendices present extraction tools,
abstraction tools and presentation tools as referred to throughout the
thesis. The last appendix presents all the RPA operators in a nutshell.

The thesis contains many examples, which we have slightly modified to
retain Philips' competitive edge. In my opinion, this does not affect
the illustrative value of these examples.

x Preface Acknowledgements During my work at the Philips Research
Laboratories I had the opportunity to analyse software architectures.
This work eventually resulted in this thesis, which could not have been
written without the support of my group leader, Jaap van der Heijden,
and my cluster leader, Henk Obbink. I also want to thank the director
of the Information and Software Technology, Eric van Utteren, who gave
me this opportunity.

Loe Feijs encouraged me to write a thesis on my research performed
since 1994. Without his work on Relation Partition Algebra my work would
never have reached the current level of maturity. He was always willing
to discuss subjects and he has always supported my work in many cases.

I would like to thank Jan Bergstra, my promotor at the University of
Amsterdam, for his support in discussing many topics and giving advice in
writing this thesis. Comments on earlier versions of my thesis were very
useful and always utmost to the point. It was a pleasure to work with Jan.
The ready knowledge of Chris Verhoef about reverse engineering research
in the world was of great help in relating it to software architecture
reconstruction research. Discussions with Chris were fruitful and they
improved the quality of this thesis.

Throughout the years, I had the opportunity to analyse many systems
at Philips. Many Philips' sites (in Europe) were willing to discuss the
ins and outs of their systems with me. I appreciate their dedication and
the time they have invested despite their often very busy daily work.
Although many people were involved, I would like to thank especially
Lothar Baumbauer (Germany/Nuremberg), Ad Zephat and Jan Willem Dijkstra
(the Netherlands/Best), Paul Schenk (Austria/Vienna), Reinder Bril
and Thijs Winter (the Netherlands/Hilversum), and Paul Krabbendam (the
Netherlands/Eindhoven). Research of software architectures can indeed only
be performed in cooperation with people who actually build large systems.

xii Acknowledgements

Therefore, it is of great importance to have access to such systems.
Without the support of these people, research into this topic is not
practicable.

In various projects in which I participated over the past years I worked
together with a number of colleagues for some amount of time. First of
all, I would like to thank Jan Gerben Wijnstra with whom I spent about
eight months in Nuremberg in Germany. As colleagues we worked together in
the Building Block project which aimed to establish an abstraction of the
software architecting method used to develop telecommunication systems.
During our stay in Nuremberg we were sentenced to spend a lot of spare
time together and I still cherish good memories of that time. The initial
idea to develop a more structured method for reverse engineering large
systems originated in that time.

In 1996 and 1997 Jeroen Medema participated in two reverse architecting
projects (Med and Switch). Jeroen carried out a good deal of practical
work and was of great help in pushing our research in the right direction.
Jeroen is also responsible for the Java implementation of Relation
Partition Algebra described in the appendix. This implementation proved
most valuable in the introduction of our research results at various
development sites at Philips.

I thank Henk Obbink who often participated in our discussions of software
architecture. I also want to thank my colleague J"urgen M"uller with whom
I discussed a variety of related and unrelated topics. Rob van Ommering
supported the work by discussing with me his experiences in creating
software for televisions. He was also one of the persons behind the
mathematical foundation of Relation Partition Algebra, and participated
in the AWK implementation of this theory.

Initial versions of (parts of) the thesis have been reviewed by Reinder
Bril, Loe Feijs, Robert Jagt, Jeroen Medema, Andr'e Postma and Marc
Stroucken. I thank them all for their critical and constructive comments.
I also want to thank Maarten Pennings, who helped me with the pecularities
of LATEX [GMS93, Lam85], Noor Krikhaar for correcting the Dutch grammar,
Philips Translation Services for correcting the English grammar of an
earlier version of this thesis, Frans Willemsen for his text writing
advices and Aad Knikman, who helped to create the picture on the cover.

I want to thank the reading committee for reading and for approving
my thesis: Peter van Emde Boas (University of Amsterdam), Loe Feijs
(Eindhoven University of Technology), Rick Kazman (Carnegie Mellon
University and Waterloo University, USA) and Paul Klint (University
of Amsterdam).

Acknowledgements xiii Finally, I would like to thank all the people I
have not mentioned so far, but who have also supported my work and life
in a spiritual or technical way.

Ren'e L. Krikhaar

xiv Acknowledgements

Contents Preface i Acknowledgements xi Contents xv List Of Figures xix
1 Software Architecture 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 1.2 Definitions of Software Architecture . . . . . . . . . . . . .
2 1.3 Business Goals . . . . . . . . . . . . . . . . . . . . . . . . .
8 1.4 Architectural Objectives . . . . . . . . . . . . . . . . . . . .
8 1.5 Architectural Patterns . . . . . . . . . . . . . . . . . . . . .
11 1.6 Relating Goals-Objectives-Patterns . . . . . . . . . . . . .
17 1.7 Final Remarks . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 Overview of the SAR Method 19

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
19 2.2 Forward Software Architecting . . . . . . . . . . . . . . . .
20 2.3 Reverse Software Architecting . . . . . . . . . . . . . . . . .
21 2.4 Architecture Improvement . . . . . . . . . . . . . . . . . . .
24 2.5 The SAR Method . . . . . . . . . . . . . . . . . . . . . . . . 25

xvi Contents

3 Relation Partition Algebra 29

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
29 3.2 Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30 3.3 Binary Relations . . . . . . . . . . . . . . . . . . . . . . . .
32 3.4 Part-Of relations . . . . . . . . . . . . . . . . . . . . . . . .
37 3.5 Introducing multiplicity in RPA . . . . . . . . . . . . . . . .
39 3.6 RPA Formulas in Action . . . . . . . . . . . . . . . . . . . .
46 3.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
48

4 Described Architecture 51

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
51 4.2 ArchiSpect: Software Concepts Model . . . . . . . . . . . .
54 4.3 ArchiSpect: Source Code Organisation . . . . . . . . . . . .
58 4.4 ArchiSpect: Build Process . . . . . . . . . . . . . . . . . . .
62 4.5 InfoPack: Files . . . . . . . . . . . . . . . . . . . . . . . . .
65 4.6 InfoPack: Import . . . . . . . . . . . . . . . . . . . . . . . .
68 4.7 InfoPack: Part-Of . . . . . . . . . . . . . . . . . . . . . . .
76 4.8 InfoPack: Depend . . . . . . . . . . . . . . . . . . . . . . .
78 4.9 ArchiSpect: Component Dependency . . . . . . . . . . . . .
80 4.10 ArchiSpect: Using and Used Interfaces . . . . . . . . . . . .
88 4.11 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . 95

5 Redefined Architecture 99

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
99 5.2 ArchiSpect: Component Coupling . . . . . . . . . . . . . . .
102 5.3 ArchiSpect: Cohesion and Coupling . . . . . . . . . . . . .
109 5.4 InfoPack: Aspect Assignment . . . . . . . . . . . . . . . . .
116 5.5 ArchiSpect: Aspect Coupling . . . . . . . . . . . . . . . . .
118 5.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . 122

6 Managed Architecture 125

Contents xvii

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
125 6.2 ArchiSpect: Layering Conformance . . . . . . . . . . . . . .
126 6.3 ArchiSpect: Usage Conformance . . . . . . . . . . . . . . .
131 6.4 ArchiSpect: Aspect Conformance . . . . . . . . . . . . . . .
134 6.5 ArchiSpect: Generics and Specifics Conformance . . . . . . 137 6.6
Architecture Verification in Action . . . . . . . . . . . . . . 140

7 Concluding Remarks 143

7.1 Recommendations for Application . . . . . . . . . . . . . . 143 7.2
Relation Partition Algebra . . . . . . . . . . . . . . . . . . . 145

A Extraction Tools 149

A.1 file-exts.pl . . . . . . . . . . . . . . . . . . . . . . . . . . . .
149 A.2 units.pl . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
149 A.3 comment-strip.pl . . . . . . . . . . . . . . . . . . . . . . . .
150 A.4 C-imports.pl . . . . . . . . . . . . . . . . . . . . . . . . . .
150 A.5 J-imports.pl . . . . . . . . . . . . . . . . . . . . . . . . . . .
151 A.6 J-package.pl . . . . . . . . . . . . . . . . . . . . . . . . . . .
152 A.7 ObjC-imports.pl . . . . . . . . . . . . . . . . . . . . . . . .
152 A.8 QAC-imports.pl . . . . . . . . . . . . . . . . . . . . . . . . .
153 A.9 directory.pl . . . . . . . . . . . . . . . . . . . . . . . . . . .
153

B Abstraction Tools 155

B.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
155 B.2 RPA-Prolog . . . . . . . . . . . . . . . . . . . . . . . . . . .
155 B.3 RPA-AWK . . . . . . . . . . . . . . . . . . . . . . . . . . .
157 B.4 RPA-SQL . . . . . . . . . . . . . . . . . . . . . . . . . . . .
159 B.5 RPA-Java . . . . . . . . . . . . . . . . . . . . . . . . . . . .
160 B.6 A Brief Comparison of RPA tools . . . . . . . . . . . . . . . 162

C Presentation Tools 165

xviii Contents

C.1 Teddy-Classic . . . . . . . . . . . . . . . . . . . . . . . . . .
165 C.2 Teddy-Visio . . . . . . . . . . . . . . . . . . . . . . . . . . .
167 C.3 Teddy-PS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
168 C.4 Teddy-ArchView . . . . . . . . . . . . . . . . . . . . . . . .
168 C.5 TabVieW . . . . . . . . . . . . . . . . . . . . . . . . . . . .
168

D RPA Operators in a Nutshell 173 Bibliography 179 Glossary 189 Summary
193 Samenvatting 197 Curriculum Vitae 201

List of Figures

1.1 Business Goals, Architectural Objectives and Patterns . . .
2 1.2 The 4+1 View Model . . . . . . . . . . . . . . . . . . . . . .
3 1.3 Relationships between the Software Architectures . . . . . .
5 1.4 Architectural View Model . . . . . . . . . . . . . . . . . . .
7 1.5 Opaque Layering . . . . . . . . . . . . . . . . . . . . . . . .
13 1.6 Tele Subsystems . . . . . . . . . . . . . . . . . . . . . . . .
14 1.7 Generics and Specifics . . . . . . . . . . . . . . . . . . . . .
15 1.8 Goals, Objectives and Patterns . . . . . . . . . . . . . . . . 18

2.1 Forward and Reverse Engineering . . . . . . . . . . . . . . .
21 2.2 Extract, Abstract, and Present . . . . . . . . . . . . . . . .
22 2.3 Architecture Improvement Process . . . . . . . . . . . . . . 24

3.1 Directed Graph Representing calls Relation . . . . . . . . . 33 3.2
Transitive closure of calls . . . . . . . . . . . . . . . . . . . 36 3.3
Hasse of calls . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.4
Partitioning Functions . . . . . . . . . . . . . . . . . . . . . 38 3.5
Lifting calls . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.1 Software Architecture Reconstruction Method . . . . . . . .
52 4.2 Overview of Described Architecture . . . . . . . . . . . . . .
53 4.3 Software Concepts Model of Tele . . . . . . . . . . . . . . .
56 4.4 Software Concepts Model of Med . . . . . . . . . . . . . . . 57

xx List of Figures

4.5 Source Code Organisation of Med . . . . . . . . . . . . . . .
60 4.6 Development States and Transitions of Med Files . . . . . .
61 4.7 Build Activities . . . . . . . . . . . . . . . . . . . . . . . . .
62 4.8 Build Process Med . . . . . . . . . . . . . . . . . . . . . . .
64 4.9 Implementation of Decomposition Levels Med . . . . . . . . 77 4.10
depends Exts;Exts . . . . . . . . . . . . . . . . . . . . . . . . .
79 4.11 Component Dependency Med . . . . . . . . . . . . . . . . .
83 4.12 Import Dependency Comm . . . . . . . . . . . . . . . . . . 84 4.13
Lifting imports Files;Files . . . . . . . . . . . . . . . . . . . .
85 4.14 Using and Used interfaces . . . . . . . . . . . . . . . . . . .
89 4.15 Using and Used Interfaces of Med . . . . . . . . . . . . . . .
90 4.16 Comm Table Viewer . . . . . . . . . . . . . . . . . . . . . . 96

5.1 Architecture Improvement . . . . . . . . . . . . . . . . . . .
100 5.2 Overview of Redefined Architecture . . . . . . . . . . . . . .
101 5.3 Lifting with Multiplicity: 2-3-4-case . . . . . . . . . . . . .
103 5.4 Component Coupling Med . . . . . . . . . . . . . . . . . . .
104 5.5 Component Coupling Comm . . . . . . . . . . . . . . . . . .
105 5.6 Fan-in-oriented lifting . . . . . . . . . . . . . . . . . . . . .
108 5.7 Re-clustering . . . . . . . . . . . . . . . . . . . . . . . . . .
111 5.8 Dominating . . . . . . . . . . . . . . . . . . . . . . . . . . .
112 5.9 Cohesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
113 5.10 Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 115 5.11 Test Aspect Coupling for Med . . . . . . . . . . . . . . . . .
120 5.12 Dependencies between Aspects of Med . . . . . . . . . . . . 120

6.1 Overview of Managed Architecture . . . . . . . . . . . . . .
127 6.2 Layering Conformance of Cons . . . . . . . . . . . . . . . .
128 6.3 Layering Conformance of Tele . . . . . . . . . . . . . . . . .
129 6.4 Aspect Conformance . . . . . . . . . . . . . . . . . . . . . .
135

List of Figures xxi

6.5 Generic and Specific Components . . . . . . . . . . . . . . . 139 B.1
High Level Operations . . . . . . . . . . . . . . . . . . . . . 158 B.2
RPA Calculator . . . . . . . . . . . . . . . . . . . . . . . . . 161

C.1 Teddy-Classic . . . . . . . . . . . . . . . . . . . . . . . . . .
166 C.2 Teddy-Visio . . . . . . . . . . . . . . . . . . . . . . . . . . .
167 C.3 Teddy-PS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
169 C.4 Teddy-ArchView . . . . . . . . . . . . . . . . . . . . . . . .
170 C.5 TabVieW . . . . . . . . . . . . . . . . . . . . . . . . . . . .
172

xxii List of Figures Chapter 1 Software Architecture

In advance of discussing software architecture reconstruction, which
is the main topic of this thesis, we briefly present, in this chapter,
some issues related to software architecture (amongst others definitions
of architecture and architectural view models).

1.1 Introduction In this chapter we give an overview of definitions
of software architecture found in the literature. But we also discuss
the importance of having a good software architecture in a software
intensive system.

A software architecture must satisfy requirements from a business
point of view. These business goals lead to certain objectives for
software architecture, to be discussed in Section 1.4. A number of good
architectural patterns which may be useful in various software systems
will be presented in Section 1.5. Business goals, architectural objectives
and architectural patterns are related to each other. In Section 1.6 we
derive from specific business goals the related architectural objectives
which, in turn, lead to certain architectural patterns. A general view
on this model is illustrated in Figure 1.1.

2 Software Architecture

Business

Goals

Architectural

Objectives ArchitecturalPatterns

Figure 1.1: Business Goals, Architectural Objectives and Patterns 1.2
Definitions of Software Architecture In recent years, many definitions
of software architecture have appeared in the literature. The need
for modular structuring and explicit handling of product families was
first discussed in the late sixties [Dij68, Par76, PCW85]. Since then,
software in systems has grown tremendously in size and complexity.
Although these "old" structuring principles still hold, they have to be
transformed into principles for the products of today's sizes. The term
software architecture was introduced in the nineties to address, amongst
other things, the up-scaling of these structuring principles.

In 1992, Perry and Wolf [PW92] gave a definition of software architecture:
a set of architectural elements that have a particular form. The elements
may be processing elements, data elements or connecting elements.

According to Shaw and Garlan [SG96], the architecture of a software
system defines that system in terms of computational components
and interactions between those components. Examples of components
include clients, servers, filters and layers of a hierarchical system.
Interactions between components may consist of procedure calls, shared
variables, asynchronous events or piped streams.

Jacobson, Griss and Johnsons [JGJ97] stated that a software architecture
describes the static organization of software in subsystems interconnected
through interfaces and defines at a significant level how nodes executing
those software subsystems interact with each other.

Bass et al. [BCK98] gave an "often-heard" definition: architecture is
com1.2 Definitions of Software Architecture 3

Development View

Physical ViewProcess View Logical View

Scenarios

End users

*  functionality

Programmers

* software management

System engineers

* system topology

* delivery

* installation

* telecommunication

System integrators

* performance

* scalability

* throughput

Figure 1.2: The 4+1 View Model ponents, connectors, and constraints.
Connectors are a mechanism for transfering control and data around
the system. Constraints are definitions of the behaviour of components.

Many different structures are involved in an architecture of a software
system. In order to organize them, models have been defined that give a
certain view on software architecture. We found that the models developed
by Kruchten [Kru95] and Soni et al. [SNH95] are useful in industry.
In the next sections these so-called view models will be discussed,
including a model that combines both view models.

1.2.1 The 4 + 1 View Model Kruchten distinguishes five different views
in his 4 + 1 View Model of architecture [Kru95]. Each view addresses a
specific set of concerns which are of interest for different stakeholders.
Figure 1.2 (taken from [Kru95]) shows the views, the stakeholders and
their concerns.

4 Software Architecture

The logical view supports the functional requirements: the services
a system should provide to its end users. The designers decompose the
system into a set of key abstractions of the domain, which results in
a domain model. Kruchten suggests to use an object-oriented style to
define the logical view.

The process view addresses non-functional requirements, such as
performance and availability of resources. It takes into account
concurrency and distribution, system integrity and fault tolerance.
In this view, the control of execution is described at several levels
of abstraction.

The development view focuses on the organization of the actual software
modules in a software development environment (SDE). It concerns
the internal requirements related to ease of development and software
management. The development view is represented by module and subsystem
diagrams that show the system's export and import relations.

The physical view also takes into account the system's non-functional
requirements. It maps the various elements identified in the logical,
process and development view onto the various hardware elements.
This mapping should be highly flexible and should have a minimal impact
on the source code itself.

The scenarios help to demonstrate that the elements of the four views work
together seamlessly. The scenarios are in some respect an abstraction
of the most important requirements. A scenario acts as a driver to help
designers discover architectural elements, and also helps to illustrate
and validate the architecture design.

An example of a scenario is the description of the activation of follow
me in a telephony switching system (activation of forward direction of
incoming calls to another specified extension). Given this scenario,
one can discuss how the involved processes communicate with each other
via message communication, which components of the system are running,
and which hardware devices are involved. The scenario view is in fact
redundant with the other views, hence the "+ 1".

The various views are not completely independent of each other.
The characterization of the logical elements helps to define the proper
process elements. For example, each logical element (object) is either
active or passive (autonomy of objects); elements are transient or
permanent (persistency of objects). The autonomy and persistency of
objects have to do with the process view. The logical view and development
view are very close, but address different concerns. The logical elements
do not necessarily map

1.2 Definitions of Software Architecture 5

Conceptual  Architecture

Module  Architecture

Code  Architecture

Execution  ArchitectureHardware  Architecture

Source Code implemented_by implemented_by

located_in

resource resides on

assigned to assigned to

assigned to implemented

by

configured

by

Figure 1.3: Relationships between the Software Architectures one-to-one
on development elements. Similar arguments hold for the relation between
process view and physical view.

1.2.2 The SNH Model Soni et al. [SNH95] have investigated a number of
large systems (telecommunication, control systems, image and signal
processing systems) to determine the pragmatic and concrete issues
related to the role of software architecture. The structures they found
in the investigated systems can be divided in several broad categories.
Soni et al. distinguished five different views on architecture:

ffl conceptual architecture: describing the system in terms of its major

design elements and relationships between them. Typical elements are
components and connectors. ffl module (interconnection) architecture:
functional decomposition and

layers, which are orthogonal structures. Typical terms are subsystems,
modules, layers, imports and exports. ffl execution architecture:
describing the system's dynamic structure.

Typical elements are tasks, threads, RPC and events. ffl code
architecture: describing how the source code, binaries and libraries
are organised in the development environment. Code resides in files,
directories and libraries.

6 Software Architecture

ffl hardware architecture: describing the hardware components and their

relations as far as they are relevant for making software design
decisions. Processors, memory, networks and disks are typical hardware
elements.

The architectural views have relations with each other as depicted in
Figure 1.3 (taken from [SNH95]). A conceptual element is implemented
by one or more elements of the module architecture. Module elements
are assigned to run-time elements in the execution architecture. In
addition, each execution element is implemented by some module elements.
Module elements are implemented by elements of the code architecture.
There is also a relationship between run-time elements and executables,
resource files (e.g. help texts in different natural languages) and
configuration files in the code architecture.

1.2.3 The AV Model Kruchten described a number of design principles for
constructing elements of the various views. The SNH model was defined
after an analysis of existing systems, which comprised looking at an
architecture from a different angle. Nevertheless, the 4 + 1 View model
and the SNH model are pretty similar. A logical decision (also suggested
by [BMR+96]) is to combine the good parts of the two into a new view
model (see Figure 1.4). We have taken the 4 + 1 View model as a basis
and integrated it with good parts of the SNH model. The new model has
been baptized the Architectural View model , abbreviated as the AV model .

The logical view and conceptual architecture are more or less similar.
In both cases, the end user is the main stakeholder. The execution
architecture and process view differ only in details. Soni et al.
addressed the hardware architecture concisely, but Kruchten stressed
the physical view more explicitly.

The module architecture and code architecture maps on Kruchten's
development view of Kruchten. In our new model, we divided Kruchten's
development view into two parts: module view and code view.
The stakeholders of the module view are the programmers. The main
stakeholders of the code view are people who are responsible for tool
support. In the new model, the source code is considered part of the
code view.

Scenarios in the AV model support forward engineering as well as reverse
engineering of software architectures: scenarios play a role in defining
ar1.2 Definitions of Software Architecture 7

Module

View

Physical

View Execution

View

Logical

View

Scenarios

End users

* functionality

Programmers

* software   management

System engineers

* system topology

* delivery

* installation

System integrators

* performance

* scalability

* throughput

Code View

Tool support

* configuration   management

Figure 1.4: Architectural View Model

8 Software Architecture

chitectural elements [Kru95], and they support the analysis of software
architecture [KABC96].

The precise contents of all these views have not been described
explicitly. In practice, one has to experience which elements are most
important. In this thesis we focus on the module view, but the code view
is also required in a supporting role. It is our intention to make the
contents of the module view and code view more explicit and tangible.

1.3 Business Goals From a business perspective the following goals can be
defined for products, having impact on the software architecture within
such a product [KW95, JGJ97]:

ffl short time-to-market; ffl low cost of product; ffl high productivity
of organisation; ffl adequate predictability of process; ffl high
reliability of product; ffl high quality of product.

Which goals must be emphasized depends very much on the type of product.
The quality of a product is very important especially for medical
systems, e.g. a patient must not be exposed to too much X-ray radation.
Also important is the quality of consumer products. It is for example
impossible to provide every one of the millions of television users an
update of the software in their television every six months1. In the
currently booming market of digital videocommunication systems it is more
accepted to deliver several software updates after the first release.
In this business, time-tomarket has high priority as the aim is to remain
ahead of one's competitors. A software architect must be aware of such
trade-offs in making proper architectural decisions.

1.4 Architectural Objectives There are many architectural objectives
that justify certain architectural decisions. Bass et al. [BCK98]
distinguished different, called quality at1Although downloading of new
software to a television set is foreseen in the near future.

1.4 Architectural Objectives 9 tributes. These quality attributes
are discernable at run-time (performance, security, availability,
functionality and usability) or they are not discernable at run-time
(modifiability, portability, reusability, integrability and testability).

In this thesis we want to discuss architectural objectives in more
abstract terms. We distinguish the following architectural objectives,
which are not necessarily orthogonal:

ffl comprehension; ffl reuse; ffl evolution; ffl product family.

1.4.1 Comprehension Software changes many times during its lifetime.
A developer must understand the software well to be able to modify,
extend or fix a bug in the system. Approximately half of the time spent
on maintenance activities concerns comprehension [PZ93]. Improvement of
comprehension therefore increases a developer's productivity.

In many cases software changes are made by developers who did not
originally create the part of the software concerned. This is due to
the typical lifespans of our systems, which may be decades. An original
developer may in the mean time have moved on to another position or may
even have left the organisation. Moreover, in view of a system's size and
complexity, several developers must often have access to the same part
of the software. The nature of today's systems makes it impossible to
divide a system from the start into disjunct parts of the software that
can be assigned to a single person. Comprehension of software written
by other people is therefore necessary.

1.4.2 Reuse Reuse consists of the further use or repeated use of a
software artifact. Typically, reuse means that software artifacts are
designed for use outside their original contexts to create new systems
[JGJ97]. Proper application of reuse requires a number of precautions.
Design for reuse must be explicitly addressed in an organisation to
be able to reuse software. Component reuse is currently a hot topic in
research and practice. In general, the number

10 Software Architecture

of reusable components greatly influence productivity and quality.
Reuse of software is often hard to achieve (particularly due to the
not-inventedhere syndrome); it requires a lot of investment and it must
be managed explicitly to be successful. The benefits of a reuse-oriented
organisation start at best, two years after introduction [JGJ97].
In a business context return-on-investment times of two years are long,
especially compared with the length of time between two releases.

Reusable components can only be developed with a specified architecture
in mind. For a functional equivalent component one may request different
implementations depending on the architecture and/or satisfying different
non-functional requirements. In the world of IC design it has long been
accepted that there are different implementations for a component. In the
world of software this is less accepted. For example, a component in a
pipe-line architecture must behave differently from a component in an
event-driven system. In a pipe-line architecture a component continuously
reacts on new input data while in an event-driven system a component
is triggered before it processes data. It is impossible to combine any
arbitrary set of components into a new system. Garlan stressed this
point as the architectural mismatch [GAO95].

1.4.3 Evolution From a business perspective, software has come to be the
most profitable part of software-intensive systems. Product features
of existing systems are often related purely to software extensions.
In the past, product requirements were often assumed to be stable.
Today they are more dynamic and evolutionary. Requirements rapidly change
and product developers must allow for this fact.

The evolution of hardware also has an impact on software. Take for
example software that controls image-processing units in a medical system.
One must be able to smoothly integrate a new hardware image-processing
unit into a new system release or one may even replace such a unit
by software. So the thought of possibly having new image-processing
units in the future causes this to be explicitly covered in the software
architecture. Good intuition of possible market trends helps to define
software architectures that are future proof.

1.5 Architectural Patterns 11 1.4.4 Product Family Product family
architectures are architectures especially designed to manage (and
enhance) many product variations needed for different markets. For
example, in different parts of the world there are different television
broadcast standards, which affects the software embedded in a television.
A television's user interface is also language-dependend. Furthermore,
products may also vary in the number of features they include.
A television may be packed with or without an Electronic Programming
Guide (EPG). One must be able to switch the EPG feature on or off in
a late stage of the production process. Software architecture must be
capable of facilitating all such variations, i.e. it must be flexible.

1.5 Architectural Patterns Alexander et al. [AIS77] defined a pattern
for buildings and towns as follows:

"A pattern describes a problem which occurs over and over again in
our environment, and then describes the core of the solution to that
problem, in such a way that you can use this solution a million times
over, without ever doing it the same way twice." These patterns are
described in a consistent and uniform style.

The notion of patterns can also be applied in the construction
of software. Buschmann et al. [BMR+96] and Gamma et al [GHJV95] used
schemes to describe design patterns. Buschmann et al. categorized the
patterns into the following groups:

ffl Architectural Pattern ffl Design Pattern ffl Idiom (Code Pattern)

Conceptual integrity means that the same concept is always explicitly
applied for similar problems. Conceptual integrity supports developers
to better understand a system and it leads to programmer independence
[Bro82]. In the case of larger systems conceptual integrity is even
more important. The size of the development group is larger, which
means that developers spend more time communicating with each other.
The application of general concepts simplifies internal communication.
An architect's task is to document these concepts, but he or she is also
responsible for communicating these concepts to the development group.

12 Software Architecture

A concept may also be a typical solution to a certain problem.
Concepts must be defined for typical problems in each stage in the
development process. Design patterns are examples of typical solutions
to design problems.

To illustrate the notion of patterns, we will informally discuss three
architectural patterns which are related to the module view: layering
(Section 1.5.1), generic and specific components (Section 1.5.2) and
aspects (Section 1.5.3). While analysing the Tele system we experienced
the benefit of applying these patterns. In Chapter 6 we will return to
these patterns to discuss architecture verification.

1.5.1 Layering A layer is a group of software elements. Layers are
strictly ordered. Given the ordering, higher layers may use only lower
layers. We distinguish two types of layering:

ffl opaque layering: a layer is restricted to use only the layer directly

below it. The idea behind opaque layering is that each layer makes an
abstraction of all the layers below it and adds some extra functionality.
An example of this principle is the 7 layer OSI stack 2 [Tan76],
illustrated in Figure 1.5. ffl transparent layering; each layer is
allowed to access services of all the

layers below it see Figure 1.6. A layer abstracts functionalities in lower
layers where appropriate, but it does not encapsulate functionality that
has already reached a proper level of abstraction in a lower layer.

An advantage of opaque layering is that the user of a layer needs to
know only the layer below it. It does not have to have any knowledge of
the lowest layers. A disadvantage is that each layer must also provide
functionality from the lower layer when required by a higher layer. This
often leads to renaming of functions without adding any functionality.
Another disadvantage is that the lower layer must have knowledge of
higher layers to be able to provide proper functionality (to avoid the
risk of all the non-exported functionality of the lower layers being
provided again).

A transparent layer provides functionality to the outside world, without
paying too much attention to the layers that use the functionality.
A disadvantage is that when the interface of a layer changes, it may
affect all

2In a new edition of his book Tanenbaum defined a hybrid reference model
with only five layers.

1.5 Architectural Patterns 13

Application Layer Presentation Layer

Session Layer Transport Layer

Network Layer Data Link Layer

Physical Layer

Figure 1.5: Opaque Layering

the higher layers. Layering generally makes it possible to test a system
incrementally. Layers can be tested one by one, starting at the bottom,
i.e. when a layer of level n passes the test, one can test layer n +
1, assuming that layers 1 : : : n are functionally correct. Layers also
facilitate the control of the development process and product releases.

Layers can be defined at different levels of abstraction. The following
example of layering at the highest level of abstraction, i.e. subsystems,
is taken from telecommunication industry [KW94]. It is very common to
distinguish in a communication system the following layers, which we
call subsystems (from top to bottom):

ffl Service Management ; dealing with actual services of the system. In a

switching system e.g. it deals with redirecting a telephone call when
the follow me to another number feature is active. ffl Logical Resource
Management ; providing logical resources. These

resources are based on resources provided by Equipment Maintenance, but
they are made hardware-independent. At this level an operator configures
a communication system. ffl Equipment Maintenance; dealing with the
maintenance of peripheral

hardware. It provides virtually error-free peripheral hardware to the
higher subsystems (in a telecommunication system the functions of

14 Software Architecture

Logical Resource Management

Equipment Maintenance

Operating System

Service Management

Figure 1.6: Tele Subsystems failing hardware must be taken over by other
hardware components). Hardware specifics are hidden. This subsystem
provides an abstract representation of physical resources and their
usability. ffl Operating System; containing functionality provided by
a normal operating system. It also provides some general functionality
to higher subsystems, including e.g. software downloading, recovery and
manmachine interface procedures.

1.5.2 Generic and Specific Components Software components are currently a
hot topic in software architecture research and practice. Szyperski used
the following definition of software component [Szy97]:

"A software component is a unit of composition with contractually
specified interfaces and explicit context dependencies only.
A software component can be deployed independently and is subject to
composition by third parties." A component-based system consists of a
number of components. One can divide these components into two kinds:
generic and specific. Generic functionality, which resides in generic
components, exists in almost all the products in the family, and specific
functionality, residing in specific components, does not exist in all
products.

Generic components represent the common part of all the products of
a family. A crucial task of an architect is to distinguish generic and
specific functionalities. It is not just a matter of factoring out the
common functionality, because (yet unknown) future enhancements must
also be taken into account.

1.5 Architectural Patterns 15

Specific A Generic X

Specific P

Generic Y Figure 1.7: Generics and Specifics Generic components may
already be bound at compile and link time without the flexibility of
configuration being adversely affected. The set of generic components
form the skeleton of all products. Specific components can rely on the
availability of this skeleton, but they are not allowed to rely on the
availability of specific components.

This also means that only generic components can be responsible for
facilitating communication between specific components (see Figure 1.7).
During the system's initialisation time, specific components announce
themselves to the generics. Via a call-back mechanism the generic
component is able to access the specific component at run time.
A specific component can call a generic component's functionality, on
its turn, this generic component can call (via a call-back function)
another specific component's functionality.

Different types of generic components can be distinguished. A refinement
of generic component types has been discussed by Wijnstra [Wij96].

Example A public telephone switching system communicates with several
other switches using different protocols and different types of lines.
Each customer asks for his or her own set of hardware units and his or
her own set of protocols. The system must be configured according to
the user's needs. In a late stage of the development trajectory one must
still be able to con16 Software Architecture

figure a system. It must even be possible to extend such systems (when
they are running in the field) with new hardware and/or protocols.
Explicit handling of generic and specific functionalities (combined with
late binding) satisfies this list of requirements [KW94].

1.5.3 Aspects In addition to object-oriented system modelling [Boo91],
one can also simultaneously address a functional view on the system.
In the case of large systems it is even necessary to apply another
structuring mechanism for comprehension reasons. We call the means used
for this structuring approach aspects. Before developing the separate
components, one must define aspects which are in principle applicable
to each component. Such a set of aspects is fixed for the whole system.

As an example we give the aspects of a typical telecommunication system:

ffl normal operation; ffl man-machine interface; ffl recovery; ffl
configuration management; ffl fault handling; ffl performance observation;
ffl test.

The notion of aspects is relevant in the various development phases.
During system testing the aspects can be used to structure the process
and decide on the (functional) completeness of the test. Aspects should
explicitly appear in all the software artifacts (design documents, source
code). A simple, but effective, implementation of aspects at source code
level involves the use of prefixes (according to the aspect name) for
functions, variables and files. Aspects must also be handled explicitly in
design documents. For example, a reader who is interested in a certain
aspect should be guided through the document in a natural fashion.
This can e.g. be achieved by prescribing obligatory (sub)sections.

The System Infrastructure Generics (SIGs) are special generic components,
which usually reside in the lowest subsystem. They deliver some basic
functionality of the system. One must define (one or more) system
infrastructure generics to implement the basic functionality of an aspect.
For example, the man-machine interface uses basic functionality (windows,
menus, etc.) which reside in SIGs. Another example is exception handling,

1.6 Relating Goals-Objectives-Patterns 17 the basic infrastructure for
achieving exception handling (e.g. popping as many return addresses from
the call stack as required), is implemented in an exception handling SIG.

1.6 Relating Goals-Objectives-Patterns In the previous sections we have
discussed business goals, architectural objectives, and architectural
patterns. Although the business goals are very general and hold for
(almost) any business, it is obvious that some priority ordering is
necessary per system (or market). Given the ordering of business goals,
we can derive an ordering of architectural objectives, as illustrated
in Figure 1.8. For example, the cost of product is related to the amount
of reuse that can be established. Furthermore, architectural objectives
can be mapped on architectural patterns. For example, when a product
family is concerned it is good to explicitly distinguish generic and
specific components.

Making an explicit Goals-Objectives-Patterns (GOP) diagram for your
system helps to make proper trade-offs during the creation of software
architectures. The GOP diagram of Figure 1.8 (simplified version of
GOP diagram in [KW95]) should therefore be seen as just an example;
extra goals, objectives and patterns and lines could be required for
your system. The absence of a line does not necessarily mean that there is
not a relationship, but it can be seen as a relative unimportant relation.

1.7 Final Remarks Most of the discussed issues stem from the Building
Block Method used in Nuremberg for the development of telephony switching
systems (Tele). The Building Block Method and its application to large
systems have been discussed in a number of reports [KW94, KL94, Kri94,
Kri95, LM95, Wij96].

We have addressed only a few architectural patterns of the module view.
Other good architectural patterns for this view exist, but the other
views on architecture should also be covered with architectural patterns.
In this chapter it has been our intention to give a non-exhaustive
overview of the variety of issues relating to software architecture.

18 Software Architecture

Time to Market

Productivity of

Organisation

Cost of Product Reuse

Evolution Comprehension

Product Family

Predictability of

Process

Reliability of

Product

Quality of

Product

Layering

Aspects Generics and

Specifics

System Infrastructure

Generics

Business

Goals

Architectural

Objectives

Architectural

Patterns

Figure 1.8: Goals, Objectives and Patterns

Chapter 2 Overview of the SAR Method

In the previous chapter we gave an overview of software architecture.
In this chapter we present a framework required for a method to
reconstruct a software architecture of an existing system.

2.1 Introduction Here, we introduce a method to reconstruct an existing
system's software architecture: the Software Architecture Reconstruction
(SAR) method. We discuss a general framework for the SAR method, which
is also used to structure this thesis.

In general, all methods consist of four different parts [Kro93]:

ffl an underlying model; ffl a language; ffl defined steps and ordering
of these steps; ffl guidance for applying the method.

In our software architecture reconstruction method, the underlying model
consists mainly of Relation Partition Algebra (to be elaborated in Chap
20 Overview of the SAR Method

ter 3). Relation Partition Algebra consists of sets, binary relations,
part-of relations and operations on them. Besides a model, RPA is also
a language for expressing architectural information: we need graphical
and textual notations (graph diagrams, relation tables, lists) to present
a reconstructed software architecture.

The reconstruction of software architecture consists of performing the
following kinds of steps: extraction, abstraction and presentation (see
Section 2.3). Extraction steps will be discussed as parts of InfoPacks
(the notion of an InfoPack will be discussed in Section 2.5.2);
abstraction and presentation steps are contained in ArchiSpects (the
notion of an ArchiSpect will be discussed in Section 2.5.2). A guidance
describes the gaps that are not completely covered by the steps or when
the steps do not perfectly fit in the situation at hand.

In this chapter we briefly describe the engineering of software
architectures (called forward software architecting). Next, we will
discuss reverse software architecting, which is the counterpart of
forward software architecting. As we will see, improvements in existing
software architectures demands both engineering disciplines. We will
finish this chapter with a framework into which the software architecture
reconstruction method can be fitted.

2.2 Forward Software Architecting Forward software architecting,
or simply software architecting, is the discipline of engineering a
software architecture from scratch, or, if an architecture already
exists, it consists in engineering the extensions of the architecture.
An example of a method dedicated to architecture is the Building Block
method [KW94, LM95].

In chapter 1 we have discussed a number of architectural patterns that
are related to the module view of software architecture. One can also
define architecting as the process of selecting and applying proper
patterns for each of the architectural views. It is an engineering
discipline that requires a lot of experience, human sense, knowledge
of a range of good architectural patterns and the ability to define new
appropriate architectural patterns.

2.3 Reverse Software Architecting 21

Requirements Designforward

engineering

forward engineering

reverse engineering

reverse engineering

design recovery design recovery

reengineering

(renovation)

reengineering

(renovation)

restructuring restructuring redocumentation,restructuring

Implementation

Figure 2.1: Forward and Reverse Engineering 2.3 Reverse Software
Architecting Reverse software architecting is the flavour of reverse
engineering that concerns all activities for making existing (software)
architectures explicit [Kri97]. Reverse software architecting aims for:
recovery of lost architectural information, updating of architecture
documentation, supporting of maintenance (comprehension) activities,
provision of different (other) views on architecture, preparing for
another platform and facilitating impact analysis. Reverse engineering
was defined as follows by Chikofsky and Cross [CC90]:

"The process of analysing a subject system to identify the system's
components and their relationships and create representations of the
system in another form or at a higher level of abstraction." Figure 2.1
(taken from [CC90]) presents a lot of terminology within a simplified
software life-cycle. Requirements involves the specification of the
problem, design is the specification of a solution and implementation
concerns the creation of a solution which consists of coding, testing
and system delivery. Redocumentation is the simplest and oldest form of
reverse engineering. It concerns the creation or revision of a system's
documentation. However, many tools dedicated to redocumentation are only
able to generate diagrams, print code in an attractive way, or generate
cross-reference

22 Overview of the SAR Method

Repository

abstract extract present System Software

text describing the structure of the system

System History

System Experts

Figure 2.2: Extract, Abstract, and Present

listings. Restructuring is the transformation from one representation form
into another, preserving the external behaviour. The first experiments in
this area concerned the removal of `goto' statements and their replacement
by control structures like `while' loops, `if-then-else' clauses and
`for' loops. Design recovery means that one identifies meaningful higher
levels of abstraction of software. For this activity one requires domain
knowledge and designer's knowledge to add the information required to
be able to create these abstractions. Reengineering is related to the
modification of an original system to increase design quality.

Extract - Abstract - Present The process of reverse engineering (depicted
in Figure 2.2) in general consists of three activities:

ffl extract: extracting relevant information from system software, system

experts and system history; ffl abstract : abstracting extracted
information to a higher (design) level; ffl present ; presenting
abstracted information in a developer-friendly

way, taking into account his or her current topic of interest.

Tools can be used to extract information from the system software, which
includes source code, design documentation, etc. The value of the tool's
output may depend on the availability of coding standards, and of course

2.3 Reverse Software Architecting 23 on whether these coding standards
are satisfied by the developers. For example, many implementation
languages do not explicitly support a module concept (similar to modules
in Modula-2 ), but one can force a pseudomodule concept by prescribing
certain coding rules. The extraction results are stored in a database,
which is called a repository. System experts can be interviewed to obtain
architectural information with the aid of different techniques, e.g.
think-aloud sessions, structured interviews and brain-dump sessions.
History information can be extracted from the software archive or
documentation system, providing information about the system's evolution.

Because most of the extracted information is often at programming level,
one must abstract from this information and bring it to an architecture
level. In addition, some filtering of information may be required for
certain views on the system. Developers need different views on (parts of)
the system in their daily work. The requested view is to a great extent
driven by the problem at hand, so good navigation means are needed to
retrieve information.

The abstracted information can be presented in different ways.
Developers may prefer diagrams and pictures, but more fancy media may
be applicable such as sound and vision, instead of textual descriptions,
e.g. lists of items. Hyperlinks should be added to textual descriptions
to achieve good navigation means. All these types of presentation types
have already been integrated in various Web browsers, which makes this
medium a good candidate for these purposes.

Extracted information may originate from different tools, e.g. if
multiple implementation languages are used. Combining this information
from different sources may result in incomplete or even conflicting data.
One must allow for such situations, noting that incomplete data may
appear complete at higher levels of abstraction.

In the appendices we give an overview of the tools that proved to be
useful during our research. Extraction tools are discussed in Appendix A.
Abstraction tools based on Relation Partition Algebra are presented in
Appendix B. In Appendix C we discuss some proprietary presentation tools.

24 Overview of the SAR Method

Architectural

Objectives

Functional Requirements

Architecture

Definition

Ideal Architecture

Existing Software Artefacts

Architecture

Models

Reverse Architecting

Balancing

Improving Architecture

Improvement

Plan

Improved Architecture

Re-Architecting Reverse Architecting

Forward Architecting

Figure 2.3: Architecture Improvement Process 2.4 Architecture Improvement
One can improve a system by starting from scratch again and rebuilding the
complete system. However, this is hardly an option for systems containing
software of hundreds of person-years' development. Another approach
starts with the existing system as a basis and incrementally improves
(parts of) the system.

Figure 2.3 (taken from [Kri97]) shows a process for the latter approach,
comprising three typical activities:

ffl forward architecting uses architectural objectives and functional
requirements as input for the definition of an ideal architecture. ffl
reverse architecting consists of creating explicit architectural models

of an existing system, the as-built architecture. Traditional reverse
engineering techniques can be applied to extract information from
software artefacts. Appropriate abstractions must be made to obtain
the information at an architectural level. ffl re-architecting involves
balancing an ideal architecture against the

existing architecture to prioritize a list of desired improvements. The

2.5 The SAR Method 25

next step is to implement a number of these improvements. The size
of the improvement steps depends on both business-related issues and
technological facts.

Besides software architecting experience (including knowledge of
architectural patterns), a lot of domain and system knowledge is
required to define an ideal architecture. Reverse architecting can help
in extracting domain knowledge from the system, but it can also clarify
existing architectural patterns in the system. The recovered patterns
may influence choices made during forward architecting. Of course, badly
chosen patterns should not be copied by the new ideal architecture, but
should be seen as cautions. The current system can give clues for defining
a new architecture in a positive sense by recovering existing patterns.
But it can also show the design decisions that failed in the past,
which must be avoided in the new system.

The whole process is iterative, in the sense that after improvements
have been implemented reverse architecting can help to make explicit
the created architecture, which may differ from the initially defined
architecture. This process is in fact similar to any improvement
activity: define the current situation (or check the previously realized
improvement), define the desired situation, and define the path to reach
the desired situation and execute it.

2.5 The SAR Method Relation Partition Algebra, architectural views,
reconstruction levels, InfoPacks and ArchiSpects are the key elements of
our Software Architecture Reconstruction (SAR) method. Architectural views
have already been discussed in Section 1.2.3 and RPA will be presented
in Chapter 3. Before we can focus on the details of our SAR method,
we have to present the notion of reconstruction levels, InfoPacks,
ArchiSpects, and a framework in to which these notions can be fitted.

2.5.1 Software Architecture Reconstruction Levels We introduce different
levels1 of software architecture reconstruction. Each SAR level covers
a range of architectural aspects that must be reconstructed.

1We have been inspired by the levels in the Capability Maturity Model
[Hum89].

26 Overview of the SAR Method

Consider a system which is hardly documented and whose software
architecture is not known. Such systems are at the initial level of
reconstruction. By making the software architecture of such a system
explicit (i.e. reverse architecting the system), we reach the described
level of SAR2. If the gap between the ideal software architecture and
the described software architecture is too big, one must improve it by
redefining parts of it. Then, by re-architecting the system, we reach the
redefined SAR level. After the architecture improvement, one must sustain
the reached quality level. Without any precautions, the architecture will
certainly degenerate after a while. If we can continuously preserve the
software architecture in a controlled way, we reach the managed level.
Now that we have the software architecture completely under control, we
can optimise the architecture for all kinds of future extensions, which
is called the optimised level. So, the following software architecture
reconstruction levels exist:

ffl initial level; ffl described level; ffl redefined level; ffl managed
level; ffl optimised level.

2.5.2 InfoPack and ArchiSpect We introduce the terms InfoPack and
ArchiSpect as the components of our software architecture reconstruction
method. An InfoPack3 is a package of particular information extracted
from the source code, design documents or any other information source.
An InfoPack contains a description of the extraction steps to be taken to
retrieve certain software information. Alternative extraction techniques
may exist for different programming languages, which are discussed as
parts of the InfoPack. Sometimes an InfoPack is specific to a certain
(programming) language or class of languages, e.g. InfoPacks working
with the notion of inheritance are of interest only for object-oriented
languages. InfoPacks may also be domain- or application-dependent, which
makes them less widely applicable. Examples are the Import InfoPack,
import dependency extraction, and the Part-Of InfoPack, extraction of
the decomposition hierarchy. An ArchiSpect4 is a

2We may assume that each system (especially systems that exist for many
years) contains some notion of software architecture, although it has
not been explicitly documented.

3The term InfoPack is an abbreviation of the phrase information package

4The term ArchiSpect combines the words architecture and aspect.

2.5 The SAR Method 27 view on the system that makes explicit a certain
architectural structure. An ArchiSpect is more abstract than an InfoPack
and therefore more widely applicable. Most ArchiSpects build upon the
results of InfoPacks. A complete set of ArchiSpects in fact describe a
system's actual architecture; the InfoPacks serve as supporting units
to construct the ArchiSpects. Besides abstraction of information, an
ArchiSpect covers possible ways of presenting architectural information.
Examples are the Component Dependency ArchiSpect, recovery of dependency
between the components of a system, and the Layering Conformance
ArchiSpect, verifying whether a system is correctly layered.

In this thesis we describe InfoPacks and ArchiSpects according to a
fixed scheme:

ffl Name: the name of the InfoPack or ArchiSpect ; ffl Context : the
architectural view (see Section 1.2.3) to which it belongs

and the related InfoPacks and ArchiSpects (as will be clarified in
the description); ffl Description: an introduction to the InfoPack and
ArchiSpect; ffl Example: typical example(s) from Philips' systems (as
appeared after

the method had been applied); ffl Method : a description of the steps
that must be taken to construct

the InfoPack or ArchiSpect; ffl Discussion: discussion of items not
addressed in one of the above

sections (e.g. discussion of related work).

We can look at InfoPacks and ArchiSpects in different ways. The method
view focuses on the description of steps and guidance. The tool view
concerns the tools required to support the application of ArchiSpects and
InfoPacks. The representation view contains the results of ArchiSpects
and InfoPacks. We will use the terms ArchiSpect and InfoPack for each
of these views; the context in which the term is used will clarify its
actual meaning.

Software Architecture Reconstruction Framework For each of the SAR levels
(besides the initial level5), we can fill out a matrix as given in Table
2.1; the rows contain the various SAR levels and the columns contain
the architectural views. The cells have been filled with InfoPacks and
ArchiSpects. InfoPacks are closely related to dedicated ex5The initial
level refers to the situation that no reconstruction has taken place,
so the cells are empty.

28 Overview of the SAR Method

Architectural Views Logical Module Code Execution Physical SAR levels
View View View View View

Optimized . . . . . . . . . . . . . . .

ArchiSpect ArchiSpect Managed . . . InfoPack . . . . . .

ArchiSpect ArchiSpect Redefined . . . InfoPack . . . . . .

ArchiSpect ArchiSpect Described . . . InfoPack . . . . . .

Initial

Table 2.1: Software Architecture Reconstruction Framework traction means
and therefore they appear in the code view column. All architectural
views are in fact important, but in this thesis we will concentrate on
the module view and code view (the non-dotted area in the SAR matrix).
In Chapter 7 we will fill out the SAR matrix with the names of discussed
InfoPacks and ArchiSpects as we experienced to be useful for the various
SAR levels.

Chapter 3 Relation Partition Algebra In the previous chapter we presented
in general terms the SAR method. The underlying model of the SAR method
consists of Relation Partition Algebra which is introduced in this
chapter. In the succeeding chapters we heavily use Relation Partition
Algebra to describe the details of the SAR method.

3.1 Introduction In this chapter we introduce Relation Partition Algebra
(RPA). RPA is based on sets and binary relations. This chapter serves as
a brief introduction to RPA, and it is also meant to introduce notations
that will be used throughout the thesis.

RPA has been defined in order to be able to formalise descriptions
of (parts of) software architectures. Furthermore, in the context of
reverse engineering one often wants to query the software structure.
RPA offers abilities to express questions in a formal notation, which
can be executed on the actual (model of the) software. Throughout this
thesis, we will see many applications of RPA for reconstructing software
architectures or beautyfing presentations of architectural information.

We will start this chapter by discussing sets and operations on sets. In
Section 3.3 binary relations and operations upon them will be presented.
The proofs of algebraic laws relating to RPA will not be given here, but
we will refer to published work [SM77, FKO98, FK99, FO99]. In Section 3.5
we will extend RPA with multi-sets and multi-relations. RPA formulas can

30 Relation Partition Algebra

also be executed; related issues will be discussed in Section 3.6.

3.2 Sets 3.2.1 Primitives of Set Theory A set is a collection of
objects, called elements or members. If x is an element of S, given any
object x and set S, we write x 2 S. The notion of set and the relation
is-element-of are the primitive concepts of set theory. We rely on a
common understanding of the meaning of these terms.

A finite set can be specified explicitly by enumerating its elements.
The elements are separated by commas, and the enumeration is enclosed
within brackets. So, the set which contains elements a, b, and c
is denoted by fa; b; cg. Infinite sets cannot be listed explicitly,
so these sets are described implicitly. A set can be described using
a predicate with a free variable. The set fx 2 U jP (x)g, for given U
(another set playing the role of universe), denotes the set S such that
x 2 S if and only if x 2 U and P (x) holds.

We will use the logical operators . and ^ to denote the logical
(inclusive) or and the logical and, respectively. a . b holds if and only
if a is true or b is true or both a and b are true. a ^ b holds if and
only if a and b are true. Furthermore, a ) b holds if a is true then b
is true. a () b holds if and only if a ) b ^ b ) a.

At the end of each section we will illustrate the discussed operators
with a running example.

example

Subsystems = fOS ; Drivers; DB; Appg

Functions = fmain; a; b; c; dg InitFunctions = ff jf 2 Functions ^
f is called at initialisation timeg

3.2 Sets 31 3.2.2 Operations on Sets equal, subset, superset, size
Two sets S1 and S2 are equal, denoted by S1 = S2, if for each x it
holds that x 2 S1 () x 2 S2. A set S1 is contained in S2, or S1 is a
subset of S2 denoted by S1 ` S2, if for each x it holds that x 2 S1 )
x 2 S2. A similar definition holds for a superset, S1 ' S2, which is
an alternative notation for S2 ` S1. A strict subset (superset) is a
subset (superset) from which equality is excluded. It is denoted by ae
respectively oe. The number of elements in a finite set is called the
size, denoted by jSj.

union, intersection The union of two sets S1 and S2, denoted by S1 [ S2,
is the set T = fxjx 2 S1 . x 2 S2g. The intersection of two sets S1 and
S2, denoted by S1 " S2, is the set T = fxjx 2 S1 ^ x 2 S2g.

difference, complement The difference of two sets S1 and S2, denoted
by S1 n S2, is the set T = fxjx 2 S1 ^ x 62 S2g. It is also called the
relative complement of S2 with respect to S1. The complement of a set S,
denoted by S, is the set T = fxjx 62 Sg. Given that U is the universe,
containing all elements, the complement of a set S can be written as:
S = U n S.

example

Subsystems = fOS ; Drivers; DB ; Appg InitFunctions ` Functions

UpperLayers = fDB ; Appg UpperLayers = fOS ; Driversg (with respect to
Subsystems)

32 Relation Partition Algebra

3.3 Binary Relations 3.3.1 Primitives of Binary Relations Besides
the notion of sets, we need more to describe software structures.
Relationships between (software) entities play an important role in
architecture and design. Binary relations can express such relationships.
For example, function-calls within a system can be seen as the binary
relation named calls.

A binary relation, or shortly a relation, from X to Y is a subset
of the cartesian product X \Theta  Y . It is a set of tuples hx; yi
where x 2 X and y 2 Y . Tuples of a binary relation R can be denoted in
different ways. The following notations are used to refer to an element
of a binary relation:

ffl infix notation: xRy ffl prefix notation: R(x; y) ffl tuple notation:
hx; yi

In relational terms calls (main; a) is an abstraction of the following
program fragment (written in the programming language C [KR88]):

void main () -

.... a(12, i, &ref); .... ""

Besides a textual representation of relations, one can also represent
a relation in a directed graph. A directed graph, or shortly digraph,
consists of a set of elements, called vertices, and a set of ordered pairs
of these elements, called arcs [WW90]. Assume a digraph G represents the
relation R ` X \Theta  Y . The arcs of G represent the tuples of R; the
vertices represent elements of X [ Y . The vertices with outgoing arcs
are elements of X and vertices with incoming arcs are elements of Y .
The calls relation of a (fictive) program is shown in Figure 3.1.

example

calls = fhmain; ai; hmain; bi; ha; bi; ha; ci; ha; di; hb; dig

3.3 Binary Relations 33

main a b

c d Figure 3.1: Directed Graph Representing calls Relation

3.3.2 Operations on Binary Relations Relations are sets (of tuples),
so we inherit the definitions of equality, containment, size, union,
intersection, difference and complement from the previous section.

converse The converse of relation R, denoted by R\Gamma 1, is obtained
by reversing the tuples of R: R\Gamma 1 = fhy; xijhx; yi 2 Rg.

product, identity The cartesian product of two sets X and Y , denoted
by X \Theta  Y , is the relation R = fhx; yijx 2 X ^ y 2 Y g. A special
relation Id X, or just Id if the set X is obvious, is called the identity
relation. It is defined as Id X = fhx; xijx 2 Xg.

domain, range, carrier The domain of a relation R, denoted by dom (R),
is the set S = fxjhx; yi 2 Rg. The range of relation R, denoted by ran
(R), is the set S = fyjhx; yi 2 Rg. The carrier of a relation R, denoted
by car (R), is defined as dom(R) [ ran(R).

34 Relation Partition Algebra

restriction The domain restrict of a relation R with respect to a
set S, denoted by R _dom S, is a relation T = fhx; yijhx; yi 2 R ^
x 2 Sg. The range restrict of a relation R with respect to a set S,
denoted by R _ran S, is a relation T = fhx; yijhx; yi 2 R ^ y 2 Sg.
The carrier restrict of a relation R with respect to a set S, denoted
by R _car S, is a relation T = fhx; yijhx; yi 2 R ^ x 2 S ^ y 2 Sg.
The carrier restrict can also be defined as: R _car S = (R _dom S) _ran S.

exclusion A variant of restriction is exclusion. The domain exclude of
a relation R with respect to a set S, denoted by Rndom S, is a relation
T = fhx; yijhx; yi 2 R ^ x 62 Sg. The range exclude of a relation,
denoted by Rnran S, is a relation T = fhx; yijhx; yi 2 R ^ y 62 Sg.
The carrier exclude of a relation R, denoted by Rncar S, is a relation
T = fhx; yijhx; yi 2 R ^ x 62 S ^ y 62 Sg. The carrier exclude can also
be defined as Rncar S = (Rndom S)nran S.

top, bottom The top of a relation R, denoted by ?(R), is defined as dom(R)
n ran(R). Given a directed graph of relation R, the top consists of
vertices that are a root. A root is a vertex that has no incoming arcs.
Similarly, the bottom of a relation R, denoted by ?(R), is defined as
ran (R)ndom(R). They are the leaf vertices of a directed graph, which
are the vertices with no outgoing arcs.

projection The forward projection of set S in relation R, denoted by S
\Lambda  R, is the set T = fyjhx; yi 2 R ^ x 2 Sg. The backward projection
of S in R, denoted by R \Delta  S, is the set T = fxjhx; yi 2 R ^ y 2 Sg.
Forward projection can also be defined as S \Lambda  R = ran(R _dom S)
and the backward projection can be defined as R \Delta  S = dom(R _ran S).

The left image of a relation R with respect to element y, denoted by
R:y, is the set T = fxjhx; yi 2 Rg. The right image of a relation R with
respect to element x, denoted by x:R, is the set T = fyjhx; yi 2 Rg.

3.3 Binary Relations 35 composition The composition of two relations R1
and R2, denoted by R2 ffi R1, is the relation R = fha; bij9x \Pi  ha; xi
2 R1 ^ hx; bi 2 R2g. R1; R2 is an alternative notation of the composition
R2 ffi R1. One should pronounce R2 ffi R1 as "apply R2 after R1".

Composing a relation n times, R ffi R ffi : : : ffi R is denoted by Rn.
Note that composition is associative (proof is given in [FO99]), so we
may omit parentheses around each composition. Furthermore, by definition
R0 = Id .

transitive closure The transitive closure of a relation R, denoted by R+,
is the relation T =S

1 i=1 Ri, i.e. the union of all Ri. The reflexive transitive closure
R\Lambda  is

R0 [ R+ = Id [ R+.

Special algorithms have been developed to calculate the transitive
closure efficiently. In 1962 Warshall [War62] described an O(n3) algorithm
(where n is the size of the carrier of the relation):

for i in S do

for j in S do

for k in S do

T[j,k] = T[j,k] + T[j,i] x T[i,k]

explanation The array T represents the existence (boolean value) of
tuples hi; ji in the given relation. The set S equals the carrier of this
relation. Each of the for-loops enumerates the elements in the set. The +
operation is defined as the logical or operation and the x operation is
defined as the logical and. One should read the last statement as follows
(having a digraph representation in mind): if there is a path from j to
i and there is a path from i to k, then there exists a path from j to k.

As an example we present the transitive closure of the calls relation
in Figure 3.2.

reduction A relation R is cycle-free if and only if Id " R+ = ;, in
other words, in a graph representation of relation R, there is no path
from any vertex to

36 Relation Partition Algebra

main a b

c d Figure 3.2: Transitive closure of calls

main

a b

c d Figure 3.3: Hasse of calls itself that contains an arbitrary number n
(n ? 0) of edges. The transitive reduction of a cycle-free relation R,
denoted by R\Gamma , is a relation containing all tuples of R except for
short-cuts. For example, the tuple hx; zi is a short-cut if R contains the
tuples hx; yi and hy; zi. The transitive reduction of R is also called
the Hasse [SM77] of R, or the poset of R. The transitive reduction of
a cyclic-free relation R can also be defined as R\Gamma  = R n (R ffi R+).

The expression R ffi R+ represents all the pairs of elements in R that
can reach each other indirectly (so via another vertex in the digraph).
When we substract these tuples from the original relation R we retain
the tuples which are not a shortcut. The Hasse of the calls relation is
illustrated in Figure 3.3.

example

maincalls = fhmain; ai; hmain; big maincalls ` calls calls n maincalls =
fha; bi; ha; ci; ha; di; hb; dig

3.4 Part-Of relations 37

calls = fhmain ; maini; hmain ; ci; hmain ; di; ha; main i;

ha; ai; hb; main i; hb; ai; hb; bi; hb; ci; hc; main i; hc; ai; hc; bi;
hc; ci; hc; di; hd; main i; hd; ai; hd; bi; hd; ci; hd; dig

(with respect to Functions \Theta  Functions) ?(calls) = fmain g ?(calls)
= fc; dg calls _dom fmaing = fhmain ; ai; hmain ; big

calls ndom fmaing = fha; bi; ha; ci; ha; di; hb; dig

fmain g \Lambda  calls = fa; bg

calls:b = fmain ; ag

calls + = fhmain ; ai; hmain ; bi; hmain ; ci; hmain ; di;

ha; bi; ha; ci; ha; di; hb; dig Hasse(calls) = fhmain ; ai; ha; bi; ha;
ci; hb; dig

3.4 Part-Of relations A partition of a non-empty set A is a collection
of non-empty sets such that the union of these sets equals A and the
intersection of any two distinct subsets is empty. We can see a partition
as a division of a pie into different slices.

If we give each of these subsets a name we can construct a so-called
part-of relation which describes a partition. Assume that these names are
defined in a set T , then the part-of relation P is defined as follows:
P = fhx; tijt 2 T ^ x is in the subset named tg. In source code, function
definitions are contained in a (single) file, so the relation between
Functions and Files is an example of a part-of relation; see Figure 3.4.

Partitions and part-of relations are alternative views on the same
concept: decomposition. A third view on decomposition comprises
equivalence relations. One can derive an equivalence relation E from a
part-of relation P as follows: E = fhx; yij9t hx; ti 2 P ^ hy; ti 2 P g.
The equivalence relation can also be defined as E = P \Gamma 1 ffi P .

38 Relation Partition Algebra

DB Lib Appl

main

a b

c d Figure 3.4: Partitioning Functions example

T = fAppl ; DB; Libg partof = fhmain; Appli; ha; Appli; hb; DBi; hc;
Libi; hd; Libig

eqrel = partof \Gamma 1 ffi partof

= fhmain; maini; hmain; ai; ha; ai; ha; maini; hb; bi;

hc; ci; hc; di; hd; di; hd; cig

lifting, lowering Given a relation R and a part-of relation P we can
construct a new relation Q by lifting R using P , denoted by R " P .
The result is the relation Q = fhx; yij9a; b ha; bi 2 R ^ ha; xi 2 P ^
hb; yi 2 P g. Note that the carrier of relation R must be a subset of
the domain of P .

We can also construct a new relation Q by lowering R using P , denoted
by R # P . The resulting relation is defined as Q = fhx; yij9 a; b ha;
bi 2 R ^ hx; ai 2 P ^ hy; bi 2 P g. The carrier of relation R must be
a subset of the range of P .

The given definition of lifting is in fact an existential lifting.
An alternative to the above definition of lifting is universal lifting,
denoted by R "8 P . The tuple hc1; c2i is an element of R "8 P if and
only if for all x1 in c1 and for all x2 in c2 it holds that hx1; x2i 2 R.
We can now relate lifting and lowering as follows: R = (R # P ) "8 P ).
Very little use is made of universal lifting, so this alternative
definition of lifting will not be used after this section1.

1We will therefore write " instead of "

9.

3.5 Introducing multiplicity in RPA 39

DB Lib Appl

Figure 3.5: Lifting calls example Figure 3.5 shows the lifted calls
relation, which is calculated as follows:

callsLift = calls " partof

= fhAppl; Appli; hAppl; DBi; hAppl; Libi; hDB; Libig

3.5 Introducing multiplicity in RPA While reverse architecting the Med
system, we discovered that it is also useful to attribute a weight to
the tuples of a relation. This leads to the introduction of multiplicity
into RPA, by means of multi-relations. In the Chapter 5 we will see
the importance of applying multi-relations which is also illustrated
by examples.

A multi-relation is a collection of tuples in which each tuple may occur
more than once. We will represent the tuples and their corresponding
weights as a triple hx; y; ni, where n is the number of occurrences
of the tuple hx; yi. In a running system the number of calls(a; b) may
be of interest when looking at e.g. recursion: f: : : ; ha; b; 7i; : :
:g is the representation of a calls b seven times.

Multi-relations compare to relations as bags (or multi-sets) compare to
sets. Multi-sets (or bags) can be represented as sets of tuples, with the
second argument being the number of occurrences of the first argument.

40 Relation Partition Algebra

3.5.1 Calculating with weights We must first describe the basics
for calculating with weights before we can define multi-sets and
multi-relations and their operations. Weights are natural numbers (the
set f0; 1; 2; : : :g denoted by IN ) extended with an explicit value 1.
The arithmetic operations + and \Theta  work as usual if both arguments
are elements of IN . Their behaviour when applied to 1 is given by the
following rules that hold for all n 2 IN :

n + 1 = 1 + n = 1 1 + 1 = 1

0 \Theta  1 = 1 \Theta  0 = 0

n 6= 0 ) n \Theta  1 = 1 \Theta  n = 1 1 \Theta  1 = 1

Substraction of weights is also special. Take for example the rule (n
\Gamma  m) + m = n which only satisfies when n * m. If n ! m, then we
define n \Gamma  m = 0; we must note that the algebraic law (n \Gamma  m)
+ k = (n + k) \Gamma  m does not hold. Furthermore, the following rules
are given, for all n 2 IN:

n \Gamma  1 = 0 1 \Gamma  1 = 0

1 \Gamma  n = 1

The minimum of two weights is denoted by min(n1; n2), to which we add
the rules that min(1; n) = min(n; 1) = n and min(1; 1) = 1. Similarly,
we define max (n1; n2), to which we add the rules max (1; n) = max (n;
1) = 1 and max (1; 1) = 1.

By definition, elements that are not members of a multi-set or
multi-relation have a weight of 0. This simplifies the definitions of
operations on multi-sets and multi-relations.

3.5 Introducing multiplicity in RPA 41 3.5.2 Operations on Multi-Sets
mapping The n-mapping of a set T to a multi-set S, denoted by dT en,
is defined as S = fhx; nijx 2 T g. Furthermore, we define dT e = dT e1.
The mapping of a multi-set S to a set T , denoted by bSc, is defined as
T = fxjhx; ni 2 S ^ n ? 0g.

equal, subset, superset, size Two multi-sets are equal, denoted by S1 =
S2, if for each e it holds that he; ni 2 S1 () he; ni 2 S2. A multi-set
S1 is a subset of S2, denoted by S1 ` S2 if for each e it holds that he;
n1i 2 S1 ^ he; n2i 2 S2 ) n1 ^ n2. A multi-set S1 is a superset of S2,
denoted by S1 ' S2 if for each e it holds that he; n1i 2 S1 ^ he; n2i 2
S2 ) n1 * n2. The size of a multi-set S, denoted by kSk, is defined as
Phx;ni2S n.

union, addition, intersection, difference The intersection of two
multi-sets S1 and S2, denoted by S1 " S2, is defined as the multi-set T =
fhe; nijhe; n1i 2 S1 ^ he; n2i 2 S2 ^ n = min(n1; n2)g. The union of two
multi-sets S1 and S2, denoted by S1 [ S2, is defined as T = fhe; nijhe;
n1i 2 S1 . he; n2i ^ n = max (n1; n2)g.

The difference between two multi-sets, denoted by S1 n S2, is defined
as S = fhe; nijhe; n1i 2 S1 ^ he; n2i 2 S2 ^ n = n1 \Gamma  n2)g.
The addition of two multi-sets, denoted by S1 + S2, is defined as S =
fhe; nijhe; n1i 2 S1 ^ he; n2i 2 S2 ^ n = n1 + n2)g.

complement The complement of a multi-set S1 with respect to the set S
is defined as: S1 = dSe1 n S1.

42 Relation Partition Algebra

3.5.3 Operations on Multi-Relations mapping The n-mapping of a relation
R to a multi-relation M , denoted by dRen, is defined as M = fhx; y;
nijhx; yi 2 Rg. Furthermore, we define dRe = dRe1. The mapping of a
multi-relation M to a relation R, denoted by bM c, is defined as R =
fhx; yijhx; y; ni 2 M g.

equal, subset, superset, size Two multi-relations are equal, denoted by M1
= M2, if for each x and y it holds that hx; y; ni 2 M1 () hx; y; ni 2 M2.
A relation M1 is contained in M2, denoted by M1 ` M2, if for each x and
y it holds that hx; y; ni 2 M1 ^ hx; y; mi 2 M2 ) n ^ m. Similarly to
binary relations ', ae and oe are defined for multi-relations. The size
of a multi-relation M , denoted by kM k, is defined as Phx; y; ni2M n.

union, addition, intersection, difference The union of two multi-relations
M1 and M2, denoted by M1 [ M2, is the multi-relation M = fhx; y; nij(hx;
y; n1i 2 M1 . hx; y; n2i 2 M2) ^ n = max (n1; n2)g. The addition of two
multi-relations M1 and M2, denoted by M1 + M2, is the multi-relation
M = fhx; y; nij(hx; y; n1i 2 M1 . hx; y; n2i 2 M2) ^ n = n1 + n2g.
The intersection of two multi-relations M1 and M2, denoted by M1 " M2,
is the relation M = fhx; y; nij(hx; y; n1i 2 M1 ^ hx; y; n2i 2 M2) ^
min(n1; n2)g. The difference between two multi-relations M1 and M2,
denoted by M1 nM2, is the relation M = fhx; y; nij(hx; y; n1i 2 M1 ^
hx; y; n2i 2 M2) ^ n = n1 \Gamma  n2g.

converse The converse of relation M , denoted by M \Gamma 1, is obtained
by reversing the first two arguments of the triples: M \Gamma 1 = fhy;
x; nijhx; y; ni 2 M g.

3.5 Introducing multiplicity in RPA 43 product, identity The cartesian
product of two multi-sets X and Y , denoted by X \Theta  Y , is
the multi-relation M = fhx; y; nijhx; n1i 2 X ^ hy; n2i 2 Y ^ n = n1
\Theta  n2g. A special multi-relation Id X;n, or just Id n if the set X
is obvious, is called the identity relation. The identity of a set X is
defined as Id X;n = dId X en. When omitted, n must be considered to be 1.

domain, range, carrier The domain of a multi-relation M , denoted by
dom(M ), is the multi-set S = fhx; nijn = Phx; y; mi2R mg. The range of
a multi-relation M , denoted by ran(M ), is the multi-set S = fhy; nijn
= Phx; y; mi2R mg. The carrier of a multi-relation M , denoted by car
(M ), is defined as dom(M ) + ran(M ).

restriction The domain restrict of a multi-relation M with respect to
a set S, denoted by M _dom S, is a multi-relation T = fhx; y; nijhx; y;
ni 2 M ^ x 2 Sg. The range restrict of a multi-relation M with respect to
a set S, denoted by M _ran S, is a multi-relation T = fhx; y; nijhx; y;
ni 2 M ^ y 2 Sg. The carrier restrict of a multi-relation M with respect
to a set S, denoted by M _car S, is a relation T = fhx; y; nijhx; y;
ni 2 M ^ x 2 S ^ y 2 Sg. The carrier restrict can also be defined as:
M _car S = (M _dom S) _ran S.

exclusion The domain exclude of a multi-relation M with respect to a
set S, denoted by M ndom S, is a relation T = fhx; y; nijhx; y; ni 2
R ^ x 62 Sg. The range exclude of a multi-relation M with respect to
a set S, denoted by M nran S, is a relation T = fhx; y; nijhx; y; ni 2
R ^ y 62 Sg. The carrier exclude of a multi-relation M , denoted by M
ncar S, is a relation T = fhx; y; nijhx; y; ni 2 M ^ x 62 S ^ y 62 Sg.
The carrier exclude can also be defined as M ncar S = (M ndom S)nran S.

44 Relation Partition Algebra

top, bottom The top of a multi-relation M , denoted by ?(M ), is defined
as ?(M ) = dom (M _dom ?(bM c). The bottom of a multi-relation M ,
denoted by ?(M ), is defined as ?(M ) = ran(M _ran ?(bM c).

projection The forward projection of a set S in a multi-relation M ,
denoted by S \Lambda  M , is the multi-set T = fhy; nijn = Phx; y;
mi2M^x2S mg. The backward

projection of a set S in a multi-relation M , denoted by M \Delta  S,
is the set T = fhx; nijn = Phx; y; mi2M^y2S mg. Forward projection can
also be

defined as S \Lambda  M = ran (M ndom S) and the backward projection
can be defined as M \Delta  S = dom (M nran S).

The left image of a multi-relation M of y, denoted by M:y, is the
multi-set T = fhx; nijhx; y; ni 2 M g. The right image of a multi-relation
M of x, denoted by x:M , is the multi-set T = fhy; nijhx; y; ni 2 M g.

composition The composition two multi-relations M1, denoted by M2 ffi
M1, is defined as M = fhx; z; nijn = Phx; y; n1i2M1^hy; z; n2i2M2 n1
\Theta  n2g.

Given a matrix representation of M1 and M2, where the cells contain the
weight of a tuple hx; yi, the composition consists of the multiplication
of both matrices [FK99]. Given a representation of a directed graph with
weighted edges, the composition consists of the number of all possible
paths from x to z, by taking two steps: the first step in M1 and the
second step in M2.

transitive closure The transitive closure of a multi-relation M ,
denoted by M +, is defined as M + = S1i=1 M i, i.e. the union of all M i.
The reflexive transitive closure, denoted by M \Lambda , is defined as
M 0 [ M +.

Warshall's algorithm for calculating transitive closures must be adapted
a bit. Here, we give the adapted Warshall algorithm, the proof of
correctness is given in [FK99].

3.5 Introducing multiplicity in RPA 45 for i in S do

for j in S do

for k in S do

if T[i,i] == 0 then T[j,k] = T[j,k] + T[j,i] x T[i,k] else T[j,k] =
T[j,k] + INFTY x T[j,i] x T[i,k]

explanation T represents the two-dimensional (associative) array which
initially contains the multi-relation m. After completion, T contains the
multirelation m+. The value of T [i; j] represents the weight of tuple
hi; ji. The set S is the carrier of this multi-relation. Addition and
multiplication work as defined in Section 3.5.1. Comparing this algorithm
with the original one, we see that the factor INFTY is introduced when
there is a path j \Gamma  i and i \Gamma  k and T [i; i] 6= 0. If there
is a path from j to i and from i to k and there are paths from i to i
(expressed by T [i; i] 6= 0), one can reach k 1 times from j.

reduction The Hasse of a cycle-free multi-relation M , denoted by M +,
is defined as M n (M + ffi M ).

lifting, lowering Given a relation M and a part-of relation P we can
construct a new multirelation Q by lifting M using P , denoted by M "
P . The result is the multi-relation Q = fhx; y; nijn = P9a;b ha; b;
mi2M^ha;xi2P ^hb;yi2P mg.

Given a relation M and a part-of relation P we can construct a new
multirelation Q by lowering M using P , denoted by M # P . The resulting
multirelation is defined as Q = fhx; y; nijha; b; ni 2 M ^ hx; ai 2 P ^
hy; bi 2 P g.

Lifting and lowering (for a relation R as well as for a multi-relation R)
can also be defined in terms of composition:

R " P = dP e ffi R ffi dP \Gamma 1e R # P = dP \Gamma 1e ffi R ffi dP e

46 Relation Partition Algebra

\Lambda ; +; \Gamma ; \Gamma 1

ffi, ", #

\Theta

" [, + \Delta ; \Lambda ; : dom; ran ; car ; ?; ? _dom ; _ran ; _car ;
ndom ; nran ; ncar ; n

`; '; ae; oe

= d e; b c; j j; k k;

Table 3.1: RPA Operator Precedences

3.6 RPA Formulas in Action In the next chapters we will use RPA formulas
to express e.g. abstractions of software information. Before we can define
these (composed) formulas, we have to explain how we must interpret these
formulas: precedences of operators, and the notations applied for sets,
relations and multi-relations. In this section we will also discuss how
a given formula can be executed on a computer.

3.6.1 Precedences of Operations When we combine operators to construct
larger expressions, we must say something about the order in which the
operators must be applied. Precedence levels of operators indicate the
way in which an expression is implicitly grouped into separate parts.
In fact, precedence levels automatically place parentheses around
parts of the expression to prescribe the order in which the operators
are to be applied. In the case of equal precedence level, we apply
the left-associative rule, meaning that e.g. a + b + c = (a + b) + c.
The mapping, size, and complement operators already group expressions by
their notations. The precedence levels of the RPA operators are given
in Table 3.1 (at the top of the table are the operators with highest
precedence). In this thesis we will often use parentheses in formulas
for reasons of readability.

3.6 RPA Formulas in Action 47 3.6.2 Notational Aspects We will use a
special notation to distinguish various sets, multi-sets, relations
and multi-relations. For sets and multi-sets we will use the same
notation e.g. a set of functions will be denoted by Functions.
A relation representing function calls in a system, calls ` Functions
\Theta  Functions, will be denoted by callsFunctions;Functions .
For multi-relations, we will use a similar notation, except that we will
emphasize multiplicity as follows callsFunctions;Functions. Using this
notation, we can immediately qualify the relation's domain and range.
Relations with the same base names, but operating on different domains
and in different ranges can be easily identified.

3.6.3 Execution of RPA formulas We use many RPA formulas to describe
the software architecture reconstruction method. Each RPA formula can be
easily transformed into an executable code. Sets, multi-sets, relations
and multi-relations are expressed in special formatted files on the
file system. For example, the file named calls.Functions.Functions
contains the callsFunctions;Functions relation. The application of an
operator to one or more operands consists in calling the appropriate
program or function given the proper input files. A discussion of some
RPA implementations is given in Appendix B.

example Consider the following formulas (copied from Section 4.10):

imports Files;Comps = partof Files;Comps ffi imports Files;Files
importsExt Files;Comps = imports Files;Comps n partof Files;Comps

UsingExts = dom(importsExt Files;Comps ) usingFiles;Comps = partof
Files;Comps _dom UsingExts

Initially, we have the following files (representing relations), which
are results of extraction tools:

ffl imports.Files.Files representing imports Files;Files ; ffl
partof.Files.Comps representing partof Files;Comps .

48 Relation Partition Algebra

We can translate the above formulas straightforwardly into executable
code (e.g. executed in a Unix shell). We do not need any knowledge of
the semantics of the formulas to make this translation2.

rk.csh: rel.comp partof.Files.Comps imports.Files.Files "

? imports.Files.Comps rk.csh: rel.diff imports.Files.Comps
partof.Files.Comps "

? importsExt.Files.Comps rk.csh: rel.dom importsExt.Files.Comps ?
UsingExts rk.csh: rel.domR partof.Files.Comps UsingExts "

? using.Files.Comps

After we have performed the calculations we have the following files:

ffl imports.Files.Files (imports Files;Files); ffl partof.Files.Comps
(partof Files;Comps ); ffl imports.Files.Comps (imports Files;Comps );
ffl importsExt.Files.Comps (importsExt Files;Comps ); ffl UsingExts
(UsingExts); ffl using.Files.Comps (usingFiles;Comps ).

3.7 Discussion Work relating to Relation Partition Algebra has already
been discussed in [FO94, FKO98]. From [FKO98] we pick out the work of
Holt [Hol96, Hol98], as it shows a remarkable correspondence to our work.
Though RPA has been developed independently, both approaches use binary
relational algebra (Tarski Relational Algebra [Tar41]) to describe rules
in software architecture and re-engineering applications. There are
differences between both algebra's. For example, Holt [Hol98] defines
an induction operator; it is defined as C ffiRffiP ; C is a containment
relation and P is a parent relation (P = C\Gamma 1). Holt does not define
containment relation as a representation of a (hierarchical) partitioning.
This means that a module A may be contained in component X as well as
component Y. In RPA, the lift operator is more carefully defined in
this respect.

Furthermore, Holt treats a system's hierarchical decomposition as
a single containment relation, so he does not distinguish different
levels of contain2In this Unix shell the prompt is named rk csh:, the
backslash informs the shell that the command continues on the next line
and the operator ? means that the resulting output is written into the
named file.

3.7 Discussion 49 ment. We distinguish different partof relations to
represent various levels in a system's hierarchical decomposition.
Holt does not define transitive reduction, which is a useful operator
for improving the presentation of graphs with many (directed) edges.

The need for executing relational formulas (see Appendix B) is also
recognized by Holt. He calls his relational calculator grok.

50 Relation Partition Algebra Chapter 4 Described Architecture In
the next three chapters we discuss three levels of the SAR method
(respectively described, redefined and managed level). In this thesis,
as already mentioned, for each of these levels we focus on the code
view and module view of software architecture. Here, we start with the
described level of SAR.

4.1 Introduction Figure 4.1 shows an abstract view on our software
architecture reconstruction (SAR) method. The main and most explicit
source of information for reconstructing a software architecture is
the source code. The source code can be analysed and be reduced to
manageable units of information, which we call InfoPacks. Information
that cannot be extracted from the source code must be supplemented with
information from e.g. software architects. Relation Partition Algebra is
the model underlying most of the ArchiSpects. The results of InfoPacks are
expressed in a simple notation, namely the RPA-file formats introduced
in Section 3.6.3. InfoPacks yield intermediate results that are used to
construct ArchiSpects.

The described software architecture of an existing system consists of
a set of ArchiSpects, each containing a relevant aspect of the software
architecture. The main aim of the resulting described architecture is to
support comprehensability of software architecture for (new) software
developers and architects. In this chapter we will focus solely on
ArchiSpects and their required InfoPacks that are related to the module
and code view of

52 Described Architecture

Software Architects

Relation Partition Algebra

Source

Code

ArchiSpect

InfoPack

ArchiSpect ArchiSpect InfoPack InfoPackInfoPack

Figure 4.1: Software Architecture Reconstruction Method architecture.
The process of reconstructing the software architecture at a described
level is called reverse architecting [Kri97].

Figure 4.2 shows the InfoPacks (rectangles) and ArchiSpects (hexagons)
initially required to describe a software architecture. Solid lines
between InfoPacks and ArchiSpects (InfoPack) indicate that the output of
the InfoPack is input for the ArchiSpect (InfoPack). Dotted lines mean
that there is a relationship between the two, which is not expressed
in terms of input and output results (the relationship is clarified
in the description of the Infopack or ArchiSpect). In the SAR method,
InfoPacks and ArchiSpects are classified per architectural view, which
is indicated by the two large boxes: code view and module view.

In this chapter we will discuss (see also Figure 4.2) the InfoPacks
Files, Import, Part-Of and Depend of the code view, the ArchiSpects
Source Code Organisation and Build Process of the code view and the
ArchiSpects Software Concepts Model, Component Dependency and Using
and Used Interfaces of the module view. The InfoPacks and ArchiSpects
will be discussed according to the fixed scheme (context, description,
example, method, discussion) introduced in Section 2.5.2. The InfoPacks
and ArchiSpects will be discussed in the order in which they should be
applied to a system.

In recent years, we have applied these ArchiSpects to a number of real
systems; the results of some of this work will be used to illustrate
these ArchiSpects:

ffl in 1994/1995 we analysed the Tele system [KW94], which is a public

4.1 Introduction 53

Module View Code View

Part-Of Depend

Files Import

Source

Code Organisation

Build Process

Software Concepts

Model

Using and

Used Interfaces

Component Dependency

Figure 4.2: Overview of Described Architecture

54 Described Architecture

telephony switching system; ffl in 1996 we analysed the Med system
[MK97], which is a medical

imaging system; ffl in 1997/1998 we analysed the Switch system [Med98],
which is a

private telephony switching system. ffl in 1998 we analysed the Comm
system [Kri98], which is a management system for controlling digital
video communication systems.

The modular approach of our SAR method makes it possible to apply those
ArchiSpects and InfoPacks which are relevant for your case. We will
now repeat how the different sections of each ArchiSpect and InfoPack
is organised: In the first section we present the relations with other
ArchiSpects and InfoPacks, which is also indicated in Figure 4.2 by
means of lines. In the second section, a general description is given
including a motivation of why we should apply this ArchiSpect (InfoPack).
For the reader's convenience, we illustrate these ideas with examples
from practice. In the fourth section, we describe the steps to be taken
to reconstruct the ArchiSpect or InfoPack. The last section discusses
related issues or, where appropriate, related work.

4.2 ArchiSpect: Software Concepts Model 4.2.1 Context The Software
Concepts Model belongs to the module view of software architecture.
The ArchiSpect Source Code Organisation (Section 4.3) and InfoPack Part-Of
(Section 4.7) are related to this ArchiSpect.

4.2.2 Description System documentation includes many domain-specific
(or system-specific) terms. For example, in one system, a component
will be just a term for a group of smaller programming units, and,
with another, it will refer to COM components [Box98] (including its
dynamic binding machinery). A good understanding of these concepts is
needed for almost any reconstruction activity. The concepts help to
classify functionality during discussions, e.g. a subsystem is a far
more important architectural notion than a method or function.

The Software Concepts Model concentrates on the most important software

4.2 ArchiSpect: Software Concepts Model 55 concepts and their
inter-relationships. Files, functions, types and relations, such as
function accesses data, function calls function and component contains
function, are often included. Furthermore, the various abstraction levels
of software parts (decomposition levels) are described in this ArchiSpect.
We will expound this ArchiSpect by providing a number of examples from
practice.

The result of this ArchiSpect consists of a UML class diagram [Fow97]
containing these concepts and their inter-relations.

The Part-Of InfoPack fills out which of the system's entities belong to
the various decomposition levels plus the various partof relations between
these levels. The software concepts must be mapped onto real items in
the software code organisation, e.g. a subsystem resides in a directory,
which is discussed in the Software Code Organisation ArchiSpect.

4.2.3 Example Tele Figure 4.3 shows the concepts and relationships
between software concepts of the Tele telecommunication system in a UML
class diagram.

The system consists of a number of subsystems. Each building block belongs
to exactly one subsystem. A building block is an aggregation of files;
each file addresses a single aspect (see Section 1.5.3). Furthermore,
a product in the product family is described as a parts list of building
blocks (and hardware elements). building blocks reside in layers which are
strictly ordered (!); see also Section 1.5.1. The concepts at the bottom
of the class diagram describe programming concepts and relationships
between them.

Med The Software Concepts Model of the Med system is depicted in Figure
4.4. The system consists of a number of subsystems. Each subsystem is
an aggregation of components and a component is divided into several
packages. A number of files are contained in a package. A few programming
concepts have been represented at the bottom of the diagram. An archive
is a set of subsystems; this notion was introduced at a later stage in
Med 's life-cycle.

56 Described Architecture

*

type ** 1*** function

*

1 aspect* * data

*

type

**

contains * 1

istyped *

function

**

access * *

calls

* role* * **product

* file 1* addresses

* **

* building block ** plays** parts

*

1

layer 1 layer

*

resides

11 subsystem

*

system

**

Figure 4.3: Software Concepts Model of Tele

4.2 ArchiSpect: Software Concepts Model 57

system

*

archive * subsystem

* *

*

*** function

*

component

*

*

** function * *

calls

package

*

* *macrodef

* data ** access

1

file

**

* *

*

*

type * type * 1

istyped

*

** contains

Figure 4.4: Software Concepts Model of Med 4.2.4 Method The creation of
the Software Concepts Model is an iterative process. It is important to
read relevant system documentation, but often it is also necessary to
discuss various non-documented concepts. In particular, relationships
between concepts must be made explicit, which will involve discussions
with architects. The results of these activities can be presented in a
class diagram notation of the UML language [Fow97]. During this iterative
process, the class diagrams should serve as input for discussion.

4.2.5 Discussion Software concepts play an important role in the
definition of a software architecture. If the software architecture
has been properly defined, one can easily obtain the software concepts
from the documentation. As most products usually have a long life-cycle,
we may assume that new concepts were not initially foreseen and they are
consequently not completely and/or properly applied in the software today.
An example of a concept that was

58 Described Architecture

introduced only at a later stage is the archive in the Med system.
In particular, concepts that do not have a counterpart in a programming
language may become a subject of discussion. Different architects may
think differently about the semantics of such a concept. For the overall
development it is of importance to make such fuzzy concepts more concrete.
This may even result in the introduction of new concepts to achieve a
better Software Concepts Model.

The UML associations aggregation and composition represent a special
relation; they describe the decomposition tree at a generic level.
In the presented examples we do not have recursive definitions in
the decomposition tree. In practice, such recursions may exist, e.g.
a component may contain either a set of files or a set of components.

4.3 ArchiSpect: Source Code Organisation 4.3.1 Context The Source
Code Organisation belongs to the code view of software architecture.
The ArchiSpects Software Concepts Model (Section 4.2), Build Process
(Section 4.4), InfoPacks Files (Section 4.5) and Part-Of (Section 4.7)
are related to this ArchiSpect.

4.3.2 Description The Source Code Organisation ArchiSpect consists of
three parts: description of the way in which source files are stored,
the mapping of source code onto software concepts and a description of
the process of retrieving files from the configuration management system
[BHS80, Bab86].

Many deliverables (source code, design documents, etc.) are produced
during system development. These deliverables must be easily accessible
to all the developers. Sometimes, previous versions of documents
may also be requested. Different people must be able to modify the
same documents, but such concurrent access may not result in loss of
information. The functionality referred to above is offered by most of
the configuration management systems, e.g. Continuus [Con] or ClearCase
[Cle]. Most of the configuration management systems are based on a file
system (functionality is implemented by locking files, storing versions
in different directories,

4.3 ArchiSpect: Source Code Organisation 59 etc.). Source code is one of
the deliverables that is stored in the configuration management system;
see also the Build Process ArchiSpect. A list of source code files that
belong to a certain release is needed to be able to analyse source code
(see Files InfoPack). A more general description of the location of
files, taking into account different versions, is contained in Source
Code Organisation. Also included is the mapping of elements of this
ArchiSpect onto concepts of the Software Concepts Model .

During system development, files occur in different development states,
e.g. a file is reserved by a developer who extends or modifies it.
After the developer has finished, he or she consolidates the file by
restoring it in the archive. Describing these development states, their
possible transitions and the people who initiated these transitions
provides insight into the development process.

4.3.3 Example Med Figure 4.5 shows a model of the Source Code Organisation
of the Med system in a UML class diagram [Fow97]. In general, a software
archive contains various versions of the system. Each version consists
of a number of directories in which files reside. Every time the system
is released, a copy of the current version is created.

Table 4.1 shows how some concepts of the Software Concepts Model, (see
Figure 4.4) are reflected in the Source Code Organisation (Figure 4.5).
A system is reflected onto a version and a component onto a directory.
Filenames start with a special prefix of four characters which refers to
the package name. The concepts subsystem and archive are not explicitly
reflected in the Source Code Organisation.

Figure 4.6 shows the development states of files and possible transitions
in a state diagram (UML). The developer (integrator) is in control of
the states and transitions with a bold (italic) font style. For example,
a developer decides to grab a file from the archive. When he or she has
finished modifying the grab-ed file, he or she preptake-s the file which
becomes ready for archiving. The integrator take-s the file and tries
to build an alpha version of the system. In the event of problems he or
she may reject the file; the reject -ed file must be accept-ed by the
(latest preptaking) developer who

60 Described Architecture

software archive

* version

*

* directory

*

* file

*

Figure 4.5: Source Code Organisation of Med

Software Concepts Source Code

Model Organisation

system version component directory package + file file

Table 4.1: Software Concepts Model vs. Source Code Organisation of Med

4.3 ArchiSpect: Source Code Organisation 61

GRAB READYpreptake

ALPHAREJECT ARCHIVE

take

consolidate

reject accept grab

Figure 4.6: Development States and Transitions of Med Files has to
correct the file. If the integrator decides that the file satisfies,
he or she consolidate-s the file in the archive.

4.3.4 Method The Source Code Organisation is often well described in
documents. The documentation of the configuration management system (CMS)
contains additional information. The first reconstruction activity to be
performed consists of reading CMS documentation and filtering the proper
information. Additionally, one can interview system integrators and people
concerned with configuration management. The Source Code Organisation can
be described using class diagrams of the UML language [Fow97]. The next
step is to describe the mapping from software concepts onto the entities
of this ArchiSpect and identify the gaps in this mapping scheme.

The development states and a development transition diagram can also
be extracted by interviewing system integrators and/or reading the
appropriate documentation. State diagrams of the UML language can be
used to document it.

4.3.5 Discussion The top-level concepts (e.g. subsystems) in the Software
Concepts Model are often not reflected on any tangible item of the Source
Code Organisation. There is a risk of the meaning these concepts, which
exist only at a more conceptual level, degenerating with time.

62 Described Architecture

Pre-Compile Pre-Compile Compile

LinkCompilePre-Compilepre-compile compile link post-process

Figure 4.7: Build Activities Mapping between software concepts, Software
Concepts Model , and source code items, Source Code Organisation,
as depicted in e.g. Table 4.1, is relevant for analysing a system.
The results of extraction tools must be mapped onto software concepts
to be able to ascend in the system's decomposition hierarchy.

Some steps of the SAR method can be automated, but also integrated in the
development process. Knowledge of the development states and transitions
is required for integrating SAR steps in the development process.

4.4 ArchiSpect: Build Process 4.4.1 Context The Build Process ArchiSpect
belongs to the code view of software architecture. This ArchiSpect is
related to the Source Code Organisation (Section 4.3) and the Depend
InfoPack (Section 4.8).

4.4.2 Description The Build Process ArchiSpect includes a description of
how various pieces of code (see also Source Code Organisation ArchiSpect)
must be processed to derive all the executables that comprise the system.
This process can be split into a number of smaller build activities,
each describing how to create a (intermediate) result from inputs.
The different intermediate results are input for new build activities.
In Figure 4.7 the cascade of smaller build activities has been divided
into four categories: pre-compile, compile, link and post-process.

4.4 ArchiSpect: Build Process 63 The pre-compile category consists of
code generation activities, e.g., generation of scanners and parsers
from higher-level descriptions. The compile category consists of
source-code-compilation activities. Each input is either a result of
a pre-compile activity or it is created by hand. The link category
consists of linking the results of the compile activities into one or
more executables. The post-process activities consist of gathering all
the required files (executables, resource files, bitmaps, help texts,
etc.) and loading them on the target system.

If a file changes (as indicated by a file's modification date), all
its derivatives must be rebuilt. To rebuild an entire system, this
rebuild process must be recursively applied, so derivatives of modified
derivatives must also be rebuilt, and so on. There are several tools
for supporting this mechanism, of which the make [Fel79] utility is
probably the best-known. All these tools work with a build description
file that contains the dependencies between the various files and a
description of how to create the results by defining commands which
must be executed, e.g. CC -I../finance ajax.c. After some modification
a build tool will update all the (intermediate) derivatives by executing
the proper commands.

4.4.3 Example Med The Build Process of the Med system is depicted in
Figure 4.8. A build description has been distributed amongst different
files (e.g. acq.opt, acq.od and acq.tgt), each of which is responsible for
a certain task. The dotted arrows indicate how these build description
files affect the various build activities. With this system the
pre-compile activities have been merged with the compile activities
and they are therefore not explicitly depicted. The imports relation
is in fact part of a compile activity (a pre-processor of a compiler).
The post-process activity consists of target-ing various files on
the system.

Every night all the files that are in the archive or alpha development
states (see Figure 4.6) are built. The system integrator is responsible
for starting the whole build procedure, which is in fact completely
automated, every evening. Early in the morning, after a successful
build process, the system's main features are briefly tested. This test
lasts approximately half an hour. It is executed just before most of
the developers arrive at work. After the test, some files are marked as
reject -ed while others are stamped archive.

64 Described Architecture

targets acq.h

rec.c

rec.obj

acq.exe acq.exe

imports compiles

links

IncludeFiles

BodyFiles ObjectFiles

ExecFiles TargetFiles

base.h

acq.c acq.dcl

acq.comacq.obj

dutch.res

dutch.res acq.opt

acq.odacq.tgt

BuildFiles

imports

acq.com Figure 4.8: Build Process Med This process of building and
testing a system is similar to Daily Build and Smoke Test [CS95].

4.4.4 Method The build process is well documented for most systems.
One should therefore read the appropriate documents including discussions
with developers. The main task is to develop an abstract model of this
process. In addition, consulting the build description files may also
be of help in modelling build process; see also the Depend InfoPack.
We experienced that cooperation with integrators during the integration
test helps in shaping this ArchiSpect.

4.4.5 Discussion In one of the systems we investigated the build process
is described in a generic way. For most of the files a similar command
must be executed to compile the source code. In fact, one can define such
a generic command per programming language. Similar statements hold for
determining the dependencies between the various files. Per language,
a tool can determine

4.5 InfoPack: Files 65 the files on which the compilation of a file
depends; see also the Import InfoPack. Deviations from the standard way of
compilation can be defined per file. The filename, the generic command,
the programming language and the deviations from the generic compile
command can be stored in a database. A dedicated program can generate a
build description file from the records in this database. A build tool
can then execute it. An advantage of this approach is that new compilers
can be introduced simply by changing a single generic compile command.

4.5 InfoPack: Files 4.5.1 Context The Files InfoPack is part of the code
view of architecture. Knowledge of the Source Code Organisation ArchiSpect
(Section 4.3) is needed and the Depend (Section 4.8) and Import (Section
4.6) InfoPacks use the results of this InfoPack.

4.5.2 Description The Source Code Organisation ArchiSpect discusses the
organisation of files in general terms. The Files InfoPack extracts all
the source files (all the files created by humans) required to construct
a system. The results of this InfoPack serve mainly as input for other
InfoPacks. These files can be classified in different categories:

ffl Files, the files of a (version of the) system created by humans; ffl
HeaderFiles , files that specify functions, variables, etc.; ffl BodyFiles
, files that define functions, variables, etc.; ffl ResourceFiles , files
that define help texts, pictures, etc.; ffl BuildFiles, files that define
(part of) the build process; ffl Exts, extensions of file-names, e.g.
java, cpp; ffl Cats, categories of files, e.g. C-source; ffl PhFiles,
physical file-names, i.e. the complete name of the file on the

file system, e.g. //dev8/ist9/user/krikhaar/med/ver8/ajax.c.

and the following relations:

ffl typed Exts;Cats , a relation that maps elements of Exts onto
elements of

Cats;

66 Described Architecture

ffl typed Files;Exts , a relation that maps elements of Files onto
elements

of Exts; ffl located Files;PhFiles , 1-to-1 mapping of Files onto their
physical locations (PhFiles) in a file system (and vice versa).

4.5.3 Example In view of their sizes, we are unable to give the sets and
relations of this InfoPack of existing systems. However, for two systems,
we present some related information.

Switch For the Switch system it was easy to determine the system's files
involved. All the files created by humans of each version are located
in a single directory in the file system. The source files consist of C
and C++ files having the extensions .h and .hpp, respectively, for the
header files and .c and .cpp, respectively, for the body files.

Med The Med system consists of thousands of files (Files). Over 60
different file extensions were found in the system (Exts). Some of them
exist only because of the system's history (legacy). The whole list of
extensions can be grouped into ten types of extensions (Cats).

4.5.4 Method The method for extracting the results of this InfoPack
depends very much on the system at hand. The Files set consists of all
the files in the configuration management system which belong to a single
release and which were created by humans. In terms of configuration
management, these are all files that can be checked in and checked out.
How we can construct such a list will depend on the configuration
management system Other techniques consists of analysing directories
and using file-name extensions to determine whether a file was created
by human.

The extension of a file name often indicates the type of information
contained in the file. One can derive the relation typed Files;Exts ,
which describes

4.5 InfoPack: Files 67

Exts Cats

c C-source h C-source java Java-source

txt Help-text hlp Help-text

gif Picture jpeg Picture bmp Picture

Table 4.2: typed Exts;Cats the relation between files and their
extensions. The file-exts program (listed in Section A.1) creates this
relation from the set of Files.

The Files can be partitioned into a number of sets: HeaderFiles,
BodyFiles , ResourceFiles and BuildFiles. The first three sets are the
files that eventually appear in some (derived) way in the running system.
The BuildFiles are different in the sense that they indirectly belong
to the source code: they are the build description files discussed in
Section 4.4. One can calculate the various sets on the basis of file-name
extensions. For example, given that all HeaderFiles have the extension
.h or .hpp, we can calculate as follows in RPA:

HeaderExts = fh; hpp g HeaderFiles = dom (typed Files;Exts _ran
HeaderExts )

Table 4.2 gives an example of the typed Exts;Cats relation. Each extension
is assigned to a single category. This relation must be constructed by
hand, in cooperation with an architect.

For the purpose of readability, it is convenient to use short file names
instead of full file names (i.e. a device name plus directory name plus
base name of a file). It is important to ensure that the resulting file
names are unique, but one must also be able to find the file in question
in the filesystem at any requested time (i.e. physical file name).
The relation located Files;PhFiles is a function from the (unique)
file name to the physical file name. This

68 Described Architecture

relation can be derived by removing the file name's first part of the full
file name as much as possible and preserving the file name's uniqueness.

We can partition all the files according to the categories (Cats).
The partof relation (typed Files;Cats) that describes this relation can
be calculated as follows in RPA:

typed Files;Cats = typed Exts;Cats ffi typed Files;Exts

4.5.5 Discussion It is important to carefully check the extraction
results, especially when they are based on heuristics and/or line-oriented
Perl [WCS96] scripts (see also discussion in Section 4.6.5). An example
of a heuristic is that all files with the extension .hlp belong to the
set of ResourceFiles .

Some checks can be performed at an early stage of analysis already.
For example, one can check whether each existing file extension (Exts )
belongs to some category (Cats). We can express this in an RPA formula:

ran (typed Files;Exts ) ` dom(typed Exts;Cats )

4.6 InfoPack: Import 4.6.1 Context The Import InfoPack belongs to the code
view of software architecture. The Files InfoPack (Section 4.5) is used
as input and the Component Dependency (Section 4.9) and Using and Used
Interfaces (Section 4.10) ArchiSpects use the results of this InfoPack.

4.6.2 Description To be able to manage large systems, one must divide the
software into several separate compilation units. These units use each
other by e.g. calling functions, so they require knowledge of each other.
A generally accepted concept is to distinguish two parts for each unit:
a header part, containing

4.6 InfoPack: Import 69 declarations of e.g. variables and signatures
of functions, and a body part, containing the implementation of the
names declared in the header. If one unit wants to use another unit,
it will import the unit's header information.

In the programming language C [KR88] (C++ [ES90]), header information and
source code are reflected in different files. Historically, the files have
the suffixes .h and .c, but, strictly speaking, any file extension may
be used. Although not required, it is preferable to ensure a one-to-one
correspondence between the header and the body file, so that champion.h
contains declarations of names that are implemented in champion.c; nothing
less and nothing more than that. For clarity one should define such rules
in the coding standards (as this will ensure more conceptual integrity).

Although not absolutely demanded by C/C++-compilers, a header file should
contain only the following information:

ffl macro declarations ffl type declarations ffl class declarations (C++
specific) ffl function (method) declarations, i.e. signatures of functions
(methods) ffl variable declarations

Such concepts exists for other languages too. A fine example is the
Modula2 programming language [Wir83], which explicitly handles the
notions of definition modules and implementation modules. The syntax of
this language ensures that the definition module and the implementation
module both contain the right type of information.

This InfoPack results in the relations imports Files;Files and partof
Files;Units. The imports relation contains tuples hFileX ; FileY i,
where FileX imports FileY . The partof Files;Units relation groups a
header file and a body file into a single entity, named unit. The latter
relation is of use in reconstructions only if the notions of header and
body file have been properly applied (as described above).

4.6.3 Example We give a fragment of a C/C++ source file (ajax.c) as
an example.

#include "champion.h" #include !string.h? #include "../finance/stock.h"

70 Described Architecture

Files Files ajax.c champion.h ajax.c string.h ajax.c stock.h

. . . . . .

Table 4.3: imports Files;Files In this example there are three #include
statements, each with its own specifics. The three header files are
literally included before the compiler starts compiling the ajax.c file.
The compiler1 searches for the included files in the file system using
an include-path. An include-path is an ordered list of directories (in the
file system). The compiler searches for an include file by looking for it
in the directories (in the given order) as defined in the include-path.
The order of the directories in the include-path is relevant when a file
occurs more than once in the file system.

The first #include statement refers to champion.h. The compiler searches
for this file, at first in the current directory, and secondly via
the includepath. In the second #include statement, the file-name is
enclosed by angles (! ?). The compiler consequently searches for it only
via the include-path (ignoring files in the current directory). In the
third #include statement, the file-name is preceded by a relative path
(../finance/). This is in fact similar to the first statement, except
for the relative path which is taken into account during searching.

The results of the import extraction of this example are given in
Table 4.3. Table 4.4 shows the partof Files;Units relation.

4.6.4 Method The method in constructing this InfoPack comprises the
following steps:

ffl starting with a list of files belonging to the system; this list is
a result

of the Files InfoPack; ffl determining the include-path per file; the
Build Process ArchiSpect

describes where one can find this information; ffl extracting the
include statements per file and determining the included file (using
the include-path);

1In fact, a pre-processor of the compiler searches the files and literally
includes them.

4.6 InfoPack: Import 71

Files Units champion.c champion champion.h champion

ajax.c ajax ajax.h ajax stock.c stock stock.h stock

. . . . . .

Table 4.4: partof Files;Units ffl reflecting the information in the
imports Files;Files relation file; ffl determining the units and
constructing partof Files;Units .

In practice, many peculiarities make extraction slightly more difficult
than described above. For example, different operating systems have
different file systems with their own filename conventions (e.g.
Windows NT , does not distinguish cases in file names, Unix , however,
is case-sensitive with respect to file names). When different file
systems are used, case sensitivity problems must be solved first (e.g.
by converting cases).

In the following sections we will discuss the extraction of an imports
relation from source code written in different programming languages:
C/C++, Java, Objective-C and CHILL. Many parts of the systems we
investigated have been implemented in these programming languages.
The discussion of the extraction will also serve to illustrate how an
imports relation can be extracted from source code written in other
languages.

C and C++ The C++ language [ES90] is an object-oriented version of the C
language [KR88]. The import mechanisms of the two languages are the same.
First, we strip comments from the source files (the comment-strip program
is presented in Section A.3). Secondly, we extract the inclusion of
files (the C-imports program is presented in Section A.4). The #include
statement contains the file name of the included file. This file name
is enclosed between a pair of quotes (") or between angles (! and ?).
The extraction program uses these facts to filter the proper information,
which results in an imports Files;Files relation.

72 Described Architecture

One can also construct the partof Files;Units relation on the basis of
file names. The relation is based on a naming convention: the names of
header and body files are the same except for their suffixes, .h and .c,
respectively (the program is given in Section A.2).

Java The Java language [Jav, Web96] supports an import statement enabling
the use of other classes. The Java compiler searches for the imported
classes via the CLASSPATH environment variable. The mechanism is similar
to the include-path mechanism of C/C++.

We give an example (ajax.java) to illustrate various import statements
of Java:

import Player; import traffic.transport; import car.*; import
java.awt.Button;

class ajax

The first import statement asks for the Player class, which means that
the compiler searches for a Player.class2. This file must reside in one
of the directories defined in the CLASSPATH.

The second import statement defines that the transport class from the
traffic package is imported. The compiler searches for transport.class
in a traffic/ sub-directory of one of the directories in the CLASSPATH.

In the third import statement, all the classes of the car package are
imported (they are present in CLASSPATH's car/ sub-directory) by using
a wildcard (*). The fourth import statement imports from the java.awt
package the Button class. The compiler searches for a Button class in
a java/awt/ sub-directory of CLASSPATH.

The Java language also contains a class-grouping mechanism called
packages. The first statement of a Java source file may be a package
declaration, which means that all the classes defined in the file belong
to that defined package.

2In Java a class file is the compiled version of the accompanying java
source file.

4.6 InfoPack: Import 73

Classes+ Classes MyPackage.ajax Player MyPackage.ajax traffic.transport
MyPackage.ajax car.* MyPackage.ajax java.awt.Button

. . . . . .

Table 4.5: imports Classes;Classes

Classes+ Classes MyPackage.* MyPackage.ajax MyPackage.ajax MyPackage.ajax

car.* car.door car.* car.wheel car.* car.gear

. . . . . .

Table 4.6: definesClasses+;Classes

An extraction program is much simpler when comments have already been
removed from the source. Java comments are equivalent to C++ comments,
so they can be stripped with the strip-comment program (presented
in Section A.3). An imports Classes;Classes+ relation (an example is
given in Table 4.5) is the result of the J-imports extraction program
(presented in Section A.5). The Classes+ refers to an extended
set of classes; the wildcard notation has not yet been resolved.
The J-package extraction program (presented in Section A.6) generates
a definesClasses+;Classes relation (an example is given in Table 4.6).
So, given the class definitions per package, we are able to resolve the
wildcard notation resulting in an imports 3 relation:

imports Classes;Classes = imports Classes;Classes+ ffi
definesClasses+;Classes

3Public classes and file names are in fact interchangeable in Java,
so we can see it as an importsFiles;Files relation.

74 Described Architecture

Objective-C Objective-C [CN91] is an object-oriented version of the
programming language C. It is possible to translate Objective-C code into
C code with the aid of a relatively simple translator; the resulting C
file can then be compiled by a C-compiler. Various Objective-C compilers
use this strategy4.

Objective-C units can use functionalities of other units by importing
the corresponding header file (as in C). We give an example:

#import !ReportGenerator.h? #import !ReportDefinitions.h?

The ObjC-imports extraction program (presented in Section A.7) is an
adapted version of C-imports. It filters #import instead of #include
statements.

CHILL The programming language CHILL [ITU93, SMB83], CCITT HIgh Level
Language, is used particularly to build telecommunication systems.
The language contains state-oriented constructs. There are several
dialects of this language, but, fortunately, these derivatives do not
differ in their import mechanisms. We give a CHILL example:

Subscriber: MODULE GRANT

counting, connect.me;

SEIZE

make.connection;

SEIZE

PortHandler ALL;

END Subscriber;

A module can export names (such as functions, types and variables)
with a GRANT statement (counting, connect me). A module can only use names

4The first C++ compilers also used this strategy to compile C++ files.

4.6 InfoPack: Import 75 (make connection) of another module by importing
them with a SEIZE statement. It is also possible to import ALL the
(exported) names of a module (PortHandler).

If strict coding standards are applied, one may be able to extract
information with a Perl [WCS96] program, otherwise a dedicated parser
is required.

4.6.5 Discussion We have given extraction programs for import statements
in C/C++, Java and Objective-C programs. For systems that mix a number
of programming languages one can concatenate the results of the various
programs. While analysing different systems, we found that the extraction
programs sometimes had to be changed a bit. For example, when operating
system environment variables are used within C include statements,
one must interpret these variables.

#include "BAS.ENV:string.h" In the above example, the environment variable
BAS ENV must be resolved first to determine the physical location of the
file that is imported. In this particular case it was fairly easy because
this environment variable name leads directly to the directory involved
(i.e. BAS/).

The given extraction programs are implemented in Perl [WCS96]. A Perl
program is interpreted (by a Perl interpreter); one can easily modify it.
Therefore, it is very handy during the process of analysing software.
On the other hand, these programs are based on many assumptions
concerning the layout of the input, which makes them error-prone.
One should therefore carefully check the results of these programs.

Reverse engineering tools are able to extract a fair amount of
source-coderelated information (e.g. function calls, variable access,
but they do not analyse function pointers). We can process the output
of these tools to obtain, e.g. an import relation. For example, the
(intermediate) output of QAC [Pro96] (a commercial tool that checks the
quality of C programs) can easily be translated into RPA-formatted files.
The QAC-imports program (presented in Section A.8) filters appropriate
statements from QAC output and generates an imports Files;Files relation.

There is a major difference between the Perl approach and the QAC (or
any other reverse engineering tool) approach. QAC parses the complete

76 Described Architecture

source code, taking into account all kinds of pre-processor settings
(like #define). This means that for certain compiler settings parts of the
code are not parsed (e.g., code between #ifdef and #endif statements.
This may also result in skipping of the inclusion of header files.
Note that the various products in a product family are often distinguished
by including product-dependent header files (using the #ifdef construct).
For a complete analysis of the different products we have to extract
all the included files.

4.7 InfoPack: Part-Of 4.7.1 Context The Part-Of InfoPack belongs to the
code view. It is related to the Software Concepts Model (Section 4.2)
and Source Code Organisation (Section 4.3) ArchiSpects. The results
are input for the Component Dependency (Section 4.9) and Using and Used
Interfaces (Section 4.10) ArchiSpects.

4.7.2 Description Each large system is decomposed into various parts;
these parts can in turn be decomposed into smaller parts; see also the
Software Concepts Model ArchiSpect. This form of decomposition is applied
a number of times until the level of statements is reached. Programming
languages offer only a few levels of decomposition: statements are grouped
in functions and functions reside in classes or files5. We could imagine a
counterpart for each software concept in a programming language concept.
For example, layers (see Section 1.5.1) are often applied for large
systems, but programming languages do not support this concept.

All decomposition levels should be reflected in the system's code
view in some way, as already indicated in Section 4.3. For example,
one can use directories in the file system to reflect the decomposition
hierarchy. Special comments in the headers of source files can also be
used to reflect the decomposition level(s). This may easily lead to the
introduction of errors because developers may forget to maintain headers.

5Some languages offer more grouping possibilities Java e.g. includes
the concept of packages.

4.7 InfoPack: Part-Of 77

Med

Est

Sens

Acq Abb

Rec Acc

Sen Man Bot

Str

Log

Figure 4.9: Implementation of Decomposition Levels Med

/*

* Subsystem: Operating System * Component: Event Handling */

4.7.3 Example Med The subsystem and component decomposition levels of
the Med system (e.g. Est and Acq, respectively) map onto directories
in the source code organisation; see Figure 4.9. The Package level
is reflected in the applied file name convention of source code files
(encoded in the prefix of the file name).

Tele The source code files appear in a single directory. The decomposition
hierarchy is reflected in the documentation structure (which can be
automatically extracted by analysing directories on the file system).

4.7.4 Method The result of this InfoPack consists of a number of partof
relations, according to the part-of levels generally described in the
Software Concepts

78 Described Architecture

Model. If the Source Code Organisation reflects parts of the decomposition
hierarchy, one can derive the partof relations by inspecting the
directories (a program is presented in Section A.9). Comments in the
headers can moreover be analysed by simple Perl [WCS96] programs.

The other partof relations must be created by hand with the help of
software architects. After that, we have all the partof relations already
discussed at an abstract level in the Software Concepts Model ArchiSpect.
The set of partof relations embodies a system's decomposition hierarchy.

4.7.5 Discussion For reverse engineering purposes one should be able to
reconstruct the decomposition hierarchy from source code and/or from
source code organisation. In fact one should take provisions, already
during the initial creation of a system in order to make it easier to
extract information from the system's source code. Otherwise, extra
information will have to be obtained by interviewing architects and/or
dedicated heuristics will have to be applied during reconstruction.

4.8 InfoPack: Depend 4.8.1 Context The Depend InfoPack belongs to the
code view. It is related to the Build Process ArchiSpect (Section 4.4).
It uses the results of the Files InfoPack (Section 4.5). The result is
used by the Component Dependency ArchiSpect (Section 4.9).

4.8.2 Description The build description file contains knowledge relating
to how to construct the entire system is to be created. How files depend
on each other is described in the build description files. With large
systems, the whole build process often consists of executing a number
of build description files. Figure 4.7 shows a general view on the
Build Process. For this InfoPack we make explicit the various activities
in this build process in terms of files and a depends relation between
files: depends Files;Files .

4.8 InfoPack: Depend 79

o

lib

exe c

h

Figure 4.10: depends Exts;Exts This InfoPack also results in the
relation depends Exts;Exts . This relation describes dependencies at a
more abstract level. It is based on file name extensions, e.g. a .o file
depends on .c and .h files. It is interesting to analyse this relation
because some curious tuples may be found in the case of legacy systems.
These curiosities often originated in the past when different (or no)
coding standards were used. In Figure 4.10 a .exe depends on .lib and
.o files, a .lib file depends on .o files, and a .o file depends on .c
and .h files.

4.8.3 Example Med With the Med system, MMS [VAX96] is used as the
language for describing the dependencies between the various files.
It contains the commands to be executed to build the system. We give a
fragment of an MMS file:

COMP:main.exe : ,-

SUBCOMP:string.obj, - SUBCOMP:finance.obj, - COMP:main.obj, -

link COMP:main.obj SUBCOMP:string.obj SUBCOMP:finance.obj -out
COMP:main.exe

The terms COMP and SUBCOMP are VMS environment variables that refer to a
certain directory belonging to components COMP and SUBCOMP, respectively.
main.exe is created by executing the specified link command. It depends
on two object files of SUBCOMP and one object file of COMP.

80 Described Architecture

4.8.4 Method We will start with the BuildFiles created by the Files
InfoPack. The next step is to parse the build description files and
extract the dependencies between the files. The last step is to combine
the extracted information in a single relation: depends Files;Files .

For MMS we built a simple parser in Lex [LS86] and Yacc [Joh75].
The dependencies were written to a relation: depends Files;Files .
A similar strategy can be used for make [Fel79] files.

The relation depends Exts;Exts can be calculated as follows in RPA:

depends Exts;Exts = depends Files;Files " typed Files;Exts

4.8.5 Discussion In practice, the build description files are often
(partially) generated. Information about the import relation between files
is required for the generation of these files. Additional information
is required to determine the proper link commands to construct the
executables. To extract the imports Files;Files relation (outcome of the
Import InfoPack (Section 4.6)) one may tap information from this process.
However, this will be the relation for a single product in the family
(see also Section 4.6.5).

With legacy systems there may be a discrepancy between the files as
discovered in a depends Files;Files relation and the files in a system
(see the Files InfoPack). The results of this InfoPack may also help to
discover references to files that are a user's proprietary, i.e. a file
in a user directory which may suddenly disappear when he or she leaves
the organisation. The results of the Depends InfoPack should be carefully
compared (e.g. by executing RPA expressions) with the results of the Files
InfoPack in order to check the correctness of the extraction results.

4.9 ArchiSpect: Component Dependency 4.9.1 Context The Component
Dependency ArchiSpect is part of the module view. It uses the results
of the Import (Section 4.6), Part-Of (Section 4.7) and Depend

4.9 ArchiSpect: Component Dependency 81 (Section 4.8) InfoPacks.

4.9.2 Description A system's build time consists of the time consumed by
the various activities of the Build Process (pre-compiling, compiling,
linking and postprocessing). The compilation of a single-source file
consists of parsing all the imported header files and the source file
itself. Typically, each header file is parsed as many times as it is
imported in source files6. Usually, after a modification, the system is
rebuilt by compiling the directly or indirectly changed source files.
When a header file is changed, all the source files that include this
header file must be recompiled. Recompilation should be minimised by
minimising the import system's dependency. This ArchiSpect can help
to estimate the average time to recompile the system after a change.
This may affect procedures of building a system (e.g. the nightly built
should better start at 05.00 PM).

Modifying or extending part of the system requires knowledge of its
context (e.g. `neighbouring' modules). A developer must understand all
the implications of a change and he or she should therefore consider
the consequences outside the affected source, too. With fewer import
dependencies a developer need understand less a modification's context.

During development and maintenance a developer spends a lot of time
learning to comprehend the system, sometimes up to 50% of the time
spent on maintenance [PZ93]. It is hard to forecast the time needed
for such comprehension activities, and therefore it is hard to plan
modification activities. Locality of a change is inversely proportional
to the unpredictability of the required modification time. A modification
in one part may result in other modifications in other parts, which is
also known as the ripple-effect.

When software is modified, the system should always be tested again.
One can focus on the modified source, but one must always carefully
consider any software that uses a modified functionality. Component
Dependency can help in determining the parts that have to be tested again.

6Some software development environments reduce the parsing time by saving
precompiled header files in a binary format.

82 Described Architecture

4.9.3 Example Subsystems use each other's functionality to operate
properly. This usage can be presented with box-arrow diagrams (here
created by the Teddy-PS tool; see Section C.3, but there also exist
commercial and non-commercial tools, e.g. Rigi [SWM97]), but we must be
explicit about the semantics of the boxes and arrows. A box represents
a piece of software, e.g. a subsystem; an arrow from one box (importing
entity) to another box (imported entity) represents an import dependency.
The table diagram (created by the TabVieW tool; see Section C.5),
shows marks (I) in those cells for which an import dependency exists.
If there is a mark, the entity given in the leftmost column imports the
entity given in the top row.

Med The component dependency of Med is given in Figure 4.11 at subsystem
level (note that arrows from a box to that same box, i.e. the identity
relation, have not been depicted). The same information is also given
in another form in Table 4.7 (here, the identity relations appear in
the diagonal of the table).

Comm The component dependency of the Comm system is depicted in Figure
4.12.

4.9.4 Method We will start the reconstruction of Component Dependency
with the results of the following InfoPacks and relations:

ffl Import; imports Files;Files ffl Part-Of ; a chain of partof relations
starting at Files level and finishing at Subs level. More generally, we
need partof relations at all the levels in the decomposition hierarchy:
partof Files;LN , partof LN ;LN

\Gamma 1 ,. . . , partof

L2 ;L1

Knowledge of the results of the Software Concepts Model ArchiSpect is
needed to be able to correctly interpret the various partof relations,
namely the "chain" of decomposition levels versus the "chain" of partof
relations.

4.9 ArchiSpect: Component Dependency 83

Bas Bot Est Sens Pres Util Netw

UTX

Top Div

Figure 4.11: Component Dependency Med

84 Described Architecture

      Com       Std Man                    Qry

                 SQry                  LQry

Cnl Cil

Con Figure 4.12: Import Dependency Comm

4.9 ArchiSpect: Component Dependency 85

subsystem

component

package A

file   X

subsystem

component

package B

file   Y

1 2 3

Figure 4.13: Lifting imports Files;Files The next step is to make
proper abstractions of the given import information. A chain of lift
operations must be executed to derive the component dependency at the
requested level. One can derive this, in RPA, as follows, given the
decomposition hierarchy Files-Packs-Comps -Subs:

imports Packs;Packs = imports Files;Files " partof Files;Packs imports
Comps;Comps = imports Packs;Packs " partof Packs;Comps

imports Subs;Subs = imports Comps;Comps " partof Comps;Subs

explanation As illustrated in Figure 4.13: if a file X in package A
imports a file Y from package B then package A imports package B (arrow
1). This principle has been applied in the above RPA expression by means
of the " (lift) operator. Information that file X is part of package A
and file Y is part of package B is described in the relation partof Files

;Packs.We have lifted the

imports relation at Packs level to the Comps level

by lifting again (arrow 2). By lifting a third time we reach the Subs
level (arrow 3).

The last step is to present the information; a typical form of
representation is a graph. A number of graph visualisors are discussed in
Appendix C. We prefer to use a layout and format that will be familiar
to the developers. So we have applied the notation used in the system's
documentation. For example, the layout of boxes drawn in Figure 4.11
is the same as in pictures in the Med documentation. The Teddy-PS
visualisation tool (see Section C.3) can be used to create this diagram.

Another form of presentation is a table or matrix; the using subsystems
are listed in the first column and the used subsystems in the first row.
A mark appears in the cells where a using subsystem imports a used
subsystem.

86 Described Architecture

Div Top Est Sens Pres Util Netw UTX Bot Bas Div I I Top I I I I I I I

Est I I I I I I I Sens I I I I

Pres I I I

Util I I I I Netw I I I

UTX I I

Bot I I I I Bas I

Table 4.7: Component Dependency of Med The TabVieW visualisation tool
(see Section C.5) can be used to create a similar table in a Web browser.

4.9.5 Discussion The order in which the subsystems appear in Table 4.7
has been carefully chosen. The subsystems at the top of the system (see
Figure 4.11), Div and Top, are listed first, and those at the bottom,
Bot and Bas, appear in the last row and the last column, respectively.
In the case of a system with a layered structure as discussed in Section
1.5.1, one may expect marks in the upperright corner of the table (i.e.
above the diagonal). An opaque layered system may have marks only in
the diagonal cells and in exactly one cell above these cells.

Assume that a dependency between subsystem A and subsystem B is curious.
We will then want to investigate the reasons for its existence.
For example, we might want to find out which imports at component level
are responsible for it. The following RPA formulas can be used for
this purpose:

CompsA = partof Comps;Subs :A CompsB = partof Comps;Subs :B suspectsC
A;B = (imports Comps;Comps _dom CompsA) _ran CompsB

4.9 ArchiSpect: Component Dependency 87

explanation The CompsA (CompsB ) set contains all the components that
belong to subsystem A (B), i.e. the left image of partof Comps

;Subs with respectto

A (B). Taking the relation importsComps

;Comps, we must look atthe tuples that start (i.e.

domain) from components of A and end (i.e.

range) with components of B.

Analogously, we can descend the decomposition hierarchy to obtain more
specific information:

P acksA = partof Packs;Comps _ran CompsA P acksB = partof Packs;Comps
_ran CompsB suspectsP A;B = (imports Packs;Packs _dom PacksA) _ran PacksB

We have discussed this ArchiSpect by taking the imports relation as a
starting point. It is however also possible to start with the depends
relation, which will result in a slightly different interpretation of
the diagrams.

Reflexion Models We will finish this discussion by relating Component
Dependency to the reflexion models introduced by Murphy et al. [MNS95].
A so-called source model is extracted from the source code. This model
contains a use relation between source model entities (smentities), e.g.
files. Additionally, there is a mapping which describes how source model
entities are to be mapped onto high level model entities) (hlmentities).
We give an example of a mapping table:

[ file=*string*".[ch] mapTo=StringHandling ] [ file=tcpip/ip*".[ch]
mapTo=IPServices ]

All source model entities must be mapped onto high level model entities.
In fact, this mapping defines a partof SMEs;HLMEs making use of regular
expressions to reduce the number of entries in the mapping table.
From these pieces of information a mappedSourceModel can be constructed
which is a set of tuples of tuples:

mappedSourceModel = f hhh1; h2i; hs1; s2iij

88 Described Architecture

hs1; s2i 2 SourceModel ^ mapping (h1; s1) ^ mapping (h2; s2) g

The domain of the mappedSourceModel describes the use relation at a
high level. This is similar to the result obtained on lifting an imports
Files;Files relation to a subsystem level.

4.10 ArchiSpect: Using and Used Interfaces 4.10.1 Context The Using and
Used Interfaces ArchiSpect belongs to the module view. The results of
the Import (Section 4.6) and Part-Of (Section 4.7) InfoPacks are used
as input.

4.10.2 Description The interface provided by a class consists of the
methods and data which are not private. A class can be (re-)used in a
proper way when one is aware of at least its interface (both syntax and
semantics). This principle can be applied at each decomposition level,
so to reuse components one must be aware of the component's interface.

The Component Dependency ArchiSpect shows the interconnectivity of
components. It is relevant to know the constituents of a used component
that are used by other components. When a component must be replaced by
another component one should at least know the connection points of that
component with the outside world.

Good software architectures explicitly describe the interfaces of all
the components. It is also possible to reconstruct the interfaces
of components. The using interface of a component consists of the
component's elements that are using (elements of) other components.
The used interface of a component consists of the component's elements
which are used by (elements of) other components.

Figure 4.14 shows the using interface and used interface. The rounded
boxes represent files; the square boxes represent components. In this
example we will show the Files interface of component K. We must note
that

4.10 ArchiSpect: Using and Used Interfaces 89

K t u

A B C

p q s

Y Z

Used Interface

Using Interface

v

Figure 4.14: Using and Used interfaces interfaces can be calculated at
various decomposition levels, e.g. the Functions interface of subsystems.
The Files interface of a component consists of the set of files that
are using (being used-by) other components.

A poor design creates a single global include file per subsystem,
which includes all the header files of the subsystem. This minimises
the used interface of a subsystem, but it is not considered a good
design decision, because a modification of this global include file
will necessitate recompilation of all the source files that use this
subsystem. We introduce the notion of the used+ interface to overcome
this problem. The used+ interface, at Files level, consists of all the
directly or indirectly included files. Note that the used+ interface is
most relevant when we are investigating the Files interface. At higher
abstraction levels we will often obtain all the entities of the system
when we consider indirect usage. At file level the (in)direct inclusion
of files always "starts" and "stops" at header files.

4.10.3 Example Med We reconstructed the Files interface of subsystems
for the Med system. In Figure 4.15 the using and used interfaces are
expressed as ratios of a subsystem's full set of files. Boxes represent
the subsystems; the upper (shaded) part of the box represents the ratio
of the files that belong to the used interface, the lower (shaded)
part of the box represents the ratio of

90 Described Architecture

Bas Bot Est Sens Pres Util Netw

UTX

Top Div

Figure 4.15: Using and Used Interfaces of Med the files that belong to
the using interface. Note that some files may form part of the using
interface as well as the used interface. The using and used interfaces
can also be presented in a tabular format as given in Table 4.8.

4.10.4 Method The results of the Import and Part-Of InfoPacks are required
for this ArchiSpect. Knowledge of the Software Concepts Model ArchiSpect
is needed to understand the way partof relations are organised.

To be able to calculate the Files interface of components we need the
fol4.10 ArchiSpect: Using and Used Interfaces 91

Subsystem # # using ratio # used ratio

Files Files using Files used Div 13 4 0.308 0 0.000 Top 151 80 0.530
0 0.000 Est 4015 1191 0.297 52 0.013 Sens 489 105 0.215 11 0.022 Pres
326 147 0.451 46 0.141 Util 411 188 0.457 43 0.105 Netw 448 193 0.431
71 0.158 UTX 50 27 0.540 0 0.000 Bot 586 193 0.329 177 0.302 Bas 802 0
0.000 234 0.292

Table 4.8: Using and Used Interfaces Ratios of Med lowing relations:
imports Files;Files and partof Files;Comps . In general, to be able to
calculate the LX interface of LY we need the relations: partof LX ;LY
and imports LX ;LX . For clarity, we will adhere to the Files interface
of components. We will define, in RPA, the using interface step-by-step.

imports Files;Comps = partof Files;Comps ffi imports Files;Files
importsExt Files;Comps = imports Files;Comps n partof Files;Comps

explanation We construct a relation importsExtFiles

;Comps, that defines an im-port relation containing tuples as indicated
by the arrows from files

(rounded boxes) to components (square boxes) in Figure 4.14. For each
file we calculate which components it uses: importsFiles

;Comps.We are only interested in relations that pass the boundaries of
components, so from this relation we subtract the relation that contains
all the internal imports expressed by the partof Files

;Comps relation.

The second step consists of the following RPA formulas:

FilesUsingExt = dom(importsExt Files;Comps )

92 Described Architecture

usingFiles;Comps = partof Files;Comps _dom FilesUsingExt explanation
The using interface of component K consists of the files of component
K residing in the importsExtFiles

;Comps relation. The FilesUsingExtset contains all the files that are
used by components other than their

containers. The usingFiles

;Comps relation assigns the FilesUsingExtset to the components to which
they belong. This is a subset of the

partof relation, so we restrict this relation to its domain with respect
to FilesUsingExt.

The next step consists of the following RPA formulas:

imports Comps;Files = imports Files;Files ffi partof \Gamma 1 Files;Comps
importsExt Comps;Files = imports Comps;Files n partof \Gamma 1 Files;Comps

FilesUsedExt = ran(importsExt Comps;Files ) used Files;Comps = partof
Files;Comps _dom FilesUsedExt

explanation The importsExtComps

;Files relation represents the arrows from files(rounded boxes) to
components (square boxes) indicated in Figure 4.14.

In this case we have to lift the domain part of the importsFiles

;Files ,which is performed by composing it with the conversed

partof relation. We calculate the rest in a similar manner as for the
using interface.

The ratio of the using and the used interfaces of components, i.e. the
number of files in the interface related to the total number of files
in a component, can be calculated in RPA for each component C 2 Comps:

UsingC =

jusingFiles;Comps :Cj

jpartof Files;Comps :Cj

UsedC = jused Files;Comps :Cjjpartof

Files;Comps :Cj

4.10 ArchiSpect: Using and Used Interfaces 93

explanation The number of files of component C that use "something"
from outside that component is given in the numerator part. The total
number of files of component C is given in the denominator part. The used
interface ratio is calculated in a similar manner.

The formulas of the used+ interface are the same as those of the used
interface except that we start with the transitive closure of imports .
So we arrive at the following RPA formulas:

importsPlus Comps;Files = imports+Files;Files ffi partof \Gamma
1 Files;Comps importsPlusExt Comps;Files = importsPlus Comps;Files n
partof \Gamma 1 Files;Comps

UsedPlusExts = ran (importsPlusExt Comps;Files ) usedPlusFiles;Comps =
partof Files;Comps _dom UsedPlusExts

explanation The direct or indirect use of a file is expressed by the
transitive closure of the imports relation: imports+Files

;Files. The rest of the cal-culations are carried out in a similar manner
as those for the used

interface.

As already mentioned, the above formulas can be applied at various
decomposition levels. Instead of two consecutive levels (Files and
Components ) one can also select two non-consecutive levels (Functions
and Subsystems). For two non-consecutive levels LX and LY we have to
compose a partof relation from existing ones:

partof LX ;LY = partof LY

\Gamma 1 ;LY ffi partof LY \Gamma 2 ;LY\Gamma 1 ffi : : : ffi partof
LX ;LX+1

4.10.5 Discussion The significance of addressing ratios of using and used
interfaces is also recognized in Lagu"e et al. [LLMD97, LLB+98]. In this
work information is extracted from the source code by filtering the
#include statements from the C(++) source code (probably similar to the
Import InfoPack as discussed in Section 4.6). From this information the
authors constructed different sets (not further discussed in their paper).
Each set belongs to a layer i; it contains certain types of files:

94 Described Architecture

ffl F (i) : all files ffl IF (i) : all header files ffl IM (i) : all
body files ffl D(j; i) : all files that use files from layer j ffl S(j;
i) : all files that are used by layer j

They calculated the used interface ratio of layer i with respect to
layer j as follows:

UR(j; i) = jD(j; i)jjIF (i)j

And the used interface ratio of layer i as follows:

UR(i) =

j S

j6=i

D(j; i)j

jIF (i)j

They calculated the using interface and the used+ interface ratios in
a similar manner.

Lagu"e et al. used the number of header files as the denominator in
their formulas. We used the total number of files, i.e. the header and
body files. In practice the number of header files will correspond to
the number of body files. So our ratios will be half the ratio of Lagu"e
et al. It is however possible to rewrite our using and used formulas to
obtain the same ratios:

UsingComp = jusingFiles;Comps :Compjj(partof

Files;Comps :Comp) " HeaderFilesj

UsedComp = jused Files;Comps :Compjj(partof

Files;Comps :Comp) " HeaderFilesj

Large ratios for using and used interfaces means that hardly any
information is hidden. One should therefore strive to create small
interfaces. In fact, this holds for each level of abstraction.
It is however more important to have small interfaces at the higher
levels of abstractions than lower ones. In general, the system is more
comprehensable when all the interface ratios are small.

4.11 Concluding Remarks 95 4.11 Concluding Remarks In this chapter
InfoPacks and ArchiSpects of the module view and code view at the
described level have been discussed. These InfoPacks and ArchiSpects and
their relations are represented in Figure 4.2. Reconstructing ArchiSpects
from existing systems is a very useful way of learning to comprehend
the system. The results of these ArchiSpects can be used to enhance
(or up-date) the software architecture documentation.

Over the years we have reverse architected a number of module views of
systems [Kri97]. For many of these systems we were able to reconstruct
the Component Dependency ArchiSpect in a few days with the help of
an architect. The results of this ArchiSpect helped to feed discussions
about the system's software architecture. We experienced that it is best
to have a first step of defining improvement activities relating to the
module view.

Software changes with time, so the software architecture may change,
too. Indeed, ArchiSpects have to be reconstructed every time a system
is modified. Most InfoPacks extract information from the source code
which are therefore most accurate. After an initial reconstruction,
one can think about automating this process. The various reconstruction
activities must then be integrated somewhere in the Build Process.
In this way, one can every day obtain an up-to-date set of ArchiSpects,
which may serve as part of the system documentation. We have applied this
strategy to three development sites at Philips (Med, Switch, and Comm).
Every night, the Component Dependency is reconstructed and, the next day,
it is presented to developers (on request).

Results of ArchiSpects can be presented in a Web browser. The information
to be presented must be stored on the Web server. The developers will
then have easy access to this information by means of a browser tool with
which they are already familiar. Besides just presenting ArchiSpects
in a Web browser, one can also provide user interaction. Consider the
Component Dependency ArchiSpect which can be reconstructed at different
levels in the decomposition hierarchy. A developer may want to zoom-in
and zoomout on information by clicking on boxes and arrows in diagrams
(as shown in e.g. Figure 4.12) or on entries in tables (as shown in e.g.
Table 4.7). This can be easily achieved using standard Web technology
[SQ96]: hot spots to click on boxes or arrows in diagrams and cgi scripts
to calculate more detailed (or more abstract) information on request.
TabVieW (see Figure 4.16 and Section C.5) is a presentation tool that
uses cgi scripts to

96 Described Architecture

Figure 4.16: Comm Table Viewer calculate new tables7 on request of an
architect or developer (by clicking on a hyperlink).

We will finish this chapter with a brief comparison of our approach with
the Software Bookshelf and Dali.

Software Bookshelf The Software Bookshelf [FHK+97] is a Web-based
approach for presenting software-related information. A kind of bookshelf
captures, organizes, manages and delivers comprehensive information
of a software system. It is an integrated suite of code analysers and
visualisation tools. The authors distinguish three roles: builders,
librarians and patrons. The builder develops the bookshelf's framework
and all kinds of tools to support a librarian. A librarian populates
the bookshelf with meaningful information of the software system.
A patron is the system's end user (developer, manager, architect).
Web technology is very suitable for presenting architecture information
due to its multi-media nature. Therefore, our approach could be

7One can store any table a developer may ask for on the server, but in
the case of large systems this will be many tables. Besides consuming
a lot of cpu time for generation, these tables will take a lot of disk
space on the Web server.

4.11 Concluding Remarks 97 combined with the Software Bookshelf,
especially in view of this system's open architecture. For example, the
Software Bookshelf uses Rigi as its presentation tool, but that could
be replaced by our presentation tools. It would also be possible to
extend Rigi [SWM97] with our RPA-based abstraction techniques to improve
presentations, e.g. by applying transitive reductions to remove edges
from graphs.

Dali In their Dali system, Kazman and Carri`ere [KC98] distinguish view
extraction and view fusion to support software architecture understanding.
By combining different extraction views, one can, through fusion, arrive
at more appropriate views. For example, a profiler extracts the actual
calls of a system; a static analyser extracts the potential calls of a
system relating to the modules containing these calls, which can be fused
into a view that shows actual relations between modules. Unlike our tools,
Dali uses a SQL database to store information and SQL operators to fuse
information. The authors found the expressive power of SQL operations
sufficient; but transitive closure, for example, cannot be expressed in a
single SQL query or a fixed set of queries. Note that, it is possible to
map most RPA operators on standard SQL queries; this work is discussed
in Appendix B. The need for an operator like transitive closure is
indispensable in the field of reconstructing software architectures
(as shown in different parts of this thesis). Therefore, we prefer the
RPA approach to reconstruct software architectures.

98 Described Architecture Chapter 5 Redefined Architecture In the
previous chapter we have discussed the described level of software
architecture reconstruction. The next level of the SAR method concerns
the improvement of existing software architectures: the redefined level.

5.1 Introduction A described architecture consists of the explicit
description of the software architecture of an existing system.
This is for example useful for helping developers comprehend that system.
The improvement of an existing software architecture is logically the next
subject in our discussion. Improving an existing software architecture may
be of great help in simplifying a system's maintenance and its extensions.
The realization of improvements results in a redefined architecture which
will be discussed in this chapter [KPS+99]. The process of improving a
software architecture is called (software) re-architecting.

Changing a software architecture may affect many parts of the software.
Before a change can be introduced, an architect must know exactly which
parts will be affected but also the cost of implementing the change.
Impact analysis is a technique for calculating the consequences of
a change in advance, without realizing it in the actual source code.
An architect can quietly consider all the pros and cons of a change
before he or she decides to implement it.

Figure 5.1 shows the various activities involved in architecture
improvement. A software model containing the ArchiSpects as discussed
in e.g.

100 Redefined Architecture

Software ModelSource Code Documentation System Experts

Idea

simulateevaluate Prescription

apply

transform Modification Impact Analysis

extract Figure 5.1: Architecture Improvement Chapter 4 is derived
from the source code, documentation and information obtained from
system experts. This software model is subjected to impact analysis:
an architect has an idea (e.g. moving a function to another module by
means of changing the partof relation) that can be simulated (e.g. by
means of recalculating certain ArchiSpects). This results in a modified
software model. This new model must be evaluated by the architect, which
may influence the original idea, resulting in an adapted or new idea.
After some iterations, the refined idea may be accepted or discarded.
Accepted ideas must be transformed into a prescription of modification for
the implementation. This prescription can be applied to the source code
and documentation, resulting in a new system. Note that when we extract
information from the modified system, we obtain the same software model
as we had constructed after simulating the accepted idea.

The advantages of this approach are clear. An architect can apply his
or her ideas to a software model and figure out the consequences without
involving many people and without affecting the actual system. Note that
we have described a purely architecture-driven analysis, which does
not take into account business, organisation or process-related issues.
For example, the implementation of an improvement may be beneficial from a
softwareengineering point of view, but it may have disastrous consequences
for the product's time-to-market. Besides the impact analysis described
above, considerations of the latter kind must also be taken into account
in a com5.1 Introduction 101

Module View Code View

Part-Of

Aspect Assignment

Import Component Dependency

Aspect Coupling

Component

Coupling

Cohesion and

Coupling

Using and

Used Interfaces

Figure 5.2: Overview of Redefined Architecture mercial setting. In this
chapter we will discuss some ArchiSpects that are helpful in performing
impact analysis aimed at improving the software architecture of an
existing system; see Figure 5.2. The following ArchiSpects will be
discussed in an arbitrary order:

ffl Component Coupling, the quantified dependencies between the software
parts of a system; ffl Cohesion and Coupling, metrics that describes
connectivity between

various software parts; ffl Aspect Coupling (using the Aspect Assignment
InfoPack), quantified

dependencies between various parts, taking into account a certain aspect.

102 Redefined Architecture

5.2 ArchiSpect: Component Coupling 5.2.1 Context The Component Coupling
ArchiSpect belongs to the module view. The results of the Import (Section
4.6) and PartOf (Section 4.7) InfoPacks are required. The Component
Dependency (Section 4.9) and Using and Used Interface (Section 4.10)
ArchiSpects are closely related.

5.2.2 Description The Component Coupling ArchiSpect quantifies
dependencies as depicted in the resulting diagram of the Component
Dependency ArchiSpect, e.g. see Figure 4.11. Quantification is useful for
example when we want to remove a dependency between X and Y . The size of
a relation can be of help in estimating the amount of effort that will
be involved in removing that dependency. Assume that components consist
of files that import each other. Then, the number of import statements
is a measure (or weight ) of the intensity of dependency between
the components. But, a relation can be quantified in different ways.
We define the following weights:

ffl size-oriented weight, meaning that the number of relations at the

lower level is reflected in the weight of the lifted relation at the
higher level; ffl fan-in-oriented weight, meaning that the number of
entities (e.g. files)

used by a component is reflected in the weight; ffl fan-out-oriented
weight, meaning that the relation is quantified by

the number of a component's using entities.

To illustrate this, consider the use relation depicted in Figure 5.3.
Component CompHigher uses component CompLower because High1 uses Low1,
amongst other relations. According to the above definition, we obtain
the following weights:

ffl size-oriented: 4 (the number of dashed arrows) ffl fan-in-oriented:
3 (the number of files a dashed arrow points to) ffl fan-out-oriented:
2 (the number of files at which one or more dashed

arrows start)

A combination of the three weights helps to re-architect a system. Assume
we want to remove a dependency between two components. The sizeoriented
weight indicates how many import statements must be removed to

5.2 ArchiSpect: Component Coupling 103

CompLower Low1 Low3Low2

CompHigher High1 High2

?

Figure 5.3: Lifting with Multiplicity: 2-3-4-case achieve this. On the
other hand, the fan-out-oriented weight indicates how many files of the
including component will be affected. So, it defines the number of files
that have to be changed to remove the dependency between the components.

If we want to replace a component, then the component's interface with
other components must be known. The fan-in-oriented weight defines the
number of files that will be used by the including component.

5.2.3 Example The reconstruction of Component Dependency of two
systems has been discussed in Section 4.9. In this section we present
the Component Coupling of these systems. The resulting diagram of this
ArchiSpect, created by TeddyPS, contains the same arrows as the diagram
of Component Dependency, but the arrows are of different thicknesses.
The thickness of an arrow1 is a measure of the weight of the relation.
For the corresponding relation, a tuple with a large (small) weight is
represented by a thick (thin) arrow.

The cells in the table representation of Component Coupling contain the
three different weights of the various tuples, i.e. the fan-in-oriented,
sizeoriented and fan-out-oriented weights.

1We have used a logarithmic function to map the weight onto an arrow
width of a few points.

104 Redefined Architecture

Bas Bot Est Sens Pres Util Netw

UTX

Top Div

Figure 5.4: Component Coupling Med Med The (size-oriented) Component
Coupling of Med is presented in Figure 5.4. The corresponding diagram
of Component Dependency is depicted in Figure 4.11.

A table representation of Med is given in Table 5.1 (with its related
Table 4.7). Each affected cell contains three figures, i.e. the
fan-in-oriented, size-oriented and fan-out-oriented weights. Note that
the table representation explicitly shows the identity relation (if
applicable) in contrast with the diagram representation. Furthermore,
all the variants of quantification

5.2 ArchiSpect: Component Coupling 105

      Com       Std Man                    Qry

                 SQry                  LQry

Cnl Cil

Con Figure 5.5: Component Coupling Comm are presented in a single table.
Comm The Component Dependency of the Comm system is depicted in Figure
4.12. Figure 5.5 shows the (size-oriented) Component Coupling of this
system.

5.2.4 Method The steps to be executed are similar to those of the
Component Dependency method (described in Section 4.9). The required
input consists of2:

2For clarity, we will still use the Files and Comps decomposition levels
in the description, but any other pair of levels could be used.

106 Redefined Architecture

fan-in size fan-out Div Top Est Sens Pres Util Netw UTX Bot Bas

9 34 Div 9 39

2 4

66 11 2 41 18 59 119 Top 151 11 2 52 25 204 719

75 9 2 16 16 52 80

1301 11 39 4 23 117 186 Est 8220 41 75 6 55 1623 9324

1170 34 43 6 29 522 1191 32 117 4 11 124 Sens 144 333 4 52 968

27 98 2 23 105

151 57 119 Pres 611 257 1483

147 96 147 20 193 43 144 Util 75 899 105 1773

41 188 41 188

236 108 149 Netw 1028 901 1957

193 156 193

23 32 UTX 55 197

27 27 2 46 227 147 Bot 4 98 860 1626

4 27 193 193

393 Bas 3725

338

Table 5.1: Component Coupling (fan-in, size, fan-out) of Med

5.2 ArchiSpect: Component Coupling 107

ffl imports Files;Files, a multi-relation that can be constructed
by mapping the imports relation: dimports Files;Files e; ffl partof
Files;Comps , a part-of relation which may be the result of a composition
of a chain of partof relations, for example (Med ):

partof Files;Comps = partof Packs;Comps ffi partof Files;Packs

The three variations of weight are calculated as follows (note that we
introduce two new lift operator notations "\Delta  and "\Lambda ):

size-oriented: imports Comps;Comps = imports Files;Files " partof
Files;Comps

fan-in-oriented: importsFI Comps;Comps = imports Files;Files "\Delta
partof Files;Comps

= dpartof Files;Comps e ffi

dbimports Files;Files ffi dpartof \Gamma 1Files;Comps ece

fan-out-oriented: importsFO Comps;Comps = imports Files;Files "\Lambda
partof Files;Comps

= dbdpartof Files;Comps e ffi imports Files;Filesce ffi

dpartof \Gamma 1Files;Comps e

explanation In the first definition, the importsFiles

;Files multi-relation is lifted tocomponent level. The lift operator for
multi-relations takes into account the number of dependencies at file
level in constructing the dependency at component level (as defined in
Section 3.5).

To explain fan-in-oriented and fan-out-oriented lifting, we must consider
an alternative formula for the lifting of relations: U " P j P ffi U ffi P

\Gamma 1. This alternative formula also exists for multi-relations:

U " P j dP e ffi U ffi dP

\Gamma 1e. Note that we must first map the partof

relation (which is always a binary relation) onto a multi-relation.
Furthermore, dP

\Gamma 1e j dP e\Gamma 1.

Figure 5.6 shows the various steps of fan-in-oriented lifting (this figure
is an alternative to the view presented in Figure 5.3). In the last part

108 Redefined Architecture

CompLower

Low1

Low3 Low2

CompHigher

High1 High2

importsFIComps,Comps

importsFiles,Files partof-1Files,Comps importsComps,Files
partofFiles,Comps

3

Figure 5.6: Fan-in-oriented lifting of the given formula (i.e. the second
composition), part of the lift operation is performed ("lifting in its
domain"). This intermediate result describes a relation from Comps to
Files (represented in Figure 5.6 as importsComps

;Files). We are interested in the files imported by acomponent; we are
not interested in the number of times these files

are imported. So the intermediate result is normalised, i.e. setting
its weight is set to 1 by mapping it first to a normal relation and
then back to a multi-relation. The first part of the fan-in-oriented
formula performs the rest of the lift operation ("lifting in its range").
The number of file dependencies is thus taken into account in obtaining
a fan-in-oriented weight.

The fan-out-oriented lift is defined in a similar manner. We start with
the first part of the formula: the intermediate result contains the
normalised multi-relation importsFiles

;Comps. The last step is tolift the domain of this intermediate result
to the component level, to

obtain the relation importsFOComps

;Comps with a fan-out-orientedweight.

5.2.5 Discussion We have discussed an ArchiSpect which is useful in the
context of rearchitecting. This ArchiSpect should be part of the software
model (as depicted in Figure 5.1).

5.3 ArchiSpect: Cohesion and Coupling 109 Undesired component dependencies
are intuitively considered less harmful when they have a small weight and
more harmful when they have a large weight. An architect may use this
information to detect weak spots and to analyse these spots. A large
size-oriented weight between two components and a small fan-in-oriented
weight may indicate that a file is located in the wrong component.
To check whether this is true for the system at hand we will modify
the partof relation (idea) and will recalculate the Component Coupling
ArchiSpect (simulate).

There exists a relation between the various weights described above.
Let's call the three kinds of weights (size-oriented, fan-in-oriented
and fan-outoriented) s, fi and fo, respectively. We will explain that
the following inequation holds: max (fi ; fo) ^ s ^ fi \Theta  fo.
Consider all the tuples in the imports Files;Files relation that are
responsible for the dependency between two components. The number
of tuples in this restricted imports relation (let's call it imp)
corresponds to the size-oriented weight s. The size of the range of
the imp relation describes the fan-in-oriented weight fi . The size of
the domain of imp describes the fan-out-oriented weight fo. The largest
possible imp relation consists of the cartesian product of the domain
and the range, which has a size of fi \Theta  fo. The smallest possible
imp relation contains at least the set of tuples that span the domain
(which is sized fi ) and the range (which is sized fo). Therefore,
the imp relation has a minimum size being the maximum of fi and fo.

5.3 ArchiSpect: Cohesion and Coupling 5.3.1 Context The Cohesion and
Coupling ArchiSpect belongs to the module view. It requires the results
of the Import (Section 4.6) and PartOf (Section 4.7) InfoPacks. It is
related to the Component Coupling ArchiSpect (Section 5.2).

5.3.2 Description The complexity of a system highly affects the system's
comprehensibility, maintainability and testability. Cohesion and coupling
play important roles in expressing complexity. Topics relating to module
cohesion and module coupling were discussed by Yourdon and Constantine
in the seventies [YC79, SMC74]. These measures have also been used to
develop tools

110 Redefined Architecture

that automatically cluster parts of software, e.g. [MMR+98].
A system's cohesion describes the connectivity of entities within
the part comprising them. It is defined as the ratio of the number of
actual dependencies between these entities and the number of all possible
dependencies (this agrees with the definition of intra-connectivity given
in [MMR+98]). Coupling describes the connectivity between two different
entities in terms of their comprising parts (in [MMR+98] this is called
inter-connectivity). It is defined as the ratio between the number of
actual dependencies of these entities and the number of all possible
dependencies. A general rule of thumb (and not more than that) for
achieving, amongst other things, a high degree of comprehensibility, is
that a system should minimize coupling in favour of maximising cohesion.

Consider the system depicted on the left side of Figure 5.7.
The components CompLeft , CompRight and CompLow contain some files that
import each other (dotted arrows). The components CompLeft and CompRight
import each other (solid arrow) by means of file Y . By moving this file Y
from CompRight to CompLeft , we obtain the situation depicted on the right
side of this figure. As we can see in the diagram, the degree of coupling
between the components has decreased and the degree of cohesion between
the files vof CompLeft has increased. Without having any knowledge of the
system's semantics we may conclude that the structure has been improved
and that the new system is easier to understand. We must note that a good
software architecture cannot be created simply by optimizing the cohesion
and coupling quality measures; many other aspects also play a role (e.g.
decomposing a system into parts that semantically belong together).

5.3.3 Example Comm The Cohesion and Coupling of Comm at subsystem
level are presented in Table 5.2 (\Gamma  means that there is
no cohesion/coupling). In this example we have chosen files as the
constituents of a subsystem (we can take other entity levels, too, e.g.
modules). We conclude that for all subsystems the degree of cohesion
is low. Files are small units with respect to subsystems and therefore
we may expect a low degree of cohesion. We may even state that a high
degree of cohesion would be suspect in the case of this system. For the
same reason the coupling figures are also low. This discussion shows
that a proper understanding of the system is required to be able to draw

5.3 ArchiSpect: Cohesion and Coupling 111

CompLeft CompRight

Y

Original Situation Final Situation  CompLow

CompLeft CompRight

Y

 CompLow Figure 5.7: Re-clustering

Coupling Man LQry SQry Qry Cil Cnl Con Std Com Man - - - 0.058 0.205 -
10.0 36.9 LQry - - - 0.045 - - - 147.6

SQry - - - - 0.509 - - 138.0

Qry - - - - - - - 145.2

Cil 0.058 0.045 - - 0.087 - 3.28 13.5 Cnl 0.205 - 0.509 - 0.087 0.082
9.8 26.5 Con - - - - - 0.082 - 42.2

Std 10.0 - - - 3.28 9.80 - 118.7 Com 36.9 147.6 138.0 145.2 13.5 26.5
42.2 118.7

Coh 30.7 2.04 2.15 8.90 5.06 11.6 29.4 66.4 0.000

Table 5.2: Cohesion and Coupling (\Theta 10\Gamma 3) of Components of
Comm proper conclusions from metrics. 5.3.4 Method In this section we
will discuss the calculation of Cohesion and Coupling, given the imports
and partof relations. For clarity in the discussion we will use only
two decomposition levels, namely Files and Components.

The Dominating Ratio (DR) between two components X and Y relates the
actual file imports to all possible file imports between these two
components. For example, in Figure 5.8, component X imports file y1
twice,

112 Redefined Architecture

YX y1 x1

y2 x2

x3

Figure 5.8: Dominating indicated by the solid arrows. The dashed and solid
arrows (6 in total) indicate all the possible imports between the files
of component X and those of component Y . Therefore, in this example,
the dominating ratio is DRX;Y = 26 = 0:333.

We define the Dominating Ratio of component X with respect to component
Y , denoted as DRX ;Y , as follows:

Files = dom(partof Files;Comps ) imports Comps;Comps = dimports
Files;Filese " partof Files;Comps

impAllComps;Comps = dFiles \Theta  Filese " partof Files;Comps

DRX ;Y =

kimports Comps;Comps _dom fXg _ran fY gk

kimpAllComps;Comps _dom fXg _ran fY gk

explanation The size-oriented weight of the multi-relation importsComps

;Compsrefers to the number of file import statements in the code.
The multirelation impAll describes all the possible imports between
components. The imports multi-relation is restricted in its domain with X
and it is restricted in its range with Y , resulting in a multi-relation
fhX; Y; wig. The weight w refers to the number of file imports between
components X and Y . The size of this singleton multi-relation is
equal to w. Analogously, the number of possible imports is calculated
by starting with the multi-relation impAll . We obtain the dominating
ratio DRX ;Y by dividing both sizes of doubly restricted relations.

5.3 ArchiSpect: Cohesion and Coupling 113

X x1

x2 x3

Figure 5.9: Cohesion cohesion The cohesion of a component indicates
the degree of connectivity between its comprising files. One way of
interpreting connectivity is to look at the imports relation. Cohesion is
defined as follows:

CohesionX = kimports Comps;Comps _car fXgkkimpAll

Comps;Comps _car fXgk

= kimports Comps;Comps _dom fXg _ran fXgkkimpAll

Comps;Comps _dom fXg _ran fXgk

= DRX;X

explanation The numerator defines the number of imports between files
within component X (solid arrows in Figure 5.9). In fact, the restriction
results in a singleton relation fhX; X; wig, where w indicates the
number of actually imported files. The denominator contains the number
of possible imports inside X, as we have seen above (solid and dashed
arrows in Figure 5.9). This corresponds to the dominating ratio of X
with respect to X.

The cohesion of component X, as illustrated in Figure 5.9, is 46 = 0:667.

coupling Coupling is a measure of the degree of connectivity between
two components. Given the imports relation as a connectivity artefact
we define coupling as:

114 Redefined Architecture

impX;Y = imports Comps;Comps _dom fXg _ran fY g impY;X = imports
Comps;Comps _dom fY g _ran fXg impAll X;Y = impAll Comps;Comps _dom fXg
_ran fY g impAll Y;X = impAll Comps;Comps _dom fY g _ran fXg

CouplingX ;Y = kimpX;Y [ impY;XkkimpAll

X;Y [ impAll Y;Xk

= kimpX;Yk + kimpY;XkkimpAll

X;Yk + kimpAll Y;Xk

= kimpX;Yk + kimpY;Xk2 \Theta  kimpAll

X;Yk

= 12 \Theta  ( kimpX;YkkimpAll

X;Yk +

kimpY;Xk kimpAll Y;Xk )

= DRX ;Y + DRY ;X2

explanation The numerator of CouplingX;Y counts the number of file
import statements of component X importing files from component Y and
vice versa (solid arrows in Figure 5.10). As in the cohesion definition,
the denominator contains the number of all possible imports (solid and
dashed arrows in Figure 5.10). The ratio is a measure of the degree
of coupling, which can be rewritten in terms of dominating ratios.
Because X 6= Y , and therefore impX

;Y "impY;X = ;, we can rewrite thenumerator by adding the sizes of the
two multi-relations. Analogously,

the denominator can be written as the sum of two multi-relations.
Furthermore, these latter two multi-relations are both of the same size
(due to the construction of impAll it holds that: impAll j impAll

\Gamma 1).

We note that the degree of coupling between X and Y is equal to the
degree of coupling between Y and X (due to the associative + operator).

The degree of coupling between components X and Y , illustrated in Figure
5.10, is 12 \Theta  ( 26 + 36 ) = 0:417.

5.3 ArchiSpect: Cohesion and Coupling 115

YX y1 x1

y2 x2

x3

Figure 5.10: Coupling

5.3.5 Discussion Given a set of entities and relations between them (e.g.
the imports relation between Files), one can cluster these entities
by applying the heuristic "maximise cohesion and minimise coupling".
Note that it makes no sense to increase cohesion simply by artifically
creating extra relations between entities within a cluster. Therefore we
should consider the above heuristic more carefully. By creating various
clusters one in fact divides the set of existing tuples of the relation,
e.g. imports , into two parts: a set of tuples that do not cross a
cluster's border and a set of tuples that do cross a cluster's border.
So it is better to define the heuristic as: "maximise the number of
tuples in the cohesion part and minimise it in the coupling part".

Clustering of software parts at different levels in the decomposition
hierarchy is a task which can indeed not be performed automatically
[Wig97, MMR+98]. Cohesion and coupling metrics can help an architect
to make the right decisions about clustering (a tool for software
reclustering, based upon these metrics, has been implemented by Brook
[Bro99]). Note that clustering can never be driven by optimising the
value of two metrics. Many factors play a role in the clustering process,
but only a few can be expressed in metrics.

116 Redefined Architecture

5.4 InfoPack: Aspect Assignment 5.4.1 Context The Aspect Assignment
InfoPack belongs to the code view. We may need the results of the
PartOf InfoPack (Section 4.7). The results are used by Aspect Coupling
(Section 5.5).

5.4.2 Description The notion of aspects has already been discussed in
Section 1.5.3. Aspects are a design concept, but they should also be
reflected in the implementation in some way. This can be realized in
various ways. For example, a file addresses only a single aspect of
the system: the aspect to which the file belongs can be encoded in the
file name.

The main result of this InfoPack consists of the addresses Files;Asps
relation and the Aspects set.

5.4.3 Example The Tele system explicitly engineers the notion of
aspects during system development in all its phases (i.e. in the
forward-architecting process). A file, having an aspect-related name,
addresses exactly one aspect.

We will illustrate this with an example of a system that did not initially
consider aspects.

Med During our re-architecting activities we introduced aspects into
the Med system. Although it was not possible to apply it precisely in
its full meaning, it helped us to construct a new view on the system.
We were able to identify the following aspects:

ffl Clinical : all the software that is sent to a hospital along with the

medical system; ffl Test : the software needed to test the system during
its development; ffl Development : the software that comprises all the
dedicated tools that

are required to develop the system (e.g. for code generation).

5.4 InfoPack: Aspect Assignment 117 We refine the first aspect into:

ffl Operational : software activated by an operator in the hospital;
ffl Research: software prepared for academic hospitals for clinical
research purposes; ffl Diagnostic: software relating to all the service
operations performed

by a service mechanic; ffl Installation: software required only to
install the system at a (new)

site.

So the Aspects set and the ClinicalAspects subset are defined as follows:

Aspects = fOperational ; Research ; Diagnostic;

Installation ; Test; Development g ClinicalAspects = fOperational ;
Research ; Diagnostic; Installationg

5.4.4 Method The first step is to determine the various aspects of
the system. This task is easy when aspects have been used already during
architecture design. If not, we will have to discuss the notion of aspects
with architects and designers. The second step consists of identifying
these aspects in the software. Although the notion of aspects may not
have been explicitly identified so far, it may be possible to determine
aspects in code. For example, an aspect like Logging may express itself
in the naming of functions (WriteLogMessage) and/or the naming of files
(LogUtilities). Given such naming conventions, one can assign an aspect
to most of the functions (or files). Because of the heuristic nature of
extraction, the results of this extraction should be carefully checked.
The functions (or files) that cannot be assigned to an aspect in this
way must be assigned by hand.

With the Med system, we used the names of the packages to assign aspects.
So we were able to extract the addresses Packs;Asps relation by analysing
the package name. We can lower this relation to the Files decomposition
level:

addresses Files;Asps = addresses Packs;Asps # partof Files;Packs

118 Redefined Architecture

5.4.5 Discussion This InfoPack may be hard to construct in the case
of systems in which aspects are not handled explicitly. To reconstruct
Aspect Assignment , one should take the decomposition level that fits
such an assignment best. The partof relation can be used to bring the
assignment to any requested level. For example, if it is possible to
reconstruct aspect assignment at Functions level (addresses Funcs;Asps
), one can bring it to a Files level (using composition). In that case,
the resulting addresses Files;Asps relation is not necessarily functional.

If aspects appear only at statement level, it is practically impossible
to obtain a useful addresses relation. Sometimes the notion of aspects
must be relaxed somewhat to realize aspect assignment. Although this
particular result does not comply precisely with the definition of
aspects, it may be helpful in re-architecting a system.

5.5 ArchiSpect: Aspect Coupling 5.5.1 Context The Aspect Coupling
ArchiSpect belongs to the module view. The results of the Import (Section
4.6), PartOf (Section 4.7) and Aspect Assignment (Section 5.4) InfoPacks
are required as input. This ArchiSpect is related to the Component
Coupling ArchiSpect (Section 5.2).

5.5.2 Description Consider a programmer who is working on message logging.
He is not interested in all the code, but only in the parts concerning
statements about logging. If we can offer the programmer a reduced logging
view on the system, it will be easier for him to perform his logging task.

Aspects structure a system in addition to e.g. functional structuring.
Both structuring means are more or less orthogonal, which helps to
create two completely different views on the system. Aspect structuring
plays the most important role during certain development activities,
e.g. when dealing with message logging while functional structuring is
important e.g. when adding a new feature to a system (e.g. Follow Me
into a telecommunication switching system).

5.5 ArchiSpect: Aspect Coupling 119 Design decisions relating to aspects
should also be reflected in source code. For example, an aspect can be
reflected as a set of files, which means that a single file belongs to
exactly one aspect.

A structuring mechanism is effective only when properly applied. This will
be apparent from e.g. low degree of connectivity between the parts
that result from structuring. A rule of thumb for aspect connectivity
is: an aspect A may only use functionality that belongs to aspect A.
Other dependencies between aspects could be defined by an architect,
but, for clarity, they should be minimal.

5.5.3 Example Med The Aspect Coupling of the Test aspect is presented
in Figure 5.11 (created by the Teddy-PS tool). This diagram is of the
same type shown for Component Coupling. In fact, it is a subset3 of
the diagram depicted in Figure 5.4 on page 104. The dependency between
aspects is represented in Figure 5.12.

5.5.4 Method We will start with the imports multi-relation at the proper
decomposition level, which corresponds to the decomposition level of
the domain of the addresses relation. For each aspect we construct a
diagram or table similar to the diagram shown for Component Coupling.
Assume we want to construct the diagram for an Asp aspect, then we have
to restrict the imports multi-relation for this aspect:

FilesAsp = addresses Files;Asps :Asp importsAspFiles;Files = imports
Files;Files _dom FilesAsp

explanation The FilesAsp set contains all the files assigned to the
Asp aspect. We reduce the imports relation by looking only at the files
that belong to

3An arrow can occur in the aspect diagram only if it occurs in the
component diagram, with the same or greater weight.

120 Redefined Architecture

Bas Bot Est Sens Pres Util Netw

UTX

Top Div

Figure 5.11: Test Aspect Coupling for Med Development

Research

Diagnostic

Installation

Test

Operational

Figure 5.12: Dependencies between Aspects of Med

5.5 ArchiSpect: Aspect Coupling 121

this set. This means that we have to restrict the imports relation in
its domain using FilesAsp.

The dependencies between aspects yield an alternative abstract view on
the system. Given the imports relation, we can derive the dependency
between the aspects as follows:

depends Asps;Asps = daddresses Files;Asps e ffi

imports Files;Files ffi daddresses\Gamma 1 Files;Asps e

explanation If the addresses relation defines a partition, we can lift
the imports relation to the level of Aspects. But we may not assume this,
so we must bring both the domain and the range of the imports relation
to the aspect level through composition.

The last step of the method consists of presenting this information.
We use the same presentation techniques as used for Component Dependency,
e.g. by means of the Teddy-PS tool.

5.5.5 Discussion The proper application of aspects results in a clear
division of a system into slices (each belonging to a single aspect)
that make virtually no use of each other. The Aspect Cohesion and Aspect
Coupling metrics can be defined in a similar manner to the description
in Section 5.3. The containment relation is defined by the addresses
Files;Asps relation (although it is not necessarily a partition).
Furthermore, the imports Files;Files relation represents the dependencies
between files. We define the dominating ratio (DRA;B ) between aspect
A and B as follows:

imports Asps;Asps = daddresses Files;Asps e ffi imports Files;Files ffi

daddresses\Gamma 1 Files;Asps e impAllAsps;Asps = daddresses Files;Asps
e ffi dFiles \Theta  Filese ffi

122 Redefined Architecture

daddresses\Gamma 1 Files;Asps e DRA;B = kimports Asps;Asps _dom fAg
_ran fBgkkimpAll

Asps;Asps _dom fAg _ran fBgk

So, as with cohesion and coupling, we define aspect cohesion and aspect
coupling as follows:

CohesionA = DRA;A CouplingA;B = DRA;B + DRB;A2

5.6 Concluding Remarks Improving a software architecture often involves a
lot of questions and deducing more precise questions from the answers or
defining improvements. Relation Partition Algebra offers a flexible way
of asking these questions in a formal manner; the answers are obtained
by executing the RPA formulas.

Impact analysis (see Figure 5.1 on page 100), or what-if analysis,
consists of an iterative process of defining an idea, simulating it on a
software model and evaluating the results. For example, an idea could be
to move a function from one file to another. The simulation of this idea
consists of changing the appropriate sets, relations and multi-relations
of the software model (including re-calculating the derived relations
to retain a consistent model). We have also presented a number of
quality metrics (cohesion, coupling, aspect cohesion, aspect coupling)
that can support the evaluation of a software model. But the intuition
of architects also plays an important role [Cor89]. An ArchiSpect like
Component Coupling helps an architect shape his or her intuition.

We could use our experience to develop a dedicated tool that supports
the impact analysis process (Computer-Aided Impact Analysis). An idea
can be transformed into actions that must be executed during simulation
(i.e. a script for modifying the software model). The tool could be
designed to keep the software model consistent. To support evaluation,
such a tool should be able to present various metrics and diagrams and
tables as results of various ArchiSpects.

5.6 Concluding Remarks 123 There exist clustering algorithms that e.g.
try to cluster functions into cohesive groups that are minimally coupled.
In large systems, functionalities are grouped at various levels, which
makes these algorithms hard to apply. In addition, factors that are
hard to measure play a role with respect to deciding whether to cluster
functionalities, e.g. the semantics of functions.

Accepted ideas must be implemented in the actual software. An idea,
e.g. move function f from file x to file y, must be translated into
a prescription that can be applied to the source code. Dedicated
transformation tools (often based on compiler technology to a great
extent) can automatically apply simple prescriptions to the source code.
The rest must be specified in terms of change requests and must be
performed manually by developers.

124 Redefined Architecture Chapter 6 Managed Architecture The SAR method
consists of five levels of reconstruction (initial, described, redefined,
managed and optimized). In this chapter we discuss the managed level
of SAR. We focus on architecture verification, a means to keep the
defined architecture and actual architecture consistent.

6.1 Introduction The software architecture intended by architects should
be well documented. Nevertheless, a number of implicit assumptions
relating to the architecture reside in the heads of the architects and
developers only. Sources, documents and architects' minds together in
fact embody a system's intended software architecture.

The actual software architecture, i.e. the architecture implemented by
the software developers, will definitely deviate from the intended
architecture when no precautions are undertaken to prevent such
deviations. Architecture verification is the process of revealing the
deviations between the intended architecture and the actual architecture.
Preferably, this is performed as early in the development process
as possible. The main goal of architecture verification is to achieve
architecture conformance. Bass et al. [BCK98] define architecture
conformance being the activity that is concerned with keeping developers
faithful to the structures and interaction protocols constrained by
the architecture. If architecture verification is applied consistently,
and is properly integrated in the development process, a managed software
architecture is achieved.

126 Managed Architecture

Some of the architectural decisions can be formally defined. Given that
appropriate information can be extracted, one can automatically verify
the formally defined decisions. Other architectural decisions are more
intuitive, and therefore it is hard to verify them automatically.
For example, a description of the contents of a component like All
functions relating to printing must be contained in the "Printing"
component can currently only be interpreted by humans. In this chapter
we will concentrate on the rules that can be automatically verified.
The other type of architectural rules could be verified in e.g. review
sessions.

If a system's implementation does not conform to the architecture,
this will have to be fixed. Sometimes this may lead to changes in the
architecture, but, more often, the design and/or source code will have
to be modified. Sometimes both the architecture and the source code
will have to be modified. Architecture violations describe the parts of
the implementation that do not conform to the intended architecture.
The ArchiSpects of the managed architecture define the architectural
rules and the corresponding architectural violations. Violations are
defined in such a way that they support resolving a disconformance (by
suggesting a possible solution). An architectural rule is satisfied if
there are no architectural violations.

In Chapter 1.5 we discussed a number of good architectural patterns.
Here we will discuss some ArchiSpects that correspond to these
architectural patterns. For each system, a dedicated set of ArchiSpects
must be defined to comprise the definition of the architecture. In this
chapter we will discuss the following ArchiSpects:

ffl Layering Conformance; ffl Usage Conformance; ffl Aspect Conformance;
ffl Generics and Specifics Conformance.

6.2 ArchiSpect: Layering Conformance 6.2.1 Context The Layering
Conformance ArchiSpect belongs to the module view. It requires results
of the Import (Section 4.6) and PartOf (Section 4.7) InfoPacks. It is
furthermore related to the Software Concepts Model (Section 4.2) and
Component Dependency (Section 4.9) ArchiSpects.

6.2 ArchiSpect: Layering Conformance 127

Module View Code View

Part-Of

Aspect Assignment

Import

Software Concepts

Model

Usage Conformance

Layering Conformance

Generics and

Specifics Conformance

Aspect Conformance

Aspect Coupling

Component Dependency

Figure 6.1: Overview of Managed Architecture

128 Managed Architecture

Basic Man-Machine Interface

Feature

Unit Driver

Basic Man-Machine Interface

Feature

Unit Driver Figure 6.2: Layering Conformance of Cons 6.2.2 Description
A layer is a group of software elements. Layers are strictly ordered.
Given the layer ordering, elements of a higher layer may use only elements
of lower layer(s). Layering offers the possibility to develop and test
the system incrementally: from the bottom layer towards the top layer.
The principles of layering have been discussed in Section 1.5.1.

6.2.3 Example Cons The architecture of the Cons system describes
a transparent layering of the system depicted on the left side of
Figure 6.2. The actual implementation (on the right side of the figure)
shows a different diagram. For example, some elements in Basic layer
use functionality of the Feature layer, which is not specified in the
intended architecture.

Tele The Tele system was developed with the aid of the Building Block
method [KW94]. This method requires that each Building Block resides in
a single layer. Furthermore, Building Blocks may use only Building Blocks
that reside in lower layers. Figure 6.3 shows an example of Building Block
(BB)

6.2 ArchiSpect: Layering Conformance 129

Layer

BB

Figure 6.3: Layering Conformance of Tele usage. According to the rule
described above the usage relations marked with a cross are not allowed
[Kri95, FKO98]. Note that the Tele system achieves Layering Conformance
by means of dedicated tools that support the system's development.

6.2.4 Method Each Building Block resides in a single layer, which
is defined by the residesBlocks;Layers relation1. Note that the
residesBlocks;Layers relation in fact describes a partition of all
the Building Blocks over layers (see also Figure 4.3). The layers are
strictly ordered, which is reflected in the relation !Layers;Layers .

Furthermore, we need the following relations:

ffl imports Files;Files (from the Import InfoPack) ffl partof Files;Blocks
(from the PartOf InfoPack)

We can then define the following rule with its corresponding violations
in RPA:

imports Blocks;Blocks = imports Files;Files " partof Files;Blocks

mayuseBlocks;Blocks = !+Layers;Layers # residesBlocks;Layers 1The PartOf
InfoPack can be extended to extract this information.

130 Managed Architecture

rule: imports Blocks;Blocks ` mayuseBlocks;Block

violations: v imports Blocks;Blocks = imports Blocks;Blocks n
mayuseBlocks;Blocks

v Blocks = dom(v imports Blocks;Blocks ) v imports Files;Files = imports
Files;Files n

(mayuseBlocks;Blocks # partof Files;Blocks )

explanation The mayuse relation describes the import dependencies allowed
at Blocks level. All the blocks in a layer may use the blocks of all
the lower layers, hence the transitive closure upon !Layers;Layers.
By lowering the allowed usage at Layers level to Blocks level, we get
the mayuseBlocks

;Blocks relation. The architectural rule defines that theactual import
dependencies (

importsBlocks

;Blocks) is a subset of theallowed import dependencies (

mayuseBlocks

;Blocks).

The v importsBlocks

;Blocks relation describes the violating imports be-tween Building Blocks.
It consists of the actual import dependencies

minus the allowed import dependencies. During the process of resolving
disconformance one first wants to know which Building Blocks are involved
(i.e. domain of v importsBlocks

;Blocks). After that, more pre-cise information (i.e. closer to the
source code) is required in terms of

source code files. Therefore, we lower the mayuseBlocks

;Blocks relationto the

Files level and subtract this from the actual import dependencies in
order to find the violating file import statements.

Finally, we have to present the violations in a way that will appeal to
the person who has to resolve them. The violating import dependencies can
be presented in the same way as Component Dependency (i.e. in a diagram
or tables). We can also use colours to distinguish allowed usage and
forbidden usage relations in a diagram or table.

6.3 ArchiSpect: Usage Conformance 131 6.2.5 Discussion A violation of
the layering conformance rule can be resolved in several different ways.
First, we can move a complete Building Block to another layer. Secondly,
we can remove violating import statements from the source code files
(and move the corresponding code to other files). Or, a combination of
the two options may resolve the violations.

In this ArchiSpect we have used only binary relations. The use of
multirelations offers more dedicated information when it comes to
resolving a violation. For example, a large weight in the violating
v imports Blocks;Blocks relation may indicate that the Building
Block resides in the wrong layer. Such a modification of the system
is relatively simple, especially when compared with removing import
statements (and the corresponding movements of functions or other code)
from a number of source files.

Multi-relations can also help define some exceptions to the architectural
rules. An example is a system that is strictly ordered while a single
Building Block (e.g. the Loader Building Block) may use functionality
from higher layers. This extra allowed usage can be incorporated in the
mayuse multirelation (f: : : ; hLayer1; Layer2; 1i; : : :g).

6.3 ArchiSpect: Usage Conformance 6.3.1 Context The Usage Conformance
ArchiSpect belongs to the module view. It uses the results of the Import
(Section 4.6) and PartOf (Section 4.7) InfoPacks. It is related to
Component Dependency ArchiSpect (Section 4.9).

6.3.2 Description The documentation of a software architecture often
contains a diagram that shows components and relations between those
components. Such a diagram in fact defines the allowed usage between
components. Component Usage Conformance is achieved when the actual
implementation conforms to the allowed usage defined in the documentation.

This ArchiSpect is in fact an extension of Layering Conformance.
We describe precisely which components may use each other, while Layering
Con132 Managed Architecture

Comp Comp

Acq Abb Acq Rec Acq Log

Rec Sen

Sen Log Man Str

. . . . . .

Table 6.1: Component Usage Table of Med formance is based on a more
general concept of allowed usage. When both ArchiSpects are applied to
a system, one can also check whether the Usage Conformance and Layering
Conformance are compatible, even before any code has been written.

6.3.3 Example Med The Med system formally describes the usage between
components, which is defined in a simple table (which is similar to a
RPA-formatted file). Part of the component usage table is presented in
Table 6.1 (files of the left component may import files from the right
component).

6.3.4 Method The allowed component usage can be defined manually by an
architect or it can be extracted from the architecture documentation.
In the case of the Med system, we can simply translate the component
usage table into the mayuseComps;Comps relation. The architectural rule
that must hold is defined as follows in RPA (also discussed in [FKO98]):

rule: imports Comps;Comps ` mayuseComps;Comps

6.3 ArchiSpect: Usage Conformance 133

violations: v imports Comps;Comps = imports Comps;Comps n
mayuseComps;Comps

v Comps = dom(v imports Comps;Comps ) v imports Files;Files = imports
Files;Files n

(mayuseComps;Comps # partof Files;Comps )

The explanation of these formulas is similar to that of the formulas of
Layering Conformance. We can also use the same presentation techniques
to reveal violations.

6.3.5 Discussion The build process of the Med system incorporated
controlled component usage. An architect maintains a component usage
table that describes the allowed usage between components. All the
source files of a component reside in separate directories in the file
system. Before a file is compiled, the include-path for the compiler
is automatically set by the build environment according to the entries
in the component usage table. Illegal inclusion of files consequently
results in a failure of the compiler: File olise.h not found.

This approach has a great advantage over the method described aboveand
that is that a developer gets feedback on illegal usage at a very
early stage. On the other hand, it imposes a certain organisation of
the source code files which may not hold for every system.

The coupling measure (see Section 5.3) and Usage Conformance are related.
They both say something about the coupling between a system's components.
Coupling describes a general metric for measuring usage between components
that can be applied to any system. In general, the aim is to minimise
coupling. In contrast to coupling, Usage Conformance defines exactly
which components of a specific system may use each other.

Reflexion Models Murphy et al. [MNS95, MN97] introduced reflexion
models to discuss differences between a high-level model, as defined
by an engineer, and a source model, as extracted from source code.
The high-level model describes the

134 Managed Architecture

mental model of the engineer, whereas the source model describes the
actual implementation of the system. By comparing the relations in the
two models one can partition them into three categories: convergences,
divergences and absences. Convergences occur in both models, divergences
occur only in the source model and absences occur only in the high-level
model. The authors used reflexion models to compare a design (high-level
model2) with an implementation (source model); they concluded that
divergences do not adhere to design principles (in order to achieve
design conformance).

We can easily translate these ideas into Relation Partition Algebra.
Consider a source model consisting of a relation USM and a high-level
model containing a relation UHLM . We can then define the three categories
as follows in RPA:

convergences = USM " UHLM

divergences = USM n UHLM

absences = UHLM n USM

6.4 ArchiSpect: Aspect Conformance 6.4.1 Context The Aspect Conformance
ArchiSpect belongs to the module view. It requires results of the Import
(Section 4.6), PartOf (Section 4.7) and Aspect Assignment (Section 5.4)
InfoPacks. It is related to the Aspect Coupling ArchiSpect (Section 5.5).

6.4.2 Description The notion of aspects has already been discussed in
Section 1.5.3. We would like to enforce certain dependencies between
parts of the system that belong to aspects (see also Section 5.5).
The Usage Conformance, discussed in Section 6.3, restricts the usage
between components. Aspect Conformance can be seen as an additional
means in controlling a system's

2They extracted a high-level model from design documentation (object
diagrams).

6.4 ArchiSpect: Aspect Conformance 135

O

p

e

r a

t

i o

n

a

l

R

e c o

v

e

r y

M M

I

E

r r o

r

P

e

r

f o

r m

a n

c e

Building

Block

a

s p

e c

t

Figure 6.4: Aspect Conformance

complexity, by restricting usage between software parts belonging to
certain aspects.

6.4.3 Example Tele The recovery aspect belongs to all the functions
involved in initialising a system or recovering a system from some
erroneous state (e.g. due to a hardware failure). After recovery, the
system is in a defined state, but during recovery, one cannot rely on
a defined state in other Building Blocks. During recovery, a Building
Block may therfore only access data and functionality of its own, as
illustrated in Figure 6.4.

6.4.4 Method An architect must define the maydepend Asps;Asps relation,
which defines the allowed usage between aspects. Furthermore, we require
the relations

136 Managed Architecture

imports Files;Files (Import InfoPack), partof Files;Comps (PartOf
InfoPack) and addresses Files;Asps (Aspect Assignment InfoPack).
We define the following architectural rule (also discussed in [FKO98]):

depends Asps;Asps = addresses Files;Asps ffi imports Files;Files ffi

addresses\Gamma 1 Files;Asps

rule: depends Asps;Asps ` maydepend Asps;Asps

violations: v depends Asps;Asps = depends Asps;Asps n maydepend Asps;Asps

v imports Files;Files = imports Files;Files n

(addresses\Gamma 1 Files;Asps ffi maydepend Asps;Asps ffi addresses
Files;Asps )

explanation The dependsAsps

;Asps relation is created by lifting the domain andrange of the

imports relation (as already discussed in Section 5.5).

The actual dependencies between aspects (depends Asps

;Asps ) must bea subset of the allowed dependencies (

maydepend Asps

;Asps ) to ensurecompliance with the rule. The violating dependencies
between aspects

are all actual dependencies minus the allowed ones. When solving the
problem, one wants to know how the violations occur in source code, i.e.
violating imports at Files level.

6.4.5 Discussion The following formulas are defined for the Tele system,
concerning the recovery aspect (given a calls relation and addresses
relation at Functions level):

Funcsrecovery = addresses Funcs;Asps :recovery

6.5 ArchiSpect: Generics and Specifics Conformance 137

recovers Funcs;Funcs = callsFuncs;Funcs _dom Funcsrecovery recovers
Blocks;Blocks = recovers Funcs;Funcs "

partof Funcs;Files " partof Files;Blocks

rule: recoversBlocks;Blocks n Id Blocks ` ;

There is a single exception to the recovery rule. The SysRecovery Building
Block controls the whole recovery process. Therefore, it may access all
the recovery functions of the other Building Blocks. We adapt the above
rule as follows:

rule: (recovers Blocks;Blocks ndom fSysRecovery g) n Id Blocks ` ;

In Section 5.5 we discussed Aspect Cohesion and Aspect Coupling. Aspect
Conformance and these metrics are closely related. They are the metrics
which should be maximised or minimised in any system. Aspect Conformance
defines the exact relations between the aspects of a specific system.
One may assume that the architect has considered the Aspect metrics in
defining Aspect Conformance.

6.5 ArchiSpect: Generics and Specifics Conformance

6.5.1 Context The Generics and Specifics Conformance ArchiSpect belongs
to the module view. It requires the results of Import (Section 4.6)
and PartOf (Section 4.7) InfoPacks.

138 Managed Architecture

6.5.2 Description The notions of generic and specific components have
been discussed in Section 1.5.2. There is a special relationship between
these two types of components. A generic component and a corresponding
set of specific components belong together semantically. The common
functionality resides in the generic component, while the specific
functionality resides in various specific components.

Each product in the family comprises (most of) the generic components,
while the specific components vary per product. Hence, in a system,
a component can only count on the availability of generic components.
Therefore, specific components can only be accessed via their
corresponding generic component (via a call-back mechanism).

6.5.3 Example Tele A switching system (e.g. Tele) must be able to handle
different kinds of physical lines to communicate with other switching
systems. A dedicated hardware unit (peripheral processing unit, PPU)
handles the physical communication with other systems. The central unit
of a system contains software to control proper usage of the PPUs.
During the development of Tele one does not know which products will
ultimately be configured, so one cannot rely on the availability of
software that controls a certain PPU.

At a very abstract level, each communication line performs the same
functionality, namely communication with other systems. The generic
component addresses this abstraction: hiding all the specific
characteristics of various communication lines. During initialisation
time each specific component subscribes itself to the generic component.
The allowed usage relation between generic and specific components is
illustrated in Figure 6.5.

Note that the Tele system has completely achieved Generics and Specifics
Conformance.

6.5 ArchiSpect: Generics and Specifics Conformance 139

Generic Specific

Figure 6.5: Generic and Specific Components 6.5.4 Method The input3 for
this ArchiSpect consists of the set of generic components (Generics)
and the set of specific components (Specifics ). Furthermore, we require
the relations: imports Files;Files and partof Files;Comps . Note that
the Generics and Specifics describe a partition of Comps.

Components are prohibited to import functionality from specific
components. One can define this as follows:

imports Comps;Comps = imports Files;Files " partof Files;Comps
importsSpecComps;Comps = (imports Comps;Comps _ran Specifics) n

Id Specifics

rule: importsSpecComps;Comps ` ;

violations: v imports Comps;Comps = importsSpecComps;Comps

v Comps = dom(v imports Comps;Comps ) v imports Files;Files =
(importsSpecComps;Comps #

partof Files;Comps ) "

3An InfoPack can be defined to provide this information.

140 Managed Architecture

imports Files;Files explanation The importsSpec relation represents
all the imports of specific components (excluding imports of itself).
The architectural rule is satisfied if and only if this relation is empty.
The violating components consist of the components that import a specific
component, i.e. v Comps. The violating imports at Comps level can be
lowered to Files level to obtain all the possible violating imports at
this level. The intersection of this intermediate result with the actual
imports (importsFiles

;Files )leads to the actual violating imports (

v importsFiles

;Files).

6.5.5 Discussion An alternative definition of the above rule is defined
as follows:

ExpSpecifics = dom(imports Comps;Comps n Id Comps) ExpGenerics = Comps
n ExpSpecifics

rule: ExpGenerics ` Generics

ExpSpecifics ` Specifics

explanation In fact, we define a pattern that recognizes specific
components: all the specific components import functionality only from
generic components or from themselves. Given this characteristic,
the specific components should be defined in the ExpSpecifics set;
the other components are therefore ExpGenerics. The rule consists of
verifying whether all the recognized generic (specific) components are
indeed generic (specific).

6.6 Architecture Verification in Action In the previous sections we
discussed a number of ArchiSpects relating to the managed architecture.
Architecture conformance can be achieved

6.6 Architecture Verification in Action 141 when the application of these
ArchiSpects to an existing system results in the satisfaction of the
architectural rules. The ArchiSpects discussed in this chapter should
be seen as examples of how ArchiSpects of the managed architecture
can be defined. Although the presented ArchiSpects can be useful for
many systems, each system may require its own ArchiSpects to enforce
architecture conformance. Below we will briefly discuss the introduction
of an architecture verification process in real environments.

The first step toward achieving architecture conformance consists of
formalising the architecture decisions (as described in the documentation
and/or stored in the heads of architects). As shown in this chapter,
relation partition algebra can be applied to formalise a number of
these decisions4. In addition to these rules, violations must also
be defined, in such a way that they support a developer in resolving
possible disconformances.

The second step concerns the incorporation of architecture verification
in the development process. Automation of the verification process
is required to be able to successfully introduce it. The execution of
InfoPacks and ArchiSpects should be incorporated in the Build Process5.
We can distinguish three general points in time at which architecture
conformance can be introduced:

ffl early; as soon as a developer has written some code the applicable

architectural rules are checked (in parallel to e.g. a compile job).
ffl mediate; at the time the rules are checked a module is "checked in"

in the source code management system. If errors are detected, the module
involved is not accepted by the source code management system. ffl late;
an architect initiates architecture verification at certain times

during system development (e.g. by starting a rule checking program).
If disconformance is established, the architect must submit a Problem
Report.

It will depend on the situation which of the three alternatives will
have to be applied. In general, a system should be verified as early as
possible. If possible, a developer should be given feedback immediately
after he or she has broken an architectural rule. If one is not familiar
with architecture verification, one may prefer to check architecture
conformance at a late stage. That way, the introduction of many changes
in the development

4Rules that cannot be formalised (an example is given in Section 6.1)
should be validated in e.g. review sessions.

5For example, we can create a special "thread" in the Build Process
activities to

execute InfoPacks and ArchiSpects.

142 Managed Architecture

process (and the related tools) is avoided, so the continuity of
development is guaranteed. After a while, one can shift to an early stage
(which will affect the development environment more).

The first time the architectural rules of an existing system are
checked, many violations may be expected. It will be practically
impossible to solve all the violations immediately. Therefore, one
should first only verify the architecture and identify the violations.
In a next verification session, the newly detected violations can then
be compared with the previously detected violations. But this time it
must be ensured that no new violations are introduced (in other words,
that the number of violations does not increase). The violations can
then be resolved to improve the actual architecture at a convenient time.
This way, architecture conformance can be ensured without affecting the
schedules of product deliveries.

Chapter 7 Concluding Remarks We finish this thesiswith recommendations
for application of the SAR method in a real world system. Furthermore,
the application of RPA in several contexts is elaborated once more.

7.1 Recommendations for Application In this thesis we have discussed a
framework for the Software Architecture Reconstruction method. InfoPacks
and ArchiSpects fit in various architectural views at different levels
in this framework. This modular structure of the SAR method simplifies
discussing software architecture reconstruction. For the module view and
code view of software architecture, we presented a number of InfoPacks
and ArchiSpects, summarized (I) in Table 7.1. The SAR method can be
enhanced with new ArchiSpects, which fit in the framework.

When applying SAR to an existing system, one should first consider which
ArchiSpects are most valuable to reconstruct. Most of the discussed
ArchiSpects are based on the imports relation. One imports a header file
to be able to use a function, a type definition, a macro and/or a global
variable from another file. The imports relation is in fact a mixture of
a number of relations: calls, accesses and typed , and for object-oriented
languages also the inherits relation. When reconstructing a system in more
detail, one requires a refinement of the imports relation. For example,
for each of these relations the Component Dependency ArchiSpect can be
refined, e.g. Component Dependency for function calls.

144 Concluding Remarks

Architectural Views Logical Module Code Execution Hardware SAR levels
View View View View View

Optimized

IGenerics and Specifics Conformance IAspect Conformance

IAspect Assignment

Managed IUsage Conformance

IPart-Of

ILayering Conformance

IDepend

IAspect Coupling

IAspect Assignment Redefined ICohesion

and Coupling

IPart-Of

IComponent Coupling

IDepend BResource Usage

ISource Code Organisation IBuild Process Described IUsing and

Used Interfaces

IPart-Of BProcess

Communication IComponent Dependency

IDepend BProcess

Topology

ISoftware Concepts Model

IFiles BProcesses

Initial

Table 7.1: Software Architecture Reconstruction

7.2 Relation Partition Algebra 145 For the reconstruction of the execution
view of software architecture, we suggest the following ArchiSpects:
Process Communication, Process Topology and Resource Usage. The Process
Communication ArchiSpect describes how processes (extracted by a Processes
InfoPack) communicate with each other (e.g. via TCP/IP, a database, shared
memory or shared files). The Process Topology ArchiSpect describes in
terms of a running system how and when processes are created and killed.
The Resource Usage ArchiSpect describes the usage of different resources,
e.g. RAM memory, disk memory and cpu. A first experiment in reconstructing
Resource Usage of Med is presented in [KPZ99]. The suggested InfoPack
and ArchiSpects of the execution view are presented (B) in Table 7.1.

Although separately discussed, the module view, code view and execution
view are related. In [KFJM99, BFG+99] we discussed how scenarios, applied
to the Switch system, can help developers comprehend these three views.
The importance of combining static analysis with dynamic analysis has
also been discussed by Kazman and Carri`ere [KC98].

For the described and managed level of software architecture
reconstruction, one should integrate reconstruction activities in the
development process. An up-to-date described architecture supports
developers in their activities by means of given opportunities to
comprehend the software architecture better. Web technology is the most
appropriate mechanism for presenting requested information to developers
due to its accuracy and multi-media nature. Also, for the managed
architecture, one should integrate reconstruction activities tightly in
the development process. In this way, feedback relating to architecture
conformance can be given as soon as possible. In the case of new systems,
special attention must be given to architecture verification, because it
is easier to introduce architecture conformance in an early stage of the
product's life-cycle than it is to introduce it in existing systems.
In that stage, it is easier to take special measurements and define
extra coding standards to increase the possibilities of extracting
architectural information and verifying architectural decisions.

7.2 Relation Partition Algebra Since 1994, when Relation Partition Algebra
(RPA) was defined [FO94], in 1994, we have applied it in various areas
of software architecture analysis. We experienced that RPA is suitable
for making software abstractions, embellishing the presentation of
information, expressing software metrics,

146 Concluding Remarks

performing dedicated analyses, navigating through information, verifying
architectural decisions and recognising patterns in software. We will
briefly discuss these different areas.

RPA offers filtering operators (e.g. _dom ; _ran ; ndom ) and
grouping operators (e.g. ") for abstracting information from software.
These operators make it possible to focus on specific data (i.e. to
answer a question a software analyst has in mind), and to eliminate
irrelevant data. Furthermore, information can be combined (e.g. through
composition: ffi) to obtain more dedicated information.

The presentation of a function call graph of a large system probably
results in a diagram that contains a large black area (i.e. all the
edges of the graph). Abstractions can help to reduce the amount of
information to embellish a graph presentation. For example, lifting a
large imports relation reduces the amount of information in a smart way.
Transitive reduction also improves the presentation of information.
The transitive reduction removes shortcuts1 from a (cycle-free) relation,
resulting in a more convenient graph.

We can also express software-related metrics in RPA (e.g. cohesion and
coupling). The notion of partof relations gives such metrics an extra
dimension; one can consider cohesion and coupling at different levels
in the decomposition hierarchy.

RPA is also useful for performing dedicated analyses, e.g. detecting
cyclic dependencies in a system, recognising local functions and
calculating components to be tested:

ffl To detect whether a relation (R) contains cycles, it suffices to
calculate R+ " Id . If this equals the empty relation, then R contains
no cycles [FKO98]. ffl A function is local to another function if it is
used by this function

only. Some programming languages offer concepts for defining local
functions (e.g. Pascal). To minimise a system's complexity, one should
define local functions close to their caller (preferably by limiting
the scope of the local function). ffl Given a dependency relation (D)
between components and a list of

modified components (M ), it is calculated which components must be
tested again (because of the changes): dom(D\Lambda  _ran M ), i.e.
all the components which, directly or indirectly, depend on a changed
component must be tested again.

1A tuple hx; zi is a short-cut if the relation contains the tuples hx;
y1i; hy1; y2i; : : : ; hyn; zi for some n * 1.

7.2 Relation Partition Algebra 147 Presentation can be made more dynamic
by offering some navigation mechanisms. For example, a user may want to
zoom-in or zoom-out on certain information. TabVieW is a presentation
tool that provides navigation abilities by executing RPA formulas and
re-calculating a new table.

For the managed architecture, we have defined a number of ArchiSpects
that incorporate architectural rules. RPA is suitable for formalising
architectural decisions, making it possible to automatically verify
an implementation.

In the discussion of Section 6.5 we described how the generic and specific
components can be recoginised in software. We formulated the pattern to
which generic and specific components adhere in RPA.

As indicated above, a number of different areas of software analysis can
be covered by RPA. It is a great advantage to have a single formalism
for different applications (consider e.g. the learning curve of a new
formalism). RPA offers a formal notation, but RPA formulas can also be
executed on a computer. In this way, one can easily explore different
aspects and parts of the system, using an interactive RPA calculator, see
Figure B.2. After performing various calculations, one can consolidate
these calculations by defining a new ArchiSpect which reconstructs
a certain interesting aspect of architecture. This approach in fact
roughly describes the way in which we analysed a number of systems at
Philips and the way in which we deduced various InfoPacks and ArchiSpects.

The diversity of applying RPA summarized above strenghtens our thoughts of
using RPA as a foundation for an Architecture Description Language (ADL).
The semantics of notations and operations of an ADL can be expressed in
terms of RPA. Further research into this topic is required to validate
these thoughts.

148 Concluding Remarks Appendix A Extraction Tools This appendix lists
the source codes of a number of Perl [WCS96] programs. The programs
are related to the Files, Imports and Part-Of InfoPacks discussed in
Chapter 4. We used these tools in various reconstruction activities,
but sometimes we changed these programs slightly in order to satisfy
the system at hand.

A.1 file-exts.pl #!/home/krikhaar/cadbin/perl # input: Files # standard
output: typed.Files.Exts # while (!?) -

chop; if (/.*".([^.]+)/) -

print "$. $1"n"; "" else -

print "! ERROR: unmatched filename $."n"; "" ""

A.2 units.pl #!/home/krikhaar/cadbin/perl

150 Extraction Tools

# input: Files # standard output: partof.Files.Units # while (!?) -

chop; if (/(.*)".([^".]+)/) -

print "$1 $2"n"; "" else -

print "! ERROR: unmatched filename $."n"; "" ""

A.3 comment-strip.pl #!/home/krikhaar/cadbin/perl # input: !file? #
standard output: !stripped file? #

# read whole file at once undef $/; $. = !?;

# remove comment /* ... */ with minimal ... match s-/"*.*?"*/""-""gsx;

# remove comment // ... s-//.*?"n""[]gsx;

# pre-process line extension ("") s/(^[ ]*#.*?)"""""n/$1/g; # at first
line of file s/("n[ ]*#.*?)"""""n/$1/g; # at all other lines

# print the stripped file print;

A.4 C-imports.pl #!/home/krikhaar/cadbin/perl # input: Files # standard
output: imports.Files.Files

A.5 J-imports.pl 151 # while (!?) -

if (/[ ]*("S*)/) -

$INP=$1; open INP or die "! Unable to open file $INP"n"; while (!INP?) -

if (/^[ ]*#[ ]*include[ ]*("S*)/) -

$impfile = $1; $impfile =~ s/"//g; $impfile =~ s/!//; $impfile =~ s/?//;
print "$INP $impfile"n"; "" "" "" ""

A.5 J-imports.pl #!/home/krikhaar/cadbin/perl # input: Files # standard
output: imports.Classes.Classes+ # foreach $java (@ARGV) -

print "Java: $java"n"; $java =~ /.*["/""]([^"/""]+).[Jj][Aa][Vv][Aa]/;
$class = $1;

print "Class: $class"n"; $package = ""; open JAVA, $java or die "!
Unable to open file $java"n"; while ( !JAVA? ) -

if ( /package"s+([^;]+)"s*;/ ) -

$package = $1."."; ""

if ( /import"s+([^;]+)"s*;/) -

$import = $1; print "$package$java $import"n"; "" "" ""

152 Extraction Tools

A.6 J-package.pl #!/home/krikhaar/cadbin/perl # input: Files # standard
output: defines.Classes+.Classes # foreach $java (@ARGV) -

$java =~ /.*["/""]([^"/""]+).[Jj][Aa][Vv][Aa]/; $class = $1;

open JAVA, $java or die "! Unable to open file $java"n"; while ( !JAVA?
) -

if ( /package"s+([^;]+)"s*;/ ) -

$package = $1; print "$package.* $package.$class"n"; print
"$package.$class $package.$class"n"; "" "" ""

A.7 ObjC-imports.pl #!/home/krikhaar/cadbin/perl # input: Files #
standard output: imports.Files.Files # while (!?) -

if (/[ ]*("S*)/) -

$INP=$1; open INP or die "! Unable to open file $INP"n"; while (!INP?) -

if (/^[ ]*#[ ]*import[ ]*("S*)/) -

$impfile = $1; $impfile =~ s/"//g; $impfile =~ s/!//; $impfile =~ s/?//;
print "$INP $impfile"n"; "" "" "" ""

A.8 QAC-imports.pl 153 A.8 QAC-imports.pl #!/home/krikhaar/cadbin/perl #
input: !QAC intermediate file(s)? # standard output: imports.Files.Files #
while (!?) -

chop; @fields = split/[ "t]+/; if ($fields[0] =~ /!INCL?/) -

print "$fields[1] $fields[2]"n"; "" ""

A.9 directory.pl #!/home/krikhaar/cadbin/perl # input: !list of to be
inspected directories? # standard output: partof.Files.Directories #
while (!?) -

if (/[ ]*("S*)/) -

$dir=$1; opendir DIR,$dir or die "! Unable to open file $INP"n"; @allfiles
= readdir $DIR; foreach $f (@allfiles) -

print "$f $dir"n" "" closedir; "" ""

154 Extraction Tools Appendix B Abstraction Tools

B.1 Introduction In this appendix we discuss some implementations of
Relation Partition Algebra. A number of implementations have been created
in a broad range of programming languages:

ffl functional language: Clean and Prolog; ffl scripting language: Perl
and AWK; ffl database language: SQL. ffl imperative language: Pascal,
C, C++, Basic, and Java;

For each kind of language we will briefly discuss some issues of
particular language.

One should not only think about the implementation of RPA operators, but
also about the interface of these operators. We distinguish the following
types of interfaces: API (application programmers interface) consisting
of a set of functions (e.g. Java implementation), a command line (e.g.
AWK implementation), a graphical calculator (e.g. SQL implementation)
and a sophisticated graphical interface [Pet97]. These interfaces will
not be discussed any further here.

B.2 RPA-Prolog The first implementation of RPA was written in Prolog
[SS86] using the SWI-Prolog interpreter [Wie96]. The key design decision
for almost any

156 Abstraction Tools

RPA implementation is the internal representation of sets and relations.
For RPA-Prolog [Kri95] we have chosen the following data structure:

ffl A set is implemented as a list containing the elements. ffl A relation
is a compound term obj(SetX, SetY, Rel); with an invariant

SetX = dom(Rel ) and SetY = ran(Rel ). Rel is a list of compound terms
rel(Elem, ElemRel); where Elem 2 SetX and ElemRel is a list of elements
with which Elem has a relation. Each Elem occurs only once in the Rel
list. ffl The elements in the list Rel and the elements in the list
ElemRel are

ordered according to the order in SetX and SetY, respectively.

The Prolog code of the rel dom and rel comp operators are listed below.

/* Example of facts that represent a set and relation */ set(functions,
[main,a,b,c,d]). relation(calls, obj(functions, functions,

[rel(main, [a,b]), rel(a, [b,c,d]),

rel(b,[d])])).

/*

rel.dom(Relation, Set) !-

Set is the domain of Relation */ rel.dom(obj(SetX, ., Rel), obj(SetX,
Domain)) :-

domain(Rel, Domain). domain([rel(E,.)--R], [E--Dom]) :-

domain(R, Dom). domain([], []).

/*

rel.comp(Relation1, Relation2, Relation) !-

Relation is the composition Relation2 ; Relation1 */ rel.comp(obj(NameX,
NameY, Rel1), obj(NameY, NameZ, Rel2),

obj(NameX, NameZ, Result)) :- set(NameY, SetY), set(NameZ, SetZ),
comp(Rel1, SetY, SetZ, Rel2, Result).

comp([], ., ., ., []). comp([rel(X,XList)--Rel1], SetY, SetZ,

Rel2, [rel(X,CList)--Result]) :-

B.3 RPA-AWK 157

compose(XList, SetY, SetZ, Rel2, CList), CList "== [], !, comp(Rel1,
SetY, SetZ, Rel2, Result). comp([.--Rel1], SetY, SetZ, Rel2, Result) :-

comp(Rel1, SetY, SetZ, Rel2, Result).

addlist([], [], List, List). addlist([Z--SetZ], [Z--YList], [Z--RestList],
[Z--List]) :-

!, addlist(SetZ, YList, RestList, List). addlist([Z--SetZ], YList,
[Z--RestList], [Z--List]) :-

!, addlist(SetZ, YList, RestList, List). addlist([Z--SetZ], [Z--YList],
RestList, [Z--List]) :-

!, addlist(SetZ, YList, RestList, List). addlist([.--SetZ], YList,
RestList, List) :-

addlist(SetZ, YList, RestList, List).

compose([], ., ., ., []). compose([Y--XList], [Y--SetY], SetZ,
[rel(Y,YList)--Rel], List) :-

!, compose(XList, SetY, SetZ, Rel, RestList), addlist(SetZ, YList,
RestList, List). compose(XList, [Y--SetY], SetZ, [rel(Y,.)--Rel], List) :-

!, compose(XList, SetY, SetZ, Rel, List). compose([Y--XList], [Y--SetY],
SetZ, Rel, List) :-

!, compose(XList, SetY, SetZ, Rel, List). compose(XList, [.--SetY],
SetZ, Rel, List) :-

!, compose(XList, SetY, SetZ, Rel, List).

B.3 RPA-AWK The input files for the AWK [AKW88] implementation consist
of so-called RPA files. A set file contains a single element of the
set at each line; in a relation file each line contains a tuple of two
elements separated by white space. The lines in a multi-set file and a
multi-relation file contain an

158 Abstraction Tools

rel_lift Relation-1 Relation-2

Set-3 rel_carX excluded_lift

Figure B.1: High Level Operations extra field to represent the weight.
We have implemented each RPA operator in a separate AWK script.
The input of these scripts consists of files (including standard input)
and the output is given on standard output. The unix pipe mechanism can
be used to concatenate a number of operators, implementing for example
an excluded-lift as depicted in Figure B.1:

rel.lift Relation1 Relation2 -- rel.carX - Set3 A standard wrapper is used
to parse the various arguments before the actual AWK script is called.
This wrapper is responsible for checking the command line arguments,
creating temporary files if needed (the Unix way of referring to standard
input '-' has been used to read from standard input), and some exception
handling code. For clarity the wrapper code has been removed from the
code below. Again, we give the source code for the rel dom and rel
comp operators.

#! /bin/sh # Calculates the domain of a relation Usage="Call: rel.dom
!rel?" # !wrapper code? the variable $IN1 gets a value # awk 'seen[$1]==0
- print $1; seen[$1]=1; ""

' $IN1

#! /bin/sh

B.4 RPA-SQL 159 # Composes two relations Usage="Call: rel.comp !rel1?
!rel2?" # !wrapper code? the variables $IN1 and $IN2 get a value #
awk 'flag==0 - right[$1" "nr[$1]++]=$2; ""

flag==1 - n=nr[$2]; for (i=0; i!n; i++) -

s=$1" "right[$2" "i]; if (seen[s]==0) - print s ; seen[s]=1; "" "" ""
' flag=0 $IN2 flag=1 $IN1

B.4 RPA-SQL Another implementation is built on top of a database program.
Any database that supports SQL would suffice, but we have used MS-Access
[Boe96]. The various sets, relations, multi-sets and multi-relations
are stored in separate tables. For relations, the columns in the table
are named dom and ran, respectively. The table name refers to the
relation's name.

Query: rel.dom(!rel?):

SELECT DISTINCT !rel?.dom FROM !rel?;

Query: rel.comp(!rel1?, !rel2?):

SELECT DISTINCT !rel1?.dom, !rel2?.ran FROM !rel1? AS rel1 INNER JOIN
!rel2? AS rel2

ON rel1.ran = rel2.dom GROUP BY rel1.dom, rel2.ran;

Note that the rel dom query refers to the relation name !rel?. In SQL
it is however not possible to use such a construct. Therefore we have
developed a Visual Basic program to instantiate such free variables in
our SQL description. A new SQL statement is generated in which the actual
values of these variables are filled out. After that, the generated SQL
statement is applied to the data in the database.

160 Abstraction Tools

Query: rel.dom(calls):

SELECT DISTINCT calls.dom FROM calls;

Query: rel.comp(calls, calls):

SELECT DISTINCT calls.dom, calls.ran FROM calls AS rel1 INNER JOIN calls
AS rel2

ON rel1.ran = rel2.dom GROUP BY rel1.dom, rel2.ran;

The stack-oriented RPA calculator shown in Figure B.2 has been built on
top of this program. Sets and relations can be pushed on the stack,
operations (represented by different buttons) are applied to the
element(s) on the top of the stack.

B.5 RPA-Java In our Java [Web96] implementation of RPA we made much
use of classes of standard packages. Various container classes of
the java.util package were used; e.g. relations were represented in
HashTables. We constructed an RPA package that contains the classes
Set, Relation, MultiSet, and MultiRelation. Each class defines its own
methods that perform related RPA operations. For example, the rel dom
method of Relation calculates this object's domain; the rel comp method
of Relation calculates its composition with another Relation object.
As already discussed in Section B.3, RPA files can be read and written
by calling the provided IO methods.

The command line interface of the AWK implementation proved to be very
handy in software architecture analysis. We have therefore implemented
a similar interface on top of this Java implementation, consisting of a
number of small programs, each calling a single method of the RPA package.

public class Relation -

// storage of tuples of Relation protected Hashtable tuples;

/** dom( ) returns the domain of this Relation */

B.5 RPA-Java 161

Figure B.2: RPA Calculator 162 Abstraction Tools

public Set dom( )-

Set s = new Set( ); Enumeration e = tuples.keys( ); while(
e.hasMoreElements( ) )-

s.insert( (String)( e.nextElement( ) ) ); "" return s; ""

/** comp( Relation r ) returns a Relation defined as r o 'this' */

public Relation comp( Relation r )-

Relation res = new Relation(); Enumeration e1 = r.tuples.keys( );

while( e1.hasMoreElements( ) )-

String s1 = (String)( e1.nextElement( ) ); Hashtable h1 =
(Hashtable)( r.tuples.get( s1 ) ); Enumeration e2 = h1.keys( ); while(
e2.hasMoreElements( ) )-

String s2 = (String)( e2.nextElement( ) ); Hashtable h2 = (Hashtable)(
s.tuples.get( s2 ) ); if( h2 != null )-

Enumeration e3 = h2.keys( ); int i1 = get( h1, s2 ); while(
e3.hasMoreElements( ) )-

String s3 = (String)( e3.nextElement( ) ); int i2 = get( h2, s3 );
res.insert( s1, s3, Integer( i1 * i2 ) ); "" "" "" "" return res; "" ""

B.6 A Brief Comparison of RPA tools The Prolog implementation should just
be seen as a first experiment with the aim of becoming familiar with RPA.

The AWK implementation proved to be very suitable in daily practice. It

B.6 A Brief Comparison of RPA tools 163 is easy to use (certainly for
persons familiar with unix concepts). The AWK scripts are interpreted
and can be easily combined using other shell scripts. It is easy to
incorporate these scripts, e.g. in a make facility. A disadvantage may
be the performance, the scripts may take some time in the case of large
relations.

The SQL implementation performs poorly in the case of large relations.
This holds especially for calculating a relation's transitive closure.
The transitive closure is implemented as an extra program (Visual Basic)
which iterates over a number of RPA operations (composition and union).

The advantage of Java is that it is platform-independent. It is also
easy to integrate RPA in e.g. Java applets in a Web browser.

164 Abstraction Tools Appendix C Presentation Tools In this appendix
we briefly discuss, in a chronological order, a number of proprietary
presentation tools we have developed over the years.

C.1 Teddy-Classic Teddy-Classic [Omm93, Roo94] displays components and
relations between components. Components are represented by boxes and
relations are represented by lines. Teddy-Classic requires as input a
relation file and optionally a view file and a component file. The user
can layout the components on the screen (using a mouse). The layout can
be saved in a so-called view file. Later on, the view file can be used
again, even in combination with another relation file (having the same
components as carrier). So, in Teddy-Classic, the relation and view are
separate concepts.

Components are clickable, meaning that the user can `click' on a box,
which results in a text viewer with component information. This `click'
information is described in a component file that describes the relation
between components and information files. Teddy-Classic has various types
of boxes, which represent different types of components. The lines also
have different representations, dictated by the types of components to
which they are connected.

Figure C.1 shows an example of the output of Teddy-Classic. It shows the
same information as presented in the diagram of Figure 4.11 (page 83).
A thick line between two components means that there is a relation between
the graphically lower component and the higher component. A thin line

166 Presentation Tools

Est

Netw

Bas

Bot

Div Sens

UTX

Top Util

Pres

Figure C.1: Teddy-Classic represents a relation between the higher
component and the lower component. From these rules we infer that
bi-directional relations are always represented as thick lines.

Teddy-Classic was written, in 1992, in the programming language C using
X windows. The separation of relations, click information and views is a
powerful concept: various relations can be displayed with the same view
information. A drawback of Teddy-Classic is its graphical appearance.
Bidirectional relations are not explicitly handled and all the boxes
are of the same size; lines start (end) in the middle of a box, which
reduces the possibilities of creating an appealing layout. There are
now more tools available that provide similar functionalities, e.g.
Rigi [SWM97], developed at the University of Victoria, Canada.

C.2 Teddy-Visio 167

Pres Sens UTX

Netw

Est

Util

Div

Top

Bas Bot Figure C.2: Teddy-Visio

C.2 Teddy-Visio In 1996, the functionality of Teddy-Classic was also
implemented in Visual Basic in combination with VisioTM [Vis]. The Visio
tool displays 1-D and 2-D objects, say for clarity arrows and boxes.
Each box has various connection points to which an arrow can be connected.
The connected arrows automatically re-size when a box is moved (using a
mouse). Visio calculates the best connection points for arrows (they call
it `dynamic glueing') and it automatically layouts boxes. Furthermore,
arrows may be straight, but the tool can also bend lines to beautify the
layout. An example of the output of Teddy-Visio is given in Figure C.2.

168 Presentation Tools

C.3 Teddy-PS The aim of Teddy-PS was to present architectural information
in forms resembling as closely as possible the diagrams already used
in the architecture documentation concerned. So, in 1996, Teddy-PS was
developed to resolve drawbacks of Teddy-Classic: boxes of the same sizes
and arrows that start (end) at predefined positions at the border of
these boxes.

Teddy-PS requires as input a view file and one or more relation files.
The view file is a prepared postscript file that contains the layout of
all the components (boxes) and possible relations between components
(arrows). The view file is manually created (by adapting a copy of a
template view file). Per relation, Teddy-PS filters, from the view file,
the corresponding arrows and gives them a colour. Teddy-PS also calculates
the sizes of arrows in the case of multi-relations. An example of the
output of Teddy-PS is given in Figure C.3.

C.4 Teddy-ArchView All of the Teddy tools discussed above use two
dimensions to present information. Teddy-ArchView, developed in 1997,
presents architectural information in a three-dimensional picture [FJ98].
The tool's input consists of various relations and part-of relations.
From this information, TeddyArchView generates a VRML [VRM] description.
The result is presented in a standard Web browser (using a plug-in,
a VRML viewer) or any other VRML viewer. The viewer gives the user the
opportunity to walk through the information, in a virtual-reality world.
An example of such a world is given in Figure C.4.

C.5 TabVieW TabVieW (developed in 1998) presents relations and
multi-relations in a tabular (or matrix) form in a Web browser. The input
for this tool is a use relation, e.g. imports Files;Files , and a number
of partof relations. For the sake of discussion, we will call the use
relation U1 ;1 and the chain of partof relations P1;2; P2;3; : : :.
For example, decomposition level 1 refers to Files, level 2 to Comps,
level 3 to Subs and level 4 to Systems , so partof 1 ;2 describes which
Files belong to which Comps .

C.5 TabVieW 169

Bas Bot Est Sens Pres Util Netw

UTX

Top Div

Figure C.3: Teddy-PS

170 Presentation Tools

Figure C.4: Teddy-ArchView C.5 TabVieW 171 TabVieW shows, in a Web
browser, a matrix that belongs to a tuple of focus points: a focus
point in the domain x at level d plus a focus point in the range y at
decomposition level r. In the first column the domain's focus point x
is presented, in the second column the constituents of x are listed.
In Figure C.5 the domain's focus point is Comm . The constituents of
x can be calculated with RPA: partof d\Gamma 1 ;d :x. In the given
example the constituents of x are, amongst others, Cil, Cnl, and Std.
Analogously, in the first row the range's focus point y is presented,
and in the second row its constituents ( partof r\Gamma 1 ;r :y )
are given. The cells in the matrix show whether a tuple exists in the
relation Ud\Gamma 1 ;r\Gamma 1 or, in the case of a multi-relation1,
they also show the corresponding weight in RPA:

Ud\Gamma 1 ;r\Gamma 1 = Pr\Gamma 2;r\Gamma 1 ffi : : : ffi P1;2 ffi U1
;1 ffi P \Gamma 1d\Gamma 2;d\Gamma 1 ffi : : : ffi P \Gamma 11;2

The user can navigate through the information by clicking on hyperlinks.
Zooming-in can be achieved by clicking on elements in the second column
or row. A new matrix is then calculated and presented with the clicked
element as a new focus point (preserving the other focus point).
Zoomingout can be performed by clicking on the element in the first
column (row). The parent of this element becomes the new focus point
(again preserving the other focus point). When the user clicks on the
cells, the corresponding tuples (lowered to decomposition level 1) are
presented in a table. All the calculations are performed on request
(i.e. after a user's click); a Perl script accessed via cgi [SQ96]
calculates a new matrix or table.

Since the development of this prototype, a more elaborate version of
TabVieW has been implemented by the Switch development team [Gla98,
BGKW99]. It contains a more dedicated user interface containing, amongst
other things, more zooming and hiding actions.

1In case of multi-relations, the part-of relation P of the formula should
be interpreted as a multi-relation.

172 Presentation Tools

Figure C.5: TabVieW Appendix D RPA Operators in a Nutshell In this
appendix we give an overview (quick reference guide) of all the operators
on sets, multi-sets, relations and multi-relations. The first column of
these tables contains the operators and the types of operands (all given
in the same order as discussed in Chapter 3). The second column contains
the mathematical names, and the third column contains the mnenomic,
which includes only ASCII characters (recommended as function/method
name in implementations).

174 RPA Operators in a Nutshell

=: Set \Theta  Set ! Bool equal set eq `: Set \Theta  Set ! Bool subset
set sub ': Set \Theta  Set ! Bool superset set sup ae: Set \Theta  Set !
Bool strict subset set ssub oe: Set \Theta  Set ! Bool strict superset
set ssup

[ : Set \Theta  Set ! Set union set union " : Set \Theta  Set !
Set intersection set isect

n : Set \Theta  Set ! Set difference set diff

: Set ! Set complement set compl j j : Set ! Int size set size

Table D.1: Operations on Sets

175 =: Rel \Theta  Rel ! Bool equal rel eq `: Rel \Theta  Rel !
Bool subset rel sub ': Rel \Theta  Rel ! Bool superset rel sup ae:
Rel \Theta  Rel ! Bool strict subset rel ssub oe: Rel \Theta  Rel !
Bool strict superset rel ssup

[ : Rel \Theta  Rel ! Rel union rel union " : Rel \Theta  Rel !
Rel intersection rel isect

n : Rel \Theta  Rel ! Rel difference rel diff ffi : Rel \Theta  Rel !
Rel composite rel comp \Theta  : Set \Theta  Set ! Rel cartesian product
rel times

Id X : Set ! Rel identity rel ident dom : Rel ! Set domain rel dom

ran : Rel ! Set range rel ran

car : Rel ! Set carrier rel car _dom : Rel \Theta  Set ! Rel domain
restriction rel domR

_ran : Rel \Theta  Set ! Rel range restriction rel ranR

_car : Rel \Theta  Set ! Rel carrier restriction rel carR ndom : Rel
\Theta  Set ! Rel domain exclusion rel domX

nran : Rel \Theta  Set ! Rel range exclusion rel ranX

ncar : Rel \Theta  Set ! Rel carrier exclusion rel carX

? : Rel ! Set top rel top ? : Rel ! Set bottom rel bot \Lambda  : Set
\Theta  Rel ! Set forward projection rel fproj \Delta  : Rel \Theta  Set !
Set backward projection rel bproj : : Rel \Theta  Elem ! Set left image
rel left : : Elem \Theta  Rel ! Set right image rel right

j j : Rel ! Int size rel size \Gamma 1 : Rel ! Rel converse rel conv

: Rel ! Rel complement rel compl + : Rel ! Rel transitive closure rel clos

\Lambda  : Rel ! Rel reflexive transitive closure rel rclos \Gamma  :
Rel ! Rel transitive reduction rel hasse

Table D.2: Operations on Binary Relations

176 RPA Operators in a Nutshell

": Rel \Theta  Par ! Rel lifting rel lift #: Rel \Theta  Par !
Rel lowering rel low

Table D.3: Operations on Part-Of Relations

d e : Set ! mSet mapping set 2mset d en : Set \Theta  Int ! mSet n-mapping
set n2mset

b c : mSet ! Set mapping mset 2set =: mSet \Theta  mSet ! Bool equal mset
eq `: mSet \Theta  mSet ! Bool subset mset sub ': mSet \Theta  mSet !
Bool superset mset sup ae: mSet \Theta  mSet ! Bool strict subset
mset ssub oe: mSet \Theta  mSet ! Bool strict superset mset ssup [ :
mSet \Theta  mSet ! mSet union mset union + : mSet \Theta  mSet ! mSet
addition mset sum

" : mSet \Theta  mSet ! mSet intersection mset isect

n : mSet \Theta  mSet ! Set difference mset diff

: mSet ! mSet complement mset compl k k : mSet ! Int size mset size

Table D.4: Operations on Multi-Sets

177 d e : Rel ! mRel mapping rel 2mrel d en : Rel \Theta  Int ! mRel
n-mapping rel n2mrel

b c : mRel ! Rel mapping mrel 2rel =: mRel \Theta  mRel ! Bool equal
mmrel eq `: mRel \Theta  mRel ! Bool subset mrel sub ': mRel \Theta
mRel ! Bool superset mrel sup ae: mRel \Theta  mRel ! Bool strict subset
mrel ssub oe: mRel \Theta  mRel ! Bool strict superset mrel ssup [ :
mRel \Theta  mRel ! mRel union mrel union + : mRel \Theta  mRel ! mRel
addition mrel sum

" : mRel \Theta  mRel ! mRel intersection mrel isect

n : mRel \Theta  mRel ! mRel difference mrel diff ffi : mRel \Theta  mRel
! mRel composite mrel comp \Theta  : mSet \Theta  mSet ! mRel cartesian
product mrel times

Id X : Set ! mRel identity mrel ident Id X;n : Set \Theta  Int ! mRel
identity mrel ident

dom : mRel ! mSet domain mrel dom

ran : mRel ! mSet range mrel ran

car : mRel ! mSet carrier mrel car _dom : mRel \Theta  Set ! mRel domain
restriction mrel domR

_ran : mRel \Theta  Set ! mRel range restriction mrel ranR

_car : mRel \Theta  Set ! mRel carrier restriction mrel carR ndom :
mRel \Theta  Set ! mRel domain exclusion mrel domX

nran : mRel \Theta  Set ! mRel range exclusion mrel ranX

ncar : mRel \Theta  Set ! mRel carrier exclusion mrel carX

? : mRel ! mSet top mrel top ? : mRel ! mSet bottom mrel bot \Lambda  :
Set \Theta  mRel ! mSet forward projection mrel fproj \Delta  : mRel
\Theta  Set ! mSet backward projection mrel bproj : : mRel \Theta  Elem
! mSet left image mrel left : : Elem \Theta  mRel ! mSet right image
mrel right

k k : mRel ! Int size mrel size \Gamma 1 : mRel ! mRel converse mrel conv

: mRel ! mRel complement mrel compl + : mRel ! mRel transitive closure
mrel clos

\Lambda  : mRel ! mRel reflexive transitive closure mrel rclos \Gamma  :
mRel ! mRel transitive reduction mrel hasse

": mRel \Theta  Par ! mRel lifting mrel lift #: mRel \Theta  Par !
mRel lowering mrel low

Table D.5: Operations on Multi-Relations

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188 Bibliography Glossary 4 + 1 View Model:

A specific architecture view model that partitions an architecture into
4 views (logical view, development view, process view, physical view)
plus an additional view (scenarios) that combines the four views (see
Section 1.2.1).

abstraction:

An activity that raises extracted information to a higher level of
abstraction, e.g. to an architectural level (see Section 2.3).

ArchiSpect:

A concept of the SAR method that describes how an aspect of architecture
of an existing system can be reconstructed (see Section 2.5.2).

architectural pattern:

A recurring solution to a problem relating to architecture (see Section
1.5).

architecture:

The main structures of a system, also used as a shorthand for software
architecture (see Chapter 1).

architecture conformance:

The situation in which the implementation of a system conforms to the
architecture (see Section 6.1).

architecture improvement:

The process of improving the architecture of an existing system (see
Section 2.4).

architecture verification:

The process of verifying an implementation by comparing it with its
architecture to assess architecture conformance (see Section 6.1).

190 Glossary

AV model:

An architectural view model that partitions an architecture into five
different views (logical view, module view, code view, execution view,
physical view) plus an extra view (scenarios) that combines the five views
(see Section 1.2.3).

binary relation:

A set of tuples (f: : : ; hx; yi; : : :g) representing a certain relation,
e.g. calls for function calls within a system (see Section 3.3).

Building Block method:

A dedicated software architecture method that stems from telecommunication
system development (see Section 2.2).

component:

A generic name for a piece of software; this term is sometimes used to
refer to a piece of software at a certain level of decomposition.

decomposition hierarchy:

The hierarchy of software entities of a system including their containment
relationship (see Section 4.7).

decomposition level:

A certain level in the decomposition hierarchy (see Section 4.7).

extraction:

An activity involving the retrieval of information from source code,
design documentation and/or domain experts (see Section 2.3).

Files:

An example of the notation used in this thesis for sets (see Section
3.6.2).

forward architecting:

The discipline of creating new architectures for software systems (see
Section 2.2).

impact analysis:

The process of simulating a possible idea for modification in a software
model to analyse, in advance, the possible consequences of its application
to actual source code (see Section 5.1).

imports Files;Files:

An example of the notation used in this thesis for multi-relations:
imports Files;Files represents the multi-relation: imports ` Files \Theta
Files (see Section 3.6.2).

Glossary 191 imports Files;Files :

An example of the notation used in this thesis for binary relations:
imports Files;Files represents the binary relation: imports ` Files
\Theta  Files (see Section 3.6.2).

InfoPack:

A concept of the SAR method that describes how to extract
(architecture-relevant) information from existing software (see Section
2.5.2).

lifting:

An RPA operation involving a relation and a part-of relation resulting
in a relation at a higher level of abstraction (see Section 3.4).

lowering:

An RPA operation involving a relation and a part-of relation resulting
in a relation having a finer coarse of granularity (see Section 3.4).

multi-relation:

A bag of tuples ha; bi, represented as a set of triples ha; b; ni, where n
represents the number of occurrences, called the weight (see Section 3.5).

multi-set:

A bag of entities, represented as a set of tuples ha; ni, where n
represents the number of occurrences, called the weight (see Section 3.5).

part-of relation:

A relation that describes a partition, i.e. a division of a set of
entities into various non-overlapping (named) parts (see Section 3.4).

presentation:

The activity of showing (architectural) information to developers and
architects in an appropriate way e.g. by means of diagrams, tables and/or
text (see Section 2.3).

re-architecting:

The process of modifying the software architecture of an existing system
(see Section 2.3).

repository:

A data-store containing software-related information (see Section 2.3).

reverse architecting:

Reverse engineering of software architectures (see Section 2.3).

192 Glossary

reverse engineering:

The process of analysing a subject system to identify the system's
components and their relationships and create representations of the
system in another form or at a higher level of abstraction (see Section
2.3).

RPA:

Relation Partition Algebra, an algebra based upon sets, relations and
partitions (see Chapter 3).

SAR:

Software Architecture Reconstruction, a method for reconstructing software
architectures (see Chapter 2).

SAR level:

A level of the Software Architecture Reconstruction method (see Section
2.5.1).

set:

A collection of objects, called elements or members (see Section 3.2).

SNH model:

An architectural view model that partitions an architecture into
five different views: conceptual architecture, module interconnection
architecture, execution architecture, code architecture and hardware
architecture (see Section 1.2.2).

software architecture:

A heavily overloaded term, which covers at least the main structures of
a software system (see Section 1.2).

software architecture reconstruction:

The process of recovering an existing software architecture, improving
an existing software architecture and/or verifying the architecture of
an existing system (see Section 2.5).

software architecting:

The process of creating software architectures (see Section 1.2).

Summary This thesis concerns Software Architecture Reconstruction of
large embedded systems of the kind developed at Philips (MRI scanners,
telephony switching systems, etc.). These systems typically consist of
millions of lines of code1, from which the first lines of code may have
been developed more than fifteen years ago. A complete crew of software
developers, typically sixty persons, continuously maintains and extends
the system with new functionality.

Chapter 1 discusses the term software architecture. It concerns the
design of simple and clear structures to be able to share software code
amongs different products. A well designed architecture can be reused
in dfifferent products. We must note that the term software architecture
is somewhat ambigious. Therefore, we discuss a number of definitions and
views on software architecture. Finally, a number of product's aims from
a business perspective are discussed. For example, a product should be
available in the market as soon as possible. The business goals determine
the objectives of an architecture, for example possibilities to reuse
parts of the software in other systems. On its turn these architectural
objectives can be translated into architectural patterns.

In Chapter 2 we give an overview of the SAR (Software Architecture
Reconstruction) method. First, we introduce terminology like software
architecting (the construction of a new software architecture), reverse
architecting (the process of making explicit the software architecture of
an existing system) and architecture improvement. Next, the basic notions
of the SAR method are discussed: InfoPacks, ArchiSpects and software
architecture reconstruction levels. InfoPacks describe information
extraction from software and ArchiSpects describe aspects of software
architecture. A number

1To print a system of two million lines of code, we need a pile of 300
books of the size of this thesis.

194 Summary

Architectural Views Logical Module Code Execution Hardware SAR levels
View View View View View

Optimized

IGenerics and Specifics Conformance

IAspect Conformance

IAspect Assignment

Managed IUsage Conformance

IPart-Of

ILayering Conformance

IDepend

IAspect Coupling

IAspect Assignment Redefined ICohesion and

Coupling

IPart-Of

IComponent Coupling

IDepend

ISource Code Organisation IBuild Process Described IUsing and Used

Interfaces

IPart-Of

IComponent Dependency

IDepend

ISoftware Concepts Model

IFiles

Initial

Table S.1: Software Architecture Reconstruction

of InfoPacks and ArchiSpects can be defined for each architectural view.
The following software architecture reconstruction levels are identified:
initial, described, redefined, managed and optimized. A framework
(with a focus on the module view and code view) of the SAR method is
given in Table S.1. The cells of the table contain various InfoPacks
and ArchiSpects discussed in this thesis.

In Chapter 3 Relation Partition Algebra (RPA) is discussed. Relation
Partition Algebra is an algebra based on sets, binary relations and
operations on them. Partitions play a special role in the algebra,
which can be expressed in so-called part-of relations. In particular,
dedicated operations

Summary 195 upon relations and part-of relations make the algebra very
useful for software architecture. Multi-relations are an extension of
binary relations, which were found to be very useful for architecture
analysis. In the SAR method, we consequently apply RPA to describe the
InfoPacks and ArchiSpects.

In Chapter 4 we show how the software architecture of an existing system
can be described by defining a number of InfoPacks and ArchiSpects.
The ArchiSpects Source Code Organisation and Build Process belong to
the code view and the ArchiSpects Software Concepts Model, Component
Dependency and Using and Used Interface belong to the module view of
software architecture. The InfoPacks that are required to reconstruct
these ArchiSpects are also discussed.

In Chapter 5 we describe a number of ArchiSpects that support the
improvement (or redefinition) of an existing software architecture.
The ArchiSpects Component Coupling, Cohesion and Coupling and Aspect
Coupling are discussed. These ArchiSpects can be used by an architect to
analyse the impact of the introduction of certain architectural changes.

In Chapter 6 we discuss architecture verification, i.e. the process
of checking whether the implementation agrees with the defined software
architecture. Therefore, we discuss the ArchiSpects (Layering Conformance,
Usage Control, Aspect Conformance, and Generics and Specifics) that can
help to manage a software architecture.

Chapter 7 contains some concluding remarks and recommendations.
The appendices contain the extraction, abstraction and presentation
tools used to reconstruct and present ArchiSpects of a number of Philips
systems. The last appendix contains an overview of the RPA operators.

196 Summary Samenvatting Niet alle lezers van dit proefschrift zullen
weten wat software-architecturen zijn. We zullen dit begrip proberen uit
te leggen door een vergelijking te maken met de architectuur van huizen.

Het zal duidelijk zijn dat de architectuur van een wolkenkrabber
(letterlijk) hemelhoog verschilt van die van een zomerhuisje. Een
wolkenkrabber heeft natuurlijk een heel ander soort fundering nodig,
maar ook grotere aan- en afvoerpijpen voor het water, er zal met andere
bezoekersaantallen rekening moeten worden gehouden, enzovoorts.

Toch wordt van softwaresystemen soms wel verwacht dat ze in
toepassingen worden gebruikt van een geheel andere schaalgrootte.
Dergelijke verwachtingen kunnen niet altijd worden waargemaakt.
Een televisietoestel bijvoorbeeld heeft een gesloten architectuur, wat
betekent dat nieuwe functionaliteiten, geschreven in software, via de
kabel kunnen worden binnengehaald en als die met elkaar zijn verbonden
kunnen gebruikers met elkaar communiceren.

Als dergelijke productveranderingen niet zijn voorzien en de
software-architect daarmee met zijn ontwerp geen rekening heeft gehouden,
zal het systeem niet zijn voorbereid op toekomstige uitbreidingen.
Een software-architect heeft dus ook tot taak om zoveel mogelijk rekening
te houden met toekomstige uitbreidingen.

Het is natuurlijk onpraktisch om de ontwerper van een zomerhuisje te laten
werken met allerlei specificaties die gelden voor veel grotere gebouwen,
en voor wolkenkrabber en zomerhuisje zal niet snel een gezamenlijke
architectuur te ontwerpen zijn. Maar het zou al een hele verbetering
zijn als de architectuur

198 Samenvatting

van de wolkenkrabber ook toepasbaar zou zijn voor een te ontwerpen
flatgebouw, en andersom. Ook voor software geldt dat het om allerlei
redenen, waarover hieronder meer, van belang is om bij de onderliggende
architectuur rekening te houden met toekomstige uitbreidingen en andere
toepassingen.

Dit proefschrift gaat over de reconstructie van software-architecturen van
grote systemen zoals die door Philips worden ontwikkeld zoals MRI-scanners
en telefooncentrales. De software van dit soort systemen bevat miljoenen
regels code1 waarvan de eerste code vaak al vijftien jaar oud is. Er is
een groep van zo'n zestig software-ontwikkelaars nodig om het systeem
te onderhouden en uit te breiden met nieuwe functies. Die inspanning
kan aanzienlijk worden gereduceerd als software-architecturen zodanig
worden ontworpen dat ze toepassing in vele systemen mogelijk maken.

Hoofdstuk 1 gaat over het begrip software-architectuur. Daarbij gaat het
om het ontwerpen van duidelijke en eenvoudige algemene structuren die
het mogelijk maken om softwarecode te delen met andere producten. Op die
manier kan een productfamilie ontstaan. Een goed ontworpen architectuur
kan voor vele producten worden gebruikt. Het begrip software-architectuur
is overigens niet eenduidig. Er worden verschillende definities
gehanteerd. Een aantal definities worden besproken, samen met een aantal
gezichtspunten op software-architecture. Tenslotte worden een aantal
doelen voor producten besproken vanuit bedrijfsperspectief. Zo'n doel is
bijvoorbeeld dat het product snel voor de markt beschikbaar moet zijn.
De bedrijfsdoelen bepalen uiteindelijk de eisen die aan de architectuur
worden gesteld, zoals de mogelijkheid van hergebruik. Deze eisen zijn
op hun beurt weer richtlijnen voor de architectuurpatronen.

In hoofdstuk 2 wordt een overzicht gegeven van de Software
Architectuur Reconstructie (SAR) methode. Nadat termen worden
uitgelegd zoals software architecting (de constructie van een nieuwe
software-architectuur), reverse architecting (het proces van het
expliciet maken van de softwarearchitectuur van een bestaand systeem),
en architectuurverbetering, worden de basisonderdelen van de SAR-methode
besproken: InfoPacks, ArchiSpects en niveaus van software-architectuur
reconstructie (SAR levels). InfoPacks beschrijven extractie van informatie
uit software. ArchiSpects beschrijven aspecten van software-architectuur.
Voor elk architectuurgezichtspunt kunnen andere InfoPacks en ArchiSpects
worden gedefinieerd. De

1Het afdrukken van twee miljoen regels code levert een stapel op van
300 boeken met de omvang van van dit proefschrift.

Samenvatting 199

Architectural Views Logical Module Code Execution Hardware SAR levels
View View View View View

Optimized

IGenerics and Specifics Conformance

IAspect Conformance

IAspect Assignment

Managed IUsage Conformance

IPart-Of

ILayering Conformance

IDepend

IAspect Coupling

IAspect Assignment Redefined ICohesion and

Coupling

IPart-Of

IComponent Coupling

IDepend

ISource Code Organisation IBuild Process Described IUsing and Used

Interfaces

IPart-Of

IComponent Dependency

IDepend

ISoftware Concepts Model

IFiles

Initial

Table T.1: Software-Architectuur Reconstructie

software-architectuur reconstruction levels die zijn gedefinieerd, zijn
: initieel (initial ), beschreven (described ), opnieuw gedefinieerd
(redefined ), gecontroleerd (managed ) en geoptimaliseerd (optimized ).
In tabel T.1 wordt het raamwerk van de SAR-methode (met een nadruk op
de module view en de code view ) getoond. De cellen in de tabel bevatten
de InfoPacks en ArchiSpects zoals verwoord in dit proefschrift.

In hoofdstuk 3 wordt Relatie Partitie Algebra (RPA) behandeld. RPA is
een algebra, gebaseerd op verzamelingen, binaire relaties en operaties
die hierop plaats vinden. Partities, die een aparte rol spelen in
deze algebra, kunnen worden uitgedrukt in "delen-van" relaties (partof
-relations). Het

200 Samenvatting

zijn vooral de speciale operaties op relaties en de partof-relaties die
deze algebra geschikt maken voor software-architectuur. Multi-relaties
zijn een uitbreiding op binaire relaties die uitstekend geschikt bleken
voor architectuur analyse. In de SAR-methode wordt RPA systematisch
toegepast om InfoPacks en ArchiSpects te beschrijven.

In hoofdstuk 4 wordt uitgelegd hoe een software-architectuur van een
bestaand systeem kan worden beschreven door een aantal InfoPacks en
ArchiSpects te defini"eren. De ArchiSpects Source Code Organisation en
Build Process behoren tot de code view en de ArchiSpects Software Concepts
Model, Component Dependency en Using and Used Interfaces behoren tot
de module view van software-architectuur. De InfoPacks die noodzakelijk
zijn om ArchiSpects te reconstrueren worden ook besproken.

In hoofdstuk 5 worden een aantal ArchiSpects beschreven die het verbeteren
(of opnieuw defini"eren) van een bestaande software-architectuur
ondersteunen. De ArchiSpects Component Coupling, Cohesion and Coupling
and Aspect Coupling worden besproken. Deze ArchiSpects kunnen door een
architect worden gebruikt om het effect van een bepaalde verandering in
de architectuur te analyseren.

Hoofdstuk 6 gaat over architectuurverificatie, het proces om de
implementatie te verifieren met de gedefinieerde software-architectuur.
Er worden enkele ArchiSpects (Layering Conformance, Usage Control,
Aspect Conformance, en Generics and Specifics Conformance) besproken
die bijdragen aan het beheersen van de software-architectuur.

De dissertatie wordt afgesloten met enkele concluderende opmerkingen en
aanbevelingen voor gebruik.

De appendices bevatten extractie-, abstractie- en presentatie- programma's
die zijn gebruikt om ArchiSpects van een aantal Philips-systemen te
reconstrueren en presenteren. De laatste appendix laat een overzicht
van alle RPA-operatoren zien.

Curriculum Vitae Ren'e Krikhaar was born in Nijmegen, the Netherlands,
on January 30, 1963. In 1981, he finished secondary school (Voorbereidend
Wetenschappelijk Onderwijs; pre-university education) at the Canisius
College in Nijmegen. That same year he started to study Computer
Science at the Catholic University of Nijmegen. He obtained his
university degree ("doctorandus") in 1986. After fulfilling his military
obligation in Bussum and Schaarsbergen, he started his career at Philips
Telecommunication and Data Systems in Hilversum in 1987, in a group
developing methods and tools for designing Application Specific Integrated
Circuits (ASICs). In 1991, in his spare time, he started a second study
at the Centre of Knowledge Technology in Utrecht in cooperation with the
Middlesex University in Great Britain. This resulted in a degree of Master
of Science with distinction in Knowledge Engineering in 1994. That same
year, he joined the Information and Software Technology department of the
Philips Research Laboratories in Eindhoven. In 1994 and 1995 he spent a
few months working with a group of system designers who develop public
telephony switching systems in Nuremberg, Germany. In 1999 he continued
his career at Philips Medical Systems in Best, the Netherlands. He is
currently a software architect of the Magnetic Resonance Imaging system.

202 Curriculum Vitae