Today it is more a rule than an exception that software systems have a lifecycle of more than several years. Hence, software evolution is inevitable. During the life span of a software system the domain in which the system is working evolves and changes. This causes changes to the software system, and the software system may also be evolved to satisfy new markets. The ability to evolve gracefully, and thus the long-term success of a software system, is to a large extent governed by its software architecture and the ability of the software architecture to fulfil requirements on quality attributes and to adapt to evolving requirements. In this thesis we study evolution of software architectures and what can be done to support this evolution. We focus on three particular aspects of evolution support: how to ensure that the correct blend of quality attributes is met (architecture selection), the technical means available for supporting changes in the software system (variability), and what types of changes that are likely to occur during evolution (categories of evolution). We introduce a method for architecture evaluation and selection that focus on ensuring that the selected software architecture is the architecture candidate with the most potential for fulfilling a particular blend of quality attributes. The method is based on quantification of expert opinions and focused discussions where these expert opinions differ. The architecture evaluation and selection method is studied in both an academic and in an industry setting. We also introduce a taxonomy of techniques for realising variability in a software system and study how the techniques in this taxonomy are applied in different evolution situations. The taxonomy is based on several industry case studies. Two industry cases are studied in further detail and the evolution of these systems are followed over a number of releases and generations. During this evolution it is shown how variability mechanisms are used to also support evolution, and that there are typical cases of evolution that a software system can be prepared to cope with. The contribution of this thesis is that it increases the understanding of how evolution occurs in a software system, how to create software that is flexible enough to support evolution and how to evaluate and select a software architecture that meets a particular blend of quality attributes. Together this ensures that a software system is based on a software architecture that fits the current quality requirements and that is flexible in the right places so that it is able to evolve gracefully.

Thesis (M.B.A.)--University of Hong Kong, 1991.