Acknowledgements

　　The authors would like to thank Reinder Bril, Loe Feijs, Peter van den Hamer, Jaap van der Heijden and Joachim Trescher for reviewing previous versions, and the fruitful discussions that helped us to improve this paper.
　　2 Improvement Process

　　We need to address a number of questions related to the type of software systems under investigation:

　　How do we make software architectures of existing systems explicit?
　　How do we realise and measure improvement?
　　How can we make changes in the software without introducing new defects and without spending too much time?

　　The process which in our opinion can answer these questions is depicted in Figure 1. Before we give a detailed description of the steps, we will give some definitions and assumptions on which this process is based. We adhere to the terminology proposed by Chikofsky and Cross [CC90].

　　To improve software architecture we must first have a described software architecture, which is an explicit description of requirements of the system from a software architecture point of view [SNH95,Kru95]. For large software systems a software architecture description is usually not available. Software architecture recovery or reverse architecting [Kri97,Kri99] is the process that extracts such a description from the software.

　　Rearchitecting is the process of changing the software architecture. Software architecture improvement is the process that makes changes in the architecture in such a way that it improves the software in one or more of its quality aspects. Quality aspects are for instance the comprehensibility of the software for the developers, the extensibility of the software with new features and the reusability of its parts. Quality aspects are usually accompanied by metrics. Using the proper metrics we can measure improvement by comparing the values before and after the change. Defining good metrics is a research topic beyond the scope of this paper. To experiment with different kinds of metrics and changes we need a good process, supported by tooling.

　　Changes can affect many parts of the software. Before a change is executed, an architect must know exactly which parts of the software will be affected. Also important is the cost of implementing the change. Architecture impact analysis is the process of calculating the consequences of an architecture change before applying it to the software. If the architect can track the impact of a change, then it is also possible to automate the actual changing of the software. Automation can help to increase the quality and reduce the cost of implementing the change. Since we do not want to keep modifying the software during each experiment, it is better to use an abstract model of the software.

　　At Philips we have many years of experience using Relation Partition Algebra [FO94,FKO98,FO99,FK99], which involves a mathematical model based on sets and relations and has proven to be quite adequate for impact analyses of this kind. Using a model of the existing software allows for fast feedback of the impact without the need to modify the software.

　　The software architecture improvement process that we propose makes a distinction between architecture impact analysis on the one hand and software architecture transformations on the other. Figure 1 shows the software architecture improvement process. Each of the steps in the process (corresponding to the numbers in the figure) will be described successively. An example using the process will be described in Section 3.

　　Figure 1: Software Architecture Improvement

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