Software Architecture

A good design is the key to a successful product. Almost 2000 years ago, the Roman architect Vitruvius already recorded what makes a design good: durability (rmitas), utility (utilitas) and charm (venustas). These quality requirements still hold, for buildings as well as software systems. A well-designed system is easy to implement, is understandable and reliable, and allows for smooth evolution. Badly designed systems may work at rst, but they are hard to maintain, diicult to test, and unreliable. The design phase is a most crucial step in the software development process. Design is a problem-solving activity, and as such very much a matter of trial and error. In the presentation of a mathemathical proof, subsequent steps dovetail well into each other and everything drops into place at the end. The actual discovery of the proof was probably quite diierent. The same holds for the design of software. We should not confuse the outcome of the design process with the process itself. The outcome of the design process is a `rational reconstruction' of that process. (Note that we made precisely the same remark with respect to the outcome of the requirements engineering process.) During the design phase, the system is decomposed into parts that each have a lower complexity than the system as a whole, while the parts together solve the user's problem. Since the complexity of the individual components should be surveyable, it is important that the interaction between components not be too complicated. The design problem can now be formulated as follows: how to determine this decomposition. There really is no universal method for this. The design process is a creative one, and the quality and expertise of the designers is a critical determinant for its success. Yet, during the course of the years, a number of ideas and guidelines have emerged which may serve us in designing software. These have resulted in a large number of design methods, which are the topic of chapters 12 and 13. In this chapter, the focus is on characterizing the result of the design process, the blueprint which serves as a starting point for the implementation. In the problem-solving processes that take place during software development, we employ previously acquired knowledge. In order to better understand how we go about during design, it is helpful to know how knowledge is stored and processed in human memory. Information from perception is entered in …