Software Design 2e - DIM

92Describing a design solutionNor should the association between methods and representations be regarded asparticularly rigid, so that the adoption of a method precludes in some way the use ofany representation that is not normally associated with it. The objective of design is tosolve problems, not to be ‘politically correct’ in its use of a form! (To some extent thisassociation of method with representation forms is a natural enough consequence ofusing a design method, since the method guides the designer into considering particularissues in sequence, and so into using specific notations to support these.) For thatreason the discussions of representations and methods have been separated in thisbook, so that the power of abstraction provided by different representations can beseen in a wider context than that of a particular method.Because representations provide particular abstractions of a system, they areclosely linked to the concept of a viewpoint. Many of us are familiar with the type ofplan drawing that is used for describing buildings and other physical objects. Such adrawing can be considered as describing the ‘physical’ appearance of the object.However, as demonstrated in Figure 5.2, the wiring plan will also provide an abstractrepresentation of the building, although it may look very different from the physicalplan. While these viewpoints are very different in many ways, they need to be kept consistentat the points where they ‘intersect’ through references to common attributes ofdesign objects.The notion of a viewpoint puts a particular emphasis upon the role of a representation.A representation is the means that we use to capture certain properties of adesign, and it should not be regarded as being the design itself. This concept of theviewpoint as a projection of a design model is illustrated in Figure 5.3. When using thisconceptual framework, we can regard the different viewpoints as encompassing particularsets of properties of the design model, and the representations are then the meansthat are used to describe the corresponding attributes or characteristics (Brough,1992). (There may, of course, be more than one representation that can be used todescribe the properties corresponding to a particular viewpoint, a point that will beexamined further later in this chapter.)We should note here that the use of the term ‘viewpoint’ in this book does differsomewhat from the way that it is apt to be used in requirements engineering, where theFigure 5.3The four principal viewpoints as projections from the design model.