Software Design 2e - DIM

95Design viewpoints for softwareFigure 5.6Changes of viewpoint with evolution of the overall design model.nnnnconstructional* forms, in which the viewpoint is concerned with essentially staticaspects of the system;behavioural forms, a set of viewpoints that seek to describe the causal linksbetween events and system responses during execution;functional forms, viewpoints that seek to describe what the system does in terms ofits tasks;data-modelling forms, concerned with the data objects used within the system, andthe relationships between these.Within each of these broad classes, there are a number of forms in general use, partlybecause the many possible relationships require different sets of attributes for theirdescription, and partly because no representation ever quite captures all of the propertiesthat a designer needs to consider.These representations can all be classified as ‘direct’ viewpoints, in that they arecreated directly by the designer. A further class of viewpoints can be described as‘derived’ viewpoints (Budgen and Friel, 1992), shown in Figure 5.7; in these sometransformation is applied to the design model in order to generate a ‘new’ representationform. An example of such a derived viewpoint is that produced by the use ofsome form of interpreter in order to ‘execute’ a design (Harel, 1992; Friel and Budgen,1997); here the output from the interpreter forms the derived viewpoint in termsof some sequence of states or diagrams. (This particular example is based on the* In the first edition we used the term ‘structural’ for these forms, on the basis that they described the structure ofa solution. However, this unintentionally introduced a note of ambiguity, in that all of the viewpoint descriptionscan themselves be ‘structured’. Hence for this edition we have preferred the use of ‘constructional’ to make theemphasis clear.