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4. method and tooling of POSD

4.1 definition of stepwise refinement method

A stepwise refinement approach to the development of POSD models has been introduced. Appropriate tooling for this method is being developed.

The designer starts the stepwise refinement process by giving an initial design for a system as a very simple composition of interacting component systems.

The designer may then replace one component system in this composition with its design. This design must be a composition of interacting (lower level) component systems. The designer must be confidant that the design is at least capable of the behaviour of the component system being replaced, though it may be capable of much more besides. In particular it must be capable of supporting all interactions that the replaced system had with other component systems, though it may be capable of additional interactions as well. For instance, a role in a business process may be replaced by a human capable of fulfilling that role and all its interactions, though the human is capable of behaviour and interactions well beyond that role.

The designer now has a more detailed and possibly more powerful (but certainly not less powerful) composition for the original system. The stepwise refinement process defined in the last paragraph can now be repeated for this composition. The designer may continue to refine the system composition in this way until it is defined entirely in terms of low level component systems that have already been designed (and probably implemented).

Thus the designer creates a sequence of compositions for the system being designed. Each composition is more detailed than the previous one (in the sense that it contains more components and more interactions) and is capable of at least the behaviour and interactions of the previous one.

Note that the full design of the system is represented by the full set of compositions not just by the last one in the sequence. The earlier compositions represent the significant interactions within the system, the later compositions show how these interactions are achieved in detail. Both give information about the system not available in the other.

Each step in the stepwise refinement approach described above replaces a component system with a composition of lower level component systems and ensures that the composition is at least capable of the behaviour of the replaced component system. The validity of this replacement approach is analysed in depth in the definition of POSD so is not discussed further here.

4.2 stepwise refinement example

The stepwise refinement method can best be understood by looking at an example. In looking at this example the reader should imagine a tool helping the designer. The tool has a database which records the sequence of compositions so far developed for the system being designed. The designer can ask this tool to focus on one component in its latest composition. The tool then provides the designer with a diagram showing this component and all components with which it interacts. The designer can then provide a design to replace this component. The design must be capable of all the behaviour of the replaced component though it may be capable of additional behaviour. The tool uses this design to form the next composition in the sequence recorded in its database. Many designs are so straightforward that the tool is able to provide them automatically. The example will show what is meant by this statement.

last revision: 21st June 1997