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Implinks and the Paste As Concept
The SOMT method introduces a number of different models that are used to describe different aspects of the system. In particular there are three levels formed by:
- The requirements that describe the problem and external requirements on a system
- The system analysis describing the concepts used in the system
- The system and object design that defines the structure and behavior of the system
One very common relation between objects in different models is that one object can be seen as an implementation of an object in another model. For example, an object in the object model created in the system analysis phase may be reintroduced in the design model as a process type or an ADT (abstract data type). Another example may be an object that was identified in the requirements analysis as something visible on the system boundary, and that later is reintroduced in the analysis object model and finally ends up as a signal in the SDL design. To represent this type of relations among objects the concept of implementation links (implinks) is used. An implink is a directed relation between two objects, usually (but not necessarily) in different models. Conceptually we get a picture as in Figure 615.
If used carefully, the implinks give a possibility to trace requirements all the way down into code. There are several situations where the implinks are very useful:
- What if analysis. What are the consequences if a particular requirement is changed?
- Consistency checking. Are all concepts in e.g. the system analysis model implemented?
- Understanding design by following the links backwards from implementation to requirements. What is the purpose of a particular design object? What happens if we change it?
It is important to see that the act of creating an implink is a creative design action that encapsulates a design decision. The Paste As mechanism is a special concept used in SOMT to support the task of creating implinks. The idea is that an object in one model can be copied and then pasted as a new object in another model, see Figure 616. This action serves both to create the new object and to document the design step using an implink.
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