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Analysis vs. Design


When comparing analysis to design there is always a question of where the borderline between the activities are. In practise the distinction between analysis and design is somewhat arbitrary. From a practical point of view the important decision is not to define exactly what to call a specific activity, but rather to decide where to document a decision: in the analysis documentation or in the design model. The most important input to guide this decision is to consider the purpose of the different models. The analysis model has a focus on the architecture and most important concepts in the system, abstracting away from implementation details. It is used to provide a means to understand the application as efficiently as possible. The SDL design, on the other hand, is a complete definition of all necessary details.

In the context of mapping object models to SDL in SOMT the trade-off between analysis and design, essentially is a matter of where to document a decision:

As an example of this type of question that either can be described in the analysis model or in the design model, consider the issue of the traversal direction of an association, i.e. is the association a one-way or two-way association.

If the choice is to be made in the object model a notation or convention is needed to make it possible to add the extra information. For the one-way/two-way associations a possible notation could be to use an arrow head to denote the direction of the association. Note that this is only an example. SOMT does not recommend this convention.

If the choice is to be documented in the SDL model there is a choice of how the designer expresses his/her choice:, either

The result is in both cases the same: The design choice is documented in the combination of the implink (that defines the relation to the analysis model) and in the SDL model (that defines the details of the design choice).

For the one-way/two-way association the Paste-As alternative would imply that the designer interactively entered information as a parameter to the Paste-As of a class to a SDL data type that defined weather an association would be visible in the SDL data type. The association would only be visible if the direction was from the given class (or it was a two-way association).

If the one-way/two-way design choice was to be expressed directly in SDL the designer would manually either include or not include the association in the SDL data type corresponding to the class.

SOMT makes a trade-off between the different alternatives to accomplish an as smooth as possible transition from the analysis model to the design model. The SOMT mechanism is based on the following:

The rationale behind this mechanism is mainly to be able to fulfil the objective of the analysis and design models as good as possible. The analysis model should define the high-level architecture and provide a sound basis for understanding the application as efficiently as possible. To facilitate this the analysis model should be as abstract as possible while still containing all relevant information. The implication is that design decisions that are not needed to understand the application as far as possible should be kept in the design model and that abstractions should be used as much as possible in the analysis.


http://www.ibm.com/rational
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