IBM SG24-7368-00 manual

Therefore, in the discussion that follows, much of what is said about use cases also applies to operations.

MDSD Step 5: Finding use cases

Identifying use cases is an important step in this process, and is, at the same time simple, profound, and vitally important. Use cases form the basis from which the dynamic part of the MDSD model is derived. What we are seeking to do here is to identify the complete set of planned usages of an entity when the entity is treated as a black box. This is probably the hardest part—staying to a black-box perspective of an entity. Often those doing this kind of modeling have deep knowledge of the internals of the particular entity and it is a challenge to keep the focus at a high level. We often find ourselves reassuring such teams that we will get to work on the lower-level interactions—which interest them far more—soon enough and emphasize that the purpose is to derive the lower level interactions from an analysis of the fundamental usages of the higher level entity—its reason for being. By keeping to this high level focus, the higher level use cases can be developed more quickly and the lower levels developed in due time.

Finding use cases involves stepping back and looking at the entity as a black box, and asking, how do these actors we have identified interact with the entity? What are the complete entity usages? What are the major results of value produced by the entity? When we next expand the entity to its white-box view, we will be asking the same questions of the subentities. In the case of an enterprise, we will look at the system and its actors in the white-box expansion.

What do you use your car for?

We often use this illustration in our MDSD courses to help people understand use cases. If I ask a group what they use their cars for, the first response will likely be, to get from point A to point B. I ask them where those places are because I have never seen them on a map. I also ask them if they wake up in the morning and say to themselves, today I want to get from point A to point B. They laugh and realize that getting from point A to point B is not a real usage. It is too vague. So I ask them to forget that they know anything about use cases or computers for that matter, and just answer the question: What do you use your car for?

With some thought, we come up with a number of complete usages of the car, such as:

￿Commute to work (and back home)

￿Go shopping

￿Go on vacation

￿Take the kids to school

￿Travel to a remote bike ride

48Model Driven Systems Development with Rational Products