Use Case Diagrams

As I said earlier, the UML is silent on the content of a use case but does provide a diagram format for showing them, as in Figure 9.2. Although the diagram is sometimes useful, it isn't mandatory. In your use case work, don't put too much effort into the diagram. Instead, concentrate on the textual content of the use cases.

The best way to think of a use case diagram is that it's a graphical table of contents for the use case set. It's also similar to the context diagram used in structured methods, as it shows the system boundary and the interactions with the outside world. The use case diagram shows the actors, the use cases, and the relationships between them:

• Which actors carry out which use cases

• Which use cases include other use cases

The UML includes other relationships between use cases beyond the simple includes, such as «extend». I strongly suggest that you ignore them. I've seen too many situations in which teams can get terribly hung up on when to use different use case relationships, and such energy is wasted. Instead, concentrate on the textual description of a use case; that's where the real value of the technique lies.