UML sequence diagrams are a dynamic modeling technique, as are UML communication diagrams. UML sequence diagrams are typically used to

1. ■ Validate and flesh out the logic and completeness of a usage scenario. A usage scenario is exactly what its name indicates—the description of a way that your system could be used. The logic of a usage scenario may be part of a use case, perhaps an alternate course; one entire pass through a use case, such as the logic described by the basic course of action or a portion of the basic course of action plus one or more alternate scenarios; or a pass through the logic contained in several use cases, such as when a student enrolls in the university and then immediately enrolls in three seminars.

2. ■ Explore your design because they provide a way for you to visually step through invocation of the operations defined by your classes.

3. ■ Give you a feel for which classes in your application are going to be complex, which in turn is an indication that you may need to draw state machine diagrams for those classes.

4. ■ Detect bottlenecks within an object-oriented design. By looking at what messages are being sent to an object, and by looking at roughly how long it takes to run the invoked method, you quickly get an understanding of where you need to change your design to distribute the load within your system. Naturally, you will still want to gather telemetry data from a profiling tool to detect the exact locations of your bottlenecks.