Summary

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System Behavior is documented in a use case model that illustrates the system's intended functions (use cases), its surroundings (actors), and the relationships between the use cases and actors (use case diagrams). The most important role of a use case model is to communicate the system's functionality and behavior to the customer or end user .

The use case model starts in the Inception Phase with the identification of the actors and the principal use cases for the system. The model is then matured in the Elaboration Phase.

Actors are not part of the system ”they represent anyone or anything that must interact with the system under development. Use cases represent the functionality provided by the system. They model a dialogue between an actor and the system.

Each use case contains a flow of events, which is a description of the events needed to accomplish the use case functionality. The flow of events is written in terms of what the system should do, not how the system does it. A use case diagram is a graphical representation of some or all of the actors, use cases, and their interactions for a system.

Two popular types of use case relationships are include and extend. An include relationship is drawn to show functionality that is shared by several use cases; an extend relationship depicts optional behavior of a use case.

Activity diagrams represent the dynamics of the system. They are flow charts that are used to show the workflow of a system. At this point in the life cycle, activity diagrams may be created to represent the flow across use cases or they may be created to represent the flow within a particular use case. Later in the life cycle, activity diagrams may be created to show the workflow for an operation.

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