Guidelines: Activity Diagram in the Use-Case Model

Activity Diagram

An activity diagram in the use-case model illustrates the flow of events of a use case.

The flow of events of a use case describes what needs to be done by the system to provide value to an actor. It consists of a sequence of activities that together produce something for the actor. The flow of events consists of a basic flow, and one or several alternative flows.

The flow of events of a use case can be described graphically with the help of an activity diagram. Such a diagram shows:

• Action states, which represent the execution of an activity or step within the flow of events.
• Transitions between those action states.
• Decisions for which a set of guard conditions are defined. These guard conditions control which transition that follows once the activity an action state represents has been completed.

A simplified activity diagram for the use case Withdraw Money in the use-case model of an automated teller machine (ATM).

Activity diagram is a special case of a state diagram in which all or most of the states are action states and in which all or most of the of the transitions are triggered by completion of actions in the source states.