Guidelines: Activity Diagram in the Use-Case Model
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:
- Activity states, which represent the performance of an
activity or step within the flow of events.
- Transitions
that show what activity state follows after another. This type of transition
is sometimes referred to as a completion transition, since it differs from a
transition in that it does not require an explicit trigger event, it is
triggered by the completion of the activity the activity state represents.
- Decisions
for which a set of guard conditions are defined. These
guard conditions control which transition (of a set of alternative transitions)
follows once the activity has been completed. Decisions and guard conditions
allow you to show alternative threads in the flow of
events of a use case.
- Synchronization bars which you can use to show parallel
subflows. Synchronization bars allow you to show concurrent threads
in the flow of events of a use case.
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 statechart diagram in which all or
most of the states are activity states and in which all or most of the of the
transitions are triggered by completion of actions in the source states. For
more details on activity diagrams, see Guidelines:
Activity Diagram in the Business Use-Case Model.
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