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Activity Diagrams
Activity diagrams are used to document workflows in a system, from the business
level down to the operational level. When looking at an Activity diagram,
you'll notice elements from State diagrams. In
fact, the Activity diagram is a variation of the state diagram where the
"states" represent operations, and the transitions represent the activities
that happen when the operation is complete. The general purpose of Activity
diagrams is to focus on flows driven by internal processing vs. external
events.
Notation
| Activity States |
Activity states mark an action by an object. The notation for these states are
rounded rectangles, the same notation as found in Statechart diagrams. |
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| Transition |
When an Activity State is completed, processing moves to another Activity
State. Transitions are used to mark this movement. Transitions are modeled
using arrows. |
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| Swimlane |
Swimlanes divide activities according to objects by arranging objects in column
format and placing activities by that object within that column. Objects are
listed at the top of the column, and vertical bars separate the columns to form
the swimlanes.
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| Initial State |
The Initial State marks the entry point and the initial Activity State. The
notation for the Initial State is the same as in Statechart diagrams, a solid
circle. There can only be one Initial State on a diagram. |
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| Final State |
Final States mark the end of the modeled workflow. There can be multiple Final
States on a diagram, and these states are modeled using a solid circle
surrounded by another circle.
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| Synchronization Bar |
Activities often can be done in parallel. To split processing ("fork"), or to
resume processing when multiple activites have been completed ("join"),
Synchronization Bars are used. These are modeled as solid rectangles, with
multiple transitions going in and/or out. |
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Example
Consider the following example diagram from the UML 1.4 Specification. The
Activity Diagram begins with a call to Request Service, the Initial State. Upon
completion of this operation, a synchronization bar is used to indicate
parallel processing, where the customer pays, and the sales and stockroom staff
take and fill the customer's order simultaneously. Another synchronization bar
is used to indicate that when the customer has paid and the order has been
filled, then the order is ready for delivery. After the delivery of the order,
the customer can collect the order, and the process is complete. This is marked
with the Final State icon.
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