• It is a graphical representations of workflows of stepwise activities and actions with support for choice, iteration, and concurrency.
• They are intended to model both computational and organizational processes (i.e. workflows).
  • Activity diagrams show the overall flow of control.

• The most important shape types:
  • Rounded rectangles represent actions.
  • Diamonds represent decisions.
  • Bars represent the start (split) or end (join) of concurrent activities.
  • Black circles represent the start (initial state) of the workflow.
  • Encircled black circle represents the end (final state).
• Arrows run from the start towards the end and represent the order in which activities happen.
  • Hence they can be regarded as a form of the flowchart.
• Typical flowchart techniques lack constructs for expressing concurrency.
  • However, the join and split symbols in activity diagrams only resolve this for simple cases.
  • The meaning of the model is not clear when they are arbitrarily combined with decisions or loops.

• The Activity diagram models logic-any logic-from workflow to use cases to methods.
• It borrows most of its notation from flowcharts but has added the concept of concurrency.
• The arrows trace the flow from beginning to end through decisions and loops while identifying each logical step in the process.

Notations

1. Action: A step in the activity, in which the users or software perform some task.
   • The action can start when a token has arrived at all its incoming flows.
2. Control Flow: A connector that shows the flow of control between actions.
3. Initial Node: Indicates the first action or actions in the activity.
4. Activity Final Node: An end to the activity.
5. Decision Node: A conditional branch in a flow.
   • Has one input and two or more outputs.
   • An incoming token emerges on just one of the outputs.
6. Guard: A condition that specifies whether a token can flow along with a connector.
7. Merge Node: Required to merge flows that were split with a decision node.
   • Has two or more inputs and one output.
   • A token on any input emerges on the output.
8. Comment: Provides additional information about elements to which it is linked.