What is an Activity Diagram?
An activity diagram is a type of UML (Unified Modeling Language) diagram used to model the dynamic aspects of a system. It focuses on the flow of control or data from one activity to another.
It is especially useful for describing business processes, workflow, and the logic of complex operations.
Key Characteristics:
- It’s a special form of a statechart diagram, where:
- Each state represents an action or activity.
- Transitions occur when the activity (action) is completed.
- No events are required to trigger transitions — just completion of the current action.
Core Components:
| Element | Description |
|---|---|
| Action/Activity States | Represent steps or tasks being performed. |
| Transitions | Arrows showing movement from one activity to another upon completion. |
| Start (Initial) Node | Black circle showing where the flow begins. |
| End (Final) Node | Bullseye symbol representing the end of the flow. |
| Decision Nodes | Diamonds used for branching based on conditions (like if statements). |
| Merge Nodes | Combine multiple flows into one. |
| Fork/Join Nodes | Represent parallel processing (concurrent flows). |
Example Use Cases:
- Modelling a user registration process in an app.
- Describing a purchase order approval workflow.
- Defining the control logic in an embedded system.
Comparison with Other Diagrams:
| Diagram Type | Focus | Activity Diagram Differences |
|---|---|---|
| Statechart Diagram | States and events | Activity diagram is limited to action states and uses action completion as the trigger. |
| Flowchart | General logic flow | Activity diagrams are more formal and part of UML with support for concurrency and object flows. |
| Sequence Diagram | Time-ordered interactions | Activity diagrams model control flow, not message exchange between objects. |
Why Use Activity Diagrams?
- Easy to understand.
- Good for stakeholder communication.
- Helps in identifying process improvements.
- Bridges the gap between high-level design and detailed implementation.