SysML FAQ: What is an Activity diagram (ACT) and how is it used?

What is a SysML Activity diagram?

Definitions

Activity: An Activity (notation: rounded-rectangle or "roundangle") represents a flow of functional behaviors that may include optional Object (data) Flows. Control and Object Flows can be sequential (default) or parallel (indicated by Fork & Join Nodes) depending upon conditions.

• Action = atomic Activity, which is a primitive executable behavior.
• Control Flow = flow of functional behaviors
• Object Flow = data flow of object inputs/outputs into/from an Activity or Action.

Activity diagram (act): An Activity diagram shows system dynamic behavior using a combined Control Flow and Object (data) Flow model.

• Activities (and indirectly Activity diagrams) can be recursively decomposed ("nested") by alternating between Activity definitions and Call Behavior Action usages (See Usage Notes below.)
• Activities and Actions can be Allocated via Partitions that represent Control Blocks (i.e., Blocks that represent the System, Subsystems, Sub-Subsystems, ..., atomic structures);
• Data Blocks (i.e., Blocks that represent Persistent Data Stores) and Signals that contain Data Blocks can be Allocated to Activity Parameters and Action Pins;
• compare and contrast: SA/SD DFDs, FFBDs, EFFBDs,IDEF0; BPMN BPDs.

Purpose

The purpose of Activity diagrams is to specify dynamic system behaviors that Satisfy («satisfy» Dependency) system Functional Requirements using both Control and Object (data) Flows. When properly applied (See Usage Notes below), Activity diagrams are recursively scalable and simulatable.