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SysML Activity Diagram

A SysML Activity diagram is an extension of the UML Activity diagram. The Activity diagram is a powerful tool for representing the sequence of Actions that describe the behavior of a Block or other structural element; the sequence is defined using Control Flows. Actions can contain Input and Output Pins that act as buffers for items that flow from one Action to another as the task carried out by the Action either consumes or produces them. The items can be physical materials, energy, power, data, information, or anything else that can be produced, conveyed or consumed, depending on the system and the activity being described.

Activity Diagrams can be used to define situations where parallel processing occurs in the execution of some activities. Activity diagrams are useful for engineering modeling, where they detail the processes involved in system activities.

This is an example of an Activity diagram.

This is a simplified model of driving and braking in a car that has an automatic braking system. Turning the key on has a duration constraint specifying that this action lasts no more than 0.1 seconds. Turning the key on starts two behaviors, Driving and Braking.

The SysML Activity diagram is based on the UML Activity diagram, but additional semantics have been added in two areas:

  • Continuous Flow, allowing restrictions on the rate at which entities flow along edges in an Activity, and providing mechanisms to ensure that the most recent information is available to Actions
  • Probability, introduced into Activities to include the likelihood that a value will be available to an edge or output on a parameter set


The main elements that can appear in Activity diagrams are:

  • Activity
  • Structured Activity
  • Action (various kinds)
  • Action Pin
  • Partition
  • Control Operator
  • Parameter (various kinds)
  • Object Node
  • Central Buffer Node
  • DataStore
  • Decision
  • Merge
  • Synch
  • Initial
  • Final
  • Flow Final
  • Region
  • Exception
  • Fork/Join


The main connectors  that can appear in Activity diagrams are:

  • Control Flow
  • Object Flow
  • Interrupt Flow
  • Dependency


When creating an Activity diagram as a child of an Activity:

  • Where the Activity contains Activity Parameters, on creating the child Activity diagram, these parameters are generated on to the Diagram Frame
  • Right-clicking on the diagram and selecting 'Synchronize Structural Elements' brings onto the diagram any missing Activity Parameters that might have been added later in the parent Activity

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