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Activity Diagrams

The Activity diagram is the most important behavior diagram and can be used to model flow (discrete or continuous) based behavior where inputs are converted to outputs by traversing a sequence of actions that perform work on the items. They are analogous to the common flow chart diagram but have more sophisticated semantics and also allow Activities and Actions to be related to elements such as Blocks, Requirements and Use Cases.

The Actions that appear on the Activity diagrams can contain input or output pins that represent the  interaction points where inputs are fed into an action and outputs are emitted.


The main elements that can appear in Activity diagrams are:

  • Activity
  • Action (Various kinds)
  • Action Pin
  • Partition
  • Object Node
  • Central Buffer Node
  • DataStore
  • Decision
  • Merge
  • Initial
  • Final

The main connectors  that can appear in Activity diagrams are:

  • Control Flow
  • Object Flow
  • Interrupt Flow
  • Dependency


A variety of tools can be used with behavioral modeling and Activity diagrams, including:

  • Diagram Filters - which allows a user to filter elements out of the diagram to achieve a more specific focus,
  • Pan and Zoom - which allows a modeler or viewer to easily move around large diagrams,
  • Spreadsheet (CSV) Import and Export - which allows content in spreadsheets to be imported or exported from the model,
  • Documentation - which allows formal or informal documentation to be generated from the model in a variety of formats,
  • Traceability - which provides a hierarchical view of an element's relationships to other model elements,
  • Responsibilities Window - which provides a composite view of the important responsibilities of an element, including Constraints, Requirements and Scenarios
  • Relationship Matrix - which allows the connections between Activities (or other elements) and other elements such as Blocks, Requirements and Use Cases to be visualized in a matrix.


The Activity diagram can be used to model flow based behavior and is similar to the widely-available Flow Chart or Functional Flow diagrams that had been used extensively before the SysML specification was devised. They are typically used to show the way parts of the system behave, including the input and output of items and signals.