Fork/Join

    

The Fork/Join elements can be used to:

  • Fork or split the flow into a number of concurrent flows
  • Join the flow of a number of concurrent flows
  • Both join and fork a number of incoming flows to a number of outgoing flows

These elements are used in both Activity and State Machine diagrams, in either vertical or horizontal orientation. With respect to State Machine diagrams, Forks and Joins are used as pseudo-states. Other pseudo-states include history states, entry points and exit points. Forks are used to split an incoming transition into concurrent multiple transitions leading to different target states. Joins are used to merge concurrent multiple transitions into a single transition leading to a single target. They are semantic inverses. To learn more about these individual elements see their specific topics.

Example Diagrams

Description

Diagram

Fork or split the flow into a number of concurrent flows

Join the flow of a number of concurrent flows

Join and Fork a number of incoming flows to a number of outgoing flows

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OMG UML Specification:

Fork

The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 376) states:

A fork node is a control node that splits a flow into multiple concurrent flows ... A fork node has one incoming edge and multiple outgoing edges.

Join

The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 381-382) states:

A join node is a control node that synchronizes multiple flows ... A join node has multiple incoming edges and one outgoing edge.