Fork and join

How to read it

UML 2.5.1 describes the symbol for fork and join nodes as simply a line segment, in any orientation; course notes and the specification's own state machine notation draw it as a short heavy bar. A fork has one incoming arrow and starts every outgoing path at once: whatever arrives is offered to all of them. The University of Pretoria's notes describe the paths after a fork as running at the same time, in programming terms as separate threads. A join synchronizes: unless it is labelled otherwise, it waits until a flow has arrived on every incoming arrow, then sends one flow on. A label such as {joinSpec = A and B} beside a join gives a different condition. One bar can be a join and a fork at once, with several arrows in and several out.

Also searched as: fork and join symbol, fork node, join node, synchronization bar, uml fork and join.

Fork or decision?

A fork starts every outgoing path; a decision diamond picks one of them. The specification's example has a fork after Fill Order passing control to both Ship Order and Send Invoice. Scott Ambler, in The Object Primer, marks a decision drawn where a fork was meant as a common mistake in one of his own diagrams.

Join or merge?

A join waits for all its incoming flows; a merge, drawn as a diamond, passes each one on as it arrives. Ambler's description of a join: "All flows going into the join must reach it before processing may continue."

In state machine diagrams

Fork and join pseudostates use the same short heavy bar. A fork splits one transition into transitions to states in parallel (orthogonal) regions of a composite state; a join brings them back, and all its incoming transitions have to complete before the one going out can fire.