Join or fork nodes

Activity diagrams can include join nodes or fork nodes. A join node or fork node depicts either a join operation or a fork operation. You can draw join or fork nodes in activity diagrams for objects, use cases, and operations.

The product defines activity diagrams as meaningful only if join and fork nodes are well-structured in the same sense as well-structured parentheses. In other words, they must use proper nesting. The only exception to this rule is that a join or fork node with multiple ingoing/outgoing control flows can be used in place of a number of join or fork nodes with only two ingoing or outgoing control flows each.

The product tolerates less-than-meaningful activity charts if they can be extended into meaningful ones by adding control flows (for example, a fork with control flows that never merge back).

As you draw activity diagrams, the product prevents you from drawing constructs that violate the meaningfulness of the activity diagram by displaying a "no entry" symbol.


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