Control nodes

In activity diagrams, a control node is an abstract activity node that coordinates the flow of control in an activity.

The following table describes the types of control nodes that you can use in activity diagrams.

Control node type Description Icon
Initial This node marks the point at which flow begins when the activity is invoked. An activity can have several initial nodes. A solid circle is displayed.
Decision This node represents a point in an activity where a single incoming edge branches into several outgoing edges. You typically use constraints, also called guard conditions, on the outgoing edges to determine which edge should be followed. An arrow points to the top of a diamond and two arrows point away from the bottom of the diamond.
Merge This node represents a point in an activity where several incoming edges come together into a single outgoing edge. Two arrows point to the top of a diamond and one arrow points away from the bottom of the diamond.
Fork This node represents a point in an activity where a single incoming flow is split into several outgoing flows. One arrow points to a vertical bar from the left and two arrows point away from it to the right.
Join This node represents a point in an activity where several incoming flows are synchronized into a single outgoing flow. Two arrows point to a vertical bar from the left and one arrow points away from it to the right.
Flow final This node represents a point where one flow in an activity terminates, but does not affect the other flows in the activity. A hollow circle contains an X.
Activity final This node represents a point where all flows in an activity terminate. An activity can have several activity final nodes. When the first activity final node is encountered, all flows stop and the activity terminates. A large hollow circle contains a smaller solid circle.

Parent topic: Activity diagrams

Related concepts
Activities
Structured activities
Actions
Call behaviors
Call operations
Object nodes
Activity edges

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