Initial node

How to read it

UML 2.5.1 draws an initial node as a solid circle. It is the starting point of an activity: when the activity begins, control starts at the initial node and leaves along its outgoing arrows, and no arrow ever comes into it. An activity may have more than one initial node, and then each starts a flow of its own, running at the same time as the others. In a state machine diagram the small filled circle is the initial pseudostate, the starting point of a region, with a single transition to the state the region starts in. The University of Pretoria's course notes call it the starting point of the flow shown in an activity diagram.

Also searched as: initial node symbol, initial state, start node, uml initial node.

How many start dots?

The specification allows several initial nodes in one activity, while a region of a state machine has at most one initial pseudostate. Scott Ambler, in The Object Primer, notes that an initial node isn't required, though it makes a diagram much easier to read. The University of Pretoria's 2015 course notes say an activity diagram must have exactly one initial node, which is stricter than UML 2.5.1.

Look-alikes

The same dot inside a ring is a Final node, the end of the flow. In a sequence diagram, a small filled circle at the start of a message arrow marks a found Synchronous and return messages, one whose sender isn't shown. On a flowchart the start is a rounded terminal shape instead.