Initial Node   An initial node is a type of control node which initiates flow in an invoked activity. It has no incoming flows and one or more outgoing flows. The outgoing flows may be guarded with conditions that determine if they will accept tokens. When an activity is invoked tokens are offered to all outgoing flows. There may be zero or more initial nodes in any activity. While other control nodes cannot detain tokens an initial node can retain a token if the guard on an outgoing flow prevents a token from being accepted when the activity starts.

Explanation

An initial node is the typical starting point for an activity. It is recognizable by the fact that it does not have incoming flows. An activity may have more than one initial node allowing flow to start at different points in an activity simultaneously; when the activity is invoked tokens flow from all initial nodes. Flow in an activity can originate in nodes other than initial nodes (Activity Parameter Node for example) so an initial node is not a mandatory element in an activity. In our daily lives we see situations that resemble initial nodes for example: in most games of sport there is a way to bring the ball into play. In soccer the ball is placed in the center of the field and kicked by one of the teams, thereby initializing play.

Important Points

All flows regardless of whether they are incoming or outgoing must be either all control flows or all object flows.

An initial node is a type of control node and therefore can only manage a single type of flow. So all incoming and outgoing flows must all be of a single type: either control flows carrying control tokens, or object flows carrying data or object tokens, but not a mixture of both types of flows.

Related Entries

Activity Final Node  Paired Element,  Graphically Similar An activity final is related to an initial node as they are paired elements. An initial node is the point at which token flow originates and the activity final is one of two types of final nodes where token are destroyed.

Flow Final Node  Graphically Similar,  Confused with A flow final is related to an initial node as they are paired elements. An initial node is the point at which token flow originates and the flow final is one of two types of final nodes where token are destroyed.

Initial  Graphically Similar,  Confused with An initial pseudostate is related to an initial node in that they have the same fundamental shape, a small filled circle. The two elements have very different semantics and are used on different diagrams. They both represent the starting point in the enclosing element but an initial pseudostate is used in state machine diagrams whereas an initial node is used on activity and interaction overview diagrams.