Control Flow   A control flow is a directed connection (flow) between two activity nodes. As soon as the activity node at the source (tail) end of the flow is finished it presents tokens to the control flow at the target (arrowhead) end of the flow. A control flow can only carry control tokens; it cannot carry object (data) tokens. There are rules that determine whether tokens can flow along the control flow and these are determined by the type of activity nodes at the source and target of the flow. In the case of complete activities a control flow may define a weight, which specifies the minimum number of tokens that must flow along the control flow as a group.

Explanation

A control flow is a like a conduit that transports control tokens through an activity. It is a directed connection and tokens can only flow in the direction of the arrow. It connects activity nodes and can thus have any one of a number of nodes at its tail or arrowhead end including: actions, join nodes, forks nodes and decision nodes. A number of different factors determine whether tokens will flow along the control flow. The most important factor is whether the activity node at the tail end is finished its task and presents a token to the flow. The control flow can in some circumstances be guarded and the evaluation of this guard will determine if tokens are accepted by the flow. For complete activities a weight can be specified that determines the minimum number of tokens that are required for a group and no tokens will flow until this number is reached. There is typically no delay in the transmission of control tokens by the flow, if they are accepted by the flow they are available instantaneously to the target activity node at the arrowhead end of the flow.

Important Points

A control flow can only transport control tokens.

There are two types of tokens that can flow through an activity: control and object tokens. Only control tokens can be transmitted by control flows. Object tokens, which contain objects or data, are carried by object flows.

Related Entries

Action  Used Together An activity is related to an activity transition is the sense that activities are connected by transitions.

Object Flow  Sibling,  Confused with,  Graphically Similar An object flow is related to an control flow in that they are both types of activity edges. An object flow carries object tokens that transport data and objects in an activity, whereas control flow carries control tokens that coordinate the actions in an activity.