Activations

An activation (also known as a focus of control) is a notation that can appear on a lifeline to indicate the time during which an instance (an actor instance, object, or classifier role) is active. An active instance is performing an action, such as executing an operation or a subordinate operation. The top of the activation represents the time at which the activation begins, and the bottom represents the time at which the activation ends.

For example, in a sequence diagram for a "Place Online Order" interaction, there are lifelines for a ":Cart" object and ":Order" object. An "updateTotal" message points from the ":Order" object to the ":Cart" object. Each lifeline has an activation to indicate how long it is active because of the "updateTotal" message.

Shape

An activation appears as a thin rectangle on a lifeline. You can stack activations to indicate nested stack frames in a calling sequence.

Using Activations

Activations can appear on your sequence diagrams to represent the following:

On lifelines depicting instances (actors, classifier roles, or objects), an activation typically appears as the result of a message to indicate the time during which an instance is active.
On lifelines involved in complex interactions, nested activations (also known as stacked activations or nested focuses of control) are displayed to indicate nested stack frames in a calling sequence, such as those that happen during recursive calls.
On lifelines depicting concurrent operations, the entire lifeline may appear as an activation (thin rectangles) instead of dashed lines.

Naming Conventions

An activation is usually identified by the incoming message that initiates it. However, you may add text labels that identify activations either next to the activation or in the left margin of the diagram.