In sequence diagrams, a lifeline represents a connectable element
of the enclosing classifier, including inherited properties or parameters.
Each instance in an interaction is represented by a lifeline.
As the following figure illustrates, a lifeline consists of a name in a rectangle, which is called the head, on top of a vertical dashed line, which is called the stem, in an interaction frame.
The instance name and type appears in the head of the lifeline. The stem represents the instance that the lifeline represents. Messages, which are sent and received by the instance, appear on the lifeline in a chronological order.
As the following table illustrates, lifelines can indicate several actions in sequence diagrams.
| Behavior | Description |
|---|---|
| Creation | You can create an instance during the interaction by using a create message. The "created" lifeline repositions itself at the level of the create message. Otherwise, the lifeline can start at the top of the diagram to indicate that it existed before the interaction. |
| Communication | You indicate messages with arrows between instances. The arrow originates from the source lifeline that sends it and ends at the target lifeline that terminates it. |
| Execution | An execution specification shows the time during which an instance is active, either performing an operation directly or through a subordinate operation. |
| Destruction | If you destroy an instance during the interaction with a destroy message or a stop node, its stem ends at the level of the stop node. Otherwise, its lifeline extends beyond the final message to indicate that it exists during the entire interaction. |