Lifeline   A lifeline represents the lifetime or existence of the participant located at the head of the dashed line. Regardless of whether a participant has an execution occurrence, if it has a lifeline then it has been created. Where the lifeline "disappears" beneath the execution occurrence it is assumed to continue to exist. If the participant is created or destroyed during the period of time represented by the diagram the lifeline will start or stop at the designated points.

Explanation

A sequence diagram shows how instances interact or communicate with each other. Instances are said to have a life, meaning that they can be created and destroyed during the operation of the system. The dotted line represents the existence of an instance. It does not imply that the instance is active or communicating but simply that it exists. Commonly instances are 'alive' for the entire period of time represented by the sequence diagram, in which case the lifeline runs for the entire period represented by the time axis of the diagram. An instance can also be created and destroyed by explicit messages, in which case its lifeline starts at the creation point and ends at the destruction point (denoted by a cross).

Related Entries

Message (Instance Creation)  Used Together An instance creation is related to an instance lifeline in the sense that as soon as an instance is created its lifeline is represented by a dotted line that continues until the instance is destroyed or until the end of the lifetime represented by the diagram.