Lifelines

A lifeline is a notation that represents the existence of an object or classifier role over a period of time. Lifelines appear only in sequence diagrams, where they show how each instance (object or classifier role) participates in the interaction.

For example, a "Place Online Order" interaction in an e-commerce application includes a number of lifelines in a sequence diagram, including lifelines for a ":Cart" object, ":OnlineOrder" object, and ":CheckoutCart" object. As the interaction is developed, stimuli are added between the lifelines.

Shape

A lifeline appears as a vertical dashed line in a sequence diagram.


Lifeline for an Object	Lifeline for a Classifier Role

Using Lifelines

When a classifier role or object appears in a sequence diagram, it will automatically have a lifeline. Lifelines indicate the following:

Creation – If an instance is created during the interaction, its lifeline starts at the level of the message or stimulus that creates it; otherwise, its lifeline starts at the top of the diagram to indicate that it existed prior to the interaction.
Communication – Messages or stimuli between instances are illustrated with arrows. A message or stimulus is drawn with its end on the lifeline of the instance that sends it and its arrowhead on the lifeline of the instance that receives it.
Activity – The time during which an instance is active (either executing an operation directly or through a subordinate operation) can be shown with activations.
Destruction – If an instance is destroyed during the interaction, its lifeline ends at the level of the message or stimulus that destroys it, and a destruction marker appears; otherwise, its lifeline extends beyond the final message or stimulus to indicate that it exists during the entire interaction.

Naming Conventions

A lifeline has the name of an object or classifier role. For details, see Objects or Classifier Roles.