About UML sequence diagrams

A type of interaction diagram, a sequence diagram shows the actors or objects participating in an interaction and the events they generate arranged in a time sequence. Often, a sequence diagram shows the events that result from a particular instance of a use case but a sequence diagram can also exist in a more generic form.

The vertical dimension in a sequence diagram represents time, with time proceeding down the page. The horizontal dimension represents different actors or objects.

Sequence diagram displaying how objects interact through time

The vertical dimension in a sequence diagram represents time, with time proceeding down the page. The horizontal dimension represents different actors or objects.

Callout 1 Objects are represented as object lifelines, that is, in terms of their existence at a particular time. Adjust line length to indicate an object's creation or destruction.

Callout 2 Arrows represent messages between objects. An object can also send a message to itself.

Callout 3 Add an activation (also called a focus of control) to a lifeline to indicate the time period during which an object is performing an action.

Callout 4 A return message is shown as a dashed line.

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