Sequence diagrams

In some systems, notably communications systems, the timing of things is of primary importance.

For example, if one object expects another to be present for some action, that object should exist and be in an appropriate state. It must not have been destroyed or deactivated.

A sequence diagram is a means of keeping track of the lifetimes of objects and their states, and relating that to required interactions between objects.

UML provides a way of describing graphically the order in which a system of multiple objects are to pass messages to one another.