12. Sequence diagram

We have discussed class, object and use case diagrams. Another important type of UML diagram is called the 'sequence diagram'. A sequence diagram shows the dynamic aspect of the system. It shows the interactions between objects, and as importantly, it shows the order in which they happen (unlike use case).

The sequence diagram runs horizontally to signify objects and it runs vertically to show time sequence. Like this:

UML specifies a number of graphical elements to depict different actions. Some are shown below (there are quite a few others as well)

Actor. An actor is a person, organisation or any other entity that is not part of the system but they do trigger an use case within the system
Object. A discrete part of the system
Action. An activity or process undertaken by an actor or object
Message. A communication between objects
Return message. A response from an object

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