Sequence diagram

In software engineering, a sequence diagram^[1] shows process interactions arranged in time sequence. This diagram depicts the processes and objects involved and the sequence of messages exchanged as needed to carry out the functionality. Sequence diagrams are typically associated with use case realizations in the 4+1 architectural view model of the system under development. Sequence diagrams are sometimes called event diagrams or event scenarios.

For a particular scenario of a use case, the diagrams show the events that external actors generate, their order, and possible inter-system events.^[2] The diagram emphasizes events that cross the system boundary from actors to systems. A system sequence diagram should be done for the main success scenario of the use case, and frequent or complex alternative scenarios.

There are two kinds of sequence diagrams:

• Sequence Diagram (SD): A regular version of sequence diagram describes how the system operates, and every object within a system is described specifically.
• System Sequence Diagram (SSD): All systems are treated as a black box, where all classes owned by the system are not depicted. Instead, only an object named System is depicted.