System Sequence Diagram

In software engineering, a System Sequence Diagram (SSD) is a sequence diagram that shows, for a particular scenario of a use case, the events that external actors generate, their order, and possible inter-system events.

System sequence diagrams are visual summaries of the individual use cases.

All systems are treated as a black box; the diagram places emphasis on events that cross the system boundary from actors to systems. An SSD should be done for the main success scenario of the use case, and frequent or complex alternative scenarios. An SSD should specify and show the following:

* External actors
* Messages (methods) invoked by these actors
* Return values (if any) associated with previous messages
* Indication of any loops or iteration area