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. A system sequence diagram should be done for the main success scenario of the use case, and frequent or complex alternative scenarios.
A system sequence diagram 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
Read more about System Sequence Diagram: Reading A System Sequence Diagram
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