The system sequence diagram
For one scenario of a use case, the events the actor sends to the system seen as a black box, its replies, and the alt, opt and loop fragments.
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System sequence diagram: what is it?
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A diagram that shows, for one scenario of a use case, the messages exchanged in order between the actor and the software taken as a whole.
It holds a single lifeline for the system: it describes what the system receives and answers, not the objects working inside it.
In a system sequence diagram, which of these has no place?
- The VAT calculation by an internal object
- A system event such as
validateCart() - The total displayed in reply to the customer
- A loop over each scanned item
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The VAT calculation by an internal object
A calculation done by an internal object is a design detail: the system is a black box. Only what the actor sends, what the system answers, and the structure of the dialogue (loops, choices) appear.
A system sequence diagram treats the software as a …: it shows no internal object, only the messages coming in and going out.
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A system sequence diagram treats the software as a black box: it shows no internal object, only the messages coming in and going out.
The internal structure is decided in design. In analysis, the diagram serves to check that the dialogue between the actor and the system is complete.
System event: what is it?
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A message an actor sends to the software to ask it for something: it triggers one of the system's operations.
It is named like a call, with its parameters:
identify(card),recordLoan(book). Each system event becomes an operation to implement.