Use cases and sequence diagrams

[grobledc]

Hi, I would like to know whether its valid to say that "a use case can have a sequence diagram, but a sequence diagram must have a use case". If we have a non-functional requirement not modelled as a use case, causing us to have more than one possible sequence, will this rule still hold true?  :-?

[Oliver F.]

Hi, I would like to know whether its valid to say that "a use case can have a sequence diagram, but a sequence diagram must have a use case". If we have a non-functional requirement not modelled as a use case, causing us to have more than one possible sequence, will this rule still hold true?  :-?

UseCases are describing functional requirements. There is nothing else in the UML spec which defines an alternative. A UseCase usually is the entry point under which a more detailed description can be found by having more UseCases or sequence/activity diagrams.

A functional diagram without a UseCase is an orphan. It tries to describe a functionality which does not officially exist by simply floating around and no idea where to sort it in- it has no identity.

A non-functional requirement does not describe a functionality, hence the name, hence it can not have a sequence below it.
The only thing it can do is to influence exsiting UseCases enforcing the architect to have more paths or more diagrams below an existing UC.

So the rule "a use case can have a sequence diagram, but a sequence diagram must have a use case" can of course be violated but all efforts of a disciplined architect should go into enforcing it.

[bonzo5]

The sequence diagram is the realization of the use case, and use cases capture requirements.  These relationships help to show the traceability of requirements to interaction / behavior.  IDEF0 is equivalent to a use case, although it can capture much more information, especially if the user has full ICOM (Inputs-Controls-Outputs-Mechanisms).

[Paolo F Cantoni]

The sequence diagram is the realization of the use case, and use cases capture requirements.  These relationships help to show the traceability of requirements to interaction / behavior.  IDEF0 is equivalent to a use case, although it can capture much more information, especially if the user has full ICOM (Inputs-Controls-Outputs-Mechanisms).

I think you just need to be careful as there are two kinds of sequence diagrams (I think).  Those that capture a specific scenario and those that represent a union of a number of scenarios.  I believe the latter form is what you're talking about here.

Just my AU$0.05
Paolo