Lecture - 4 - Software Engineering II Fall 2009 Click to...

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Click to edit Master subtitle style Software Engineering - II Fall 2009 Week # 3
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Use Cases Write Use Cases to discover and record functional requirements. A Use Case is a collection of related success and failure scenarios that describe actors using a system to fulfill (or support) a goal. A scenario is a specific sequence of actions and interactions between actors and the system under discussion. The most important Use Cases are those for EBPs (Elementary Business Processes).
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Use Cases (cont'd) The name of a Use Case should be the main goal of the Use Case. It should start with a verb. Each step in a scenario should be one of: An interaction between actors. A validation of data. A state change of the system. Steps are written in present tense .
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Actors An actor is something with a behavior (including the system!). Actors are roles played not only by people, but by organizations, software, and machines Each use case is written in terms of its primary actor. There are also supporting actors and offstage actors. Primary actor: identified to find user goals, which drive the use cases Supporting actor: identified to clarify external interfaces and protocols. Offstage actor: identified to ensure that all necessary interests are satisfied.
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Use Case styles Three styles of Use Cases: Brief -- a one-paragraph description of the main scenario When? During early requirements analysis, to get the quick sense of subject and scope Casual -- multiple paragraphs, no numbering When? As Above Fully dressed -- formal, detailed, with numbered paragraphs
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Use case elements A fully-dressed Use Case will include the following elements: Primary actor Calls on the system to deliver its services Stakeholders and interests Who cares about this use case Preconditions What must be true on start Success Guarantee or Postconditions What must be true on successful completion Main scenario (the "success" scenario) A typical, unconditional happy path scenario of success
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