Use Case
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Questions answered by a use case
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who are the primary and secondary actors?
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what are the actor's goals?
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what preconditions should exist before the scenario begins?
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what main tasks or functions are performed by the actor?
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what exceptions might be considered?
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what variations in the actor's interaction are possible?
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what system information will the actor acquire, produce, or change?
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does the actor need to inform the system about changes in the external environment?
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what information does the actor desire from the system?
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does the actor wish to be informed about unexpected changes?
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Kinds of actor
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primary actor — has a user goal fulfilled by the system (e.g. the cashier); used to discover use cases, since use cases exist to satisfy primary actors' goals
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supporting actor — provides a service to the system (often another computer system, e.g. a payment authorization service); used to clarify external interfaces and protocols
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offstage actor — has an interest in the use case's outcome without directly participating (e.g. a government tax agency); ensures all necessary interests are captured
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