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- A use case is a typical interaction that a user has with the system in order to achieve a goal.
- A use case does two things:
- Describe how an actor achieves a goal using the system. This is described in terms of scenarios.
- Ensures interests of stakeholder are protected by the system.
- Use cases drive the whole development process.
- Use cases provide the basis of communication between the customers and developers in planning a project.
- e.g. deciding which use cases to include in the current release.
- Use cases can be used to capture the systems functional requirements.
- An actor is someone or something external to the system and who interacts with the system.
- A scenario is a description of a sequence of actions that is a way of achieving a goal.
- The main success scenario is the usual/main/straightforward way of achieving the goal.
- All scenarios have (Note: UML does not specify any standard, this is the York recommendation)
- A precondition : required condition before starting.
- Interaction steps
- Postcondition :
- Guaranteed state of world once use case exits. successful precondition holds for successful exit.
- A minimal postcondition: Something that guarantees to protect stakeholder interests.
- For each step of main success scenario, think of:
- Secondary Scenarios:
- Alternative Scenario: A different way of achieving success (the goal)
- Exception Scenario: Something that can go wrong - and can lead to a different postcondition.
- The scenarios must be kept free of design assumptions.
- A use case is therefore is a set of scenarios tied together by a common user goal.
- ... and each scenario defines one way the goal is or is not achieved.
Structuring Use Cases
- A step in the scenario of a use case that is detailed - and might be another use case by itself.
- The main use case "calls" the inclusion use case.
- For e.g. use case for "Edit Document" - might have a step called "Modifies Text" which might be a use case by itself.
- Use when one use case is similar to another use case, but does a little bit more.
- For e.g. "Draw Square" can be a specialized use case of "Draw Rectangle".
- Sort of an interruption to to the Main use case.
- For e.g. if the main use case is "Check Balance" and if the balance > 5000$, the "Enrol as privilege customer" use case can kick in.
- The point at which the extension use case kicks in is the extension point. The extension points have to be declared in the base use case.
- Note that the main use case has no control over the special use case.
Use Case Diagrams
- Is a summary of the functionality of the whole system.
- Gives an overview of the relationships between all the actors and all the use cases.
- Not meant to record details of use cases - for that text is best, but rather as a sort of visual guide to the various use cases in the system.