Intro

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.

Actor

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.

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.

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.