Section 7.4. Use Case Scope

7.4. Use Case Scope

As mentioned previously, a use case is a distinct piece of functionality, meaning it is of sufficient granularity that the user has accomplished his desired goal. Proper scoping of use cases is an art, but UML sets several requirements to make the job a little easier:

• A use case must be initiated by an actor.

• When a use case is considered complete, there are no further inputs or outputs; the desired functionality has been performed, or an error has occurred.

• After a use case has completed, the system is in a state where the use case can be started again, or the system is in an error state.

One popular rule of thumb is to ask yourself if the user can "go to lunch" after completing the use case, meaning that a reasonably sized goal has been achieved by the initiator. For example, Add item to shopping cart is probably not the larger goal a user intends; Purchase item is likely a better scope. Purchase item can consist of adding an item to a shopping cart but typically has more functionality such as logging on, entering billing and shipping information, and confirming the order.

Above all, use cases are intended to convey desired functionality, so the exact scope of a use case may vary depending on the intended audience and purpose for modeling.