6.2. Definition: What are Actors, Scenarios, and Use Cases?
First, some informal definitions: an actor is something with behavior, such as a person (identified by role), computer system, or organization; for example, a cashier.A scenario is a specific sequence of actions and interactions between actors and the system; it is also called a use case instance . It is one particular story of using a system, or one path through the use case; for example, the scenario of successfully purchasing items with cash, or the scenario of failing to purchase items because of a credit payment denial.Informally then, a use case is a collection of related success and failure scenarios that describe an actor using a system to support a goal. For example, here is a casual format use case with alternate scenarios:
Handle Returns Main Success Scenario : A customer arrives at a checkout with items to return. The cashier uses the POS system to record each returned item …Alternate Scenarios :If the customer paid by credit, and the reimbursement transaction to their credit account is rejected, inform the customer and pay them with cash.If the item identifier is not found in the system, notify the Cashier and suggest manual entry of the identifier code (perhaps it is corrupted).If the system detects failure to communicate with the external accounting system, …
Now that scenarios (use case instances) are defined, an alternate, but similar definition of a use case provided by the RUP will make better sense:
A set of use-case instances, where each instance is a sequence of actions a system performs that yields an observable result of value to a particular actor [RUP].