The scenarios of a use case
The written description of a use case: actor, preconditions, trigger, numbered main scenario, alternative and exception extensions attached to a step, postconditions.
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Main success scenario: what is it?
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A numbered sequence of steps, alternating between the actor and the system, in which everything goes well until the goal is reached.
There is one and it has no conditions. Deviations are not slipped into it: they are described apart, attached to the step where they occur.
In “Withdraw cash”, which of these sentences is a precondition?
- The machine is in service
- The machine hands the notes to the customer
- The customer's account is debited by the amount
- The customer enters their PIN
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The machine is in service
A precondition is a state that is true before the use case. Handing over notes and entering the PIN are steps; the debit is a postcondition.
Each extension carries the … of the main scenario where it branches off, for example “3a” for the first variant of step 3.
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Each extension carries the number of the step of the main scenario where it branches off, for example “3a” for the first variant of step 3.
This attachment says exactly where the deviation occurs. Without it, nobody knows what has already happened before the branch.
Alternative scenario: what is it?
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A path that branches off a step of the main path but still leads the actor to the goal, by another route.
Withdrawing without a receipt is still a successful withdrawal. The postcondition is the same as for the main path, only the route differs.
In the description of “Withdraw cash”, which of these sentences is the trigger?
- The machine is in service
- The customer inserts their card
- The customer's account is debited
- The machine offers the amounts
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The customer inserts their card
The trigger is the event that starts the use case, here the customer's action. A machine in service is a state already true beforehand (precondition), the debit a result (postcondition), offering the amounts a later step of the main scenario.