Use case notation for login

Question

When a login is needed in a system to perfom any action. How do you express that in a Use Case Diagram?

I have a system that will allow employees to perform sells. They must be logged in, but once they are, it is not necesary to log again until the session ends (typical login).

I do not know how can I express that that login is compulsory, but only once.

Answer 1

If the login is part of the system you are creating it should be a use case in itself that is listed as a prerequisite for other use cases. if you are using an outside system (i.e. Windows authentication) you could simply list that the user has authenticated through windows authentication or whatever system you plan to use. Its generally good practice to always have a precondition and post condition sections of your use case.

Answer 2

Use Cases form a proper class hierarchy.

Must-be-logged-in Use case can be a super-class of the rest of your use cases.

Some folks like to use <<extends>> to show that a use case extends the login use case.

See this diagram: http://yuml.me/diagram/scruffy/usecase/draw