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Now, let's look at use cases in a little more detail.

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And start by defining exactly what an actor is. An actor

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represents an entity, which can be a human or a device,

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that plays a role within my system, so that interacts with my

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system. It's some entity from the outside world, with respect to

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my system, that interacts with my system. It is important to

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clarify that an entity can play more than one role. For

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example, you might have somebody working in a bank that can be

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both an employee of the bank, or a customer of

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the bank, depending on how it interacts with the banking

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system. And, obviously, more than one entity can play the

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same role. Using the same example, we can have both an

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employee of the bank and just a regular customer, playing

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the role of the customer. So again, it all depends on

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what the entity does, how the entity interacts with the

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system, what kind of functionality of the system the entity uses.

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And finally, actors may appear in more than one use case.

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So it's fairly normal for the same actor to interact with

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the system in different ways. And therefore, to appear in more

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than one use case. Just think about the use cases in

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scenarios of usage. If the same actor can interact with the

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system in different ways, that actor will appear in multiple use

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cases. Now let's go back to the description of our course

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management system, and see how we can identify actors in the system.

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And as we did for the class diagram before, I encourage

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you to stop the video and try to identify the actors

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in the system yourself, before I do it.

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If we look at the description, we can see that, for example, the Registration

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Manager is clearly an actor for the system. Students are actors

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for the system. Professors are actors for the system. And notice

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that we're not doing the same thing that we were doing

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when identifying classes. Here we're identifying

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entities that are from the outside

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world, and have an active role in interacting with my

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system. Again, Registration Manager, that we will just call registrar for

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simplicity, students, and professors. So once we have identified the

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actors for our example, we can simply draw them, using the

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notation that we just introduced. So we have the registrar,

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and notice how for every actor we clarify the role that

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the actor plays. We have the professor, and we have

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the student. So here, these are the three actors that we

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identified for our system.