Subtitles of the Movie

Let's revisit the use case diagram we saw in an earlier movie so that we can see how all of these elements and relationships work together to give us a snapshot of what a system or some part of a system can do. So here we have in the rectangle our system and up at the top we have the title, it's an online reservation system. Interacting with that system over here we have an actor who is the customer and the customer has associations with all of these different use cases, each use case appears inside the system in an ellipse. So the customer is the primary actor who initiates all of these different use cases, the customer interacts with the system, in other words whether the customer wants to search for flights, make a reservation, check flight status and so on, it's the customer who gets the ball rolling. Over here we have a secondary actor in this case the secondary actor is a system which is a payment processor. Now the difference between a primary actor and a secondary actor is the primary actor is the one who initiates the use case, the secondary actor comes into play once the use case is under way. So for example when the validate credit card use case starts that use case brings the payment processor, this actor into play. The besides relationship between actors and use cases we also see relationships between actual use cases inside the system and a good example here is the purchase ticket use case. The purchase ticket use case includes the validate credit card use case and as you'll recall the include dependency is shown by a dotted arrow that points from the dependent use case to the use case that it depends on. So what include means here is that in order for the purchase use ticket, use case to be complete the validate credit card use case also has to be complete. So this must happen before the purchase ticket use case can be complete. The purchase ticket use case also has an extend relationship with this use case right here select seat. In this example the dependency arrow points from the extending use case back to the purchase ticket use case because the select seat use case depends on the purchase ticket use case. It's not going to happen all by itself, so the extend relationship means as you know that this use case is optional before this use case is complete. Now you can probably think of other use cases and you can probably think of ways in which we might be able to generalize some of these use cases but you want to keep in mind that your goal is to keep your diagram simple. This diagram as it stands is simple, it's easy to understand and it doesn't go into too much detail that's going to clutter up the diagram and make it harder to read.