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Now that we have seen use cases, the next behavioral
2
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diagram I want to discuss is the sequence diagram. So
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what is a sequence diagram? It is an interaction diagram
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that emphasizes how objects communicate and the time ordering of
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the messages between objects. To illustrate sequence diagrams in a
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practical way, and hopefully in a clear way, I will
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introduce them by creating an actual sequence diagram using an
8
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example taken from our course management system. So let's see what
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are the steps needed to build such a sequence diagram. The first
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thing we want to do is place the objects that participate in
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the interaction at the top of the diagram along the x-axis, and
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you also want to place them in a specific way. You want
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to place objects that initiate the interaction at the left, and place
14
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increasingly more subordinate objects to the
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right. So basically, this should reflect
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the way the events will flow for the majority of the interactions
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in the system. Next thing you want to do is to add
18
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what is called the object lifeline. It's a vertical line that
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shows the existence of objects over a period of time. And
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it's normally represented with a dashed line, except for the outermost
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object for which it is a solid line. Now that you
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have your object lifeline you can start placing messages that these
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objects send and receive. You want to put them along the
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y-axis in order of increasing time, from top to bottom. And
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you can also put a number on the message to further
26
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clarify the sequence. So in this case what we're showing
27
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is that the student will send the fill in info
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message to the registration form. And this is the first
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message in the sequence diagram, the first interaction. Then the student
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might submit the form and this is also a message
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that goes to the registration form. At this point, when
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the submission takes place, the registration form will send the
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message, so it will invoke some functionality in the registration manager.
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Specifically you will invoke the add course functionality
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and pass Joe, the name of the student and
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Math 101 which is the specific course for which
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Joe is registering. Then the registration manager will ask
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the Math 101 course whether it accepts registrations,
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and the interaction will continue. So that Math 101
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will actually check for a specific offering, if everything
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goes fine, you will receive an ack, you'll send
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back the act to the registration manager and so
43
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on. Until at the end, Joe will be registered for
44
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Math 101. As you can see, it is very
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easy to see how the interaction occurs between these different
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objects at run time, dynamically. So what the behavior
47
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of the system is for this specific scenario. So the
48
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last notational element that I want to add to this
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diagram is the focus of control. Which is this tall
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thin rectangle, that shows the period of time
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that an object is performing an action, either
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directly or indirectly. So if we look at
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the registration form, this is telling us that
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the registration form is active for this amount
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of time. And the same thing we can
56
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do for the registration manager, the Math 101
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course offering, and the Math 101 specific section.
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