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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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+00:00:12,710 --> 00:00:15,940
+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
+
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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
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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
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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
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+clarify the sequence. So in this case what we're showing
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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
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+on. Until at the end, Joe will be registered for
+
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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
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+of the system is for this specific scenario. So the
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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
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+do for the registration manager, the Math 101
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+course offering, and the Math 101 specific section.