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