Introduction to Sequence Diagrams
Sequence diagrams and collaboration diagrams are basically interaction diagrams. So first we should understand what are these interaction diagrams.
Interaction diagrams shown an interaction consisting of a set of objects and their relationship including the messages that may be dispatched among them.
So sequence diagrams is an interaction diagram that emphasize the structural organization of the objects. They send receive messages. They model the dynamic aspects of a system. The context of the sequence diagrams ( or interacting diagrams ) can be understand by a simple analogy.
The analogy of sequence Diagrams
Consider a system which is in running state. For gaining insight to the system, a section of money has to be observed for the change that take place over the time. With a bit of more focus several objects of interest may be revealed over a period of time. The creation of objects changes in their attributes and then the destruction of some of the objects may be seen.
Concept of sequence Diagrams
Sequence diagram emphasizes basically the time ordering of message. It can be represented by a table with,
X - axis : represents objects
Y - axis: to have messages arranged in increasing time sequence diagram to contain objects links and messages
It sequence diagram objects that initiate the interaction are placed at the left and more subordinate objects are placed increasingly to the right. A sequence diagram for elevator system is shown in fig 8.24,
The message that the objects send and receive are placed along the y axis in the order of increasing times from top to bottom.
This ordering of objects and message gives a clear view of control over to time.
In fig 8.25, a sequence diagram is shown for the flow from the database to browser.
Consider one more example of sequence diagram. In a simple two party phone call system we can assume followings things.
1. There are two objects of class caller that stands for sender and r that for receiver. There is an objects d of the switch an lastly there is an object c class conversation. It is shown in fig 8.26.
2. Every objects has a lifeline shown in fig 8.27, by a vertical dashed line. This line represents existence of an objects over a period of time. An objects may be either present from the very beginning or it may possible that an objects has been created during an interaction. So an objects can be either be created or can be destroyed and the life of the objects is thus represented by a lifeline.
3. Another important part of a sequence diagram is focus of control. It is shown by a thin rectangle that is drawn over the lifeline. The focus of control shows the period during which an objects is performing an action.
4. Returning to the example of two party call system. The sequence begins with the action taken by the caller by picking up the receiver. Thus a message lift receiver goes from the objects s to the objects d. Now in return the objects d calls a method set dial tone of class caller. Then again the objects s sends message dial digit d to d. The objects s now creates an objects c of class conversation. The objects c created by s now calls a method ring of class caller. And when the person at the other end hears the ring and picks up the receiver the objects r sends messages to the objects c, the it calls a method connect of class switch to connect the two parties.
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