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copied!<blockquote> <p>The "lifeline1" represents an object execute during a period of time.</p> </blockquote> <p>The dotted vertical line represents the object's life time. An 'execution specification' box is drawn on the line when the object is active / executing / has control. </p> <blockquote> <p>During lifelin1 execution and exactly at The red circle "lifeline1" needs something from "lifeline2" to be able to complete its execution</p> </blockquote> <p>If it needed something to begin execution, it would not be shown with an execution specification to start with and would instead be shown as becoming active after it received a message. </p> <p>Instead, the block shows lifeline1 is shown as active, and sending a synchronous message.</p> <blockquote> <p>1)the "lifeline1" pause its execution and then</p> </blockquote> <p>No pause (or other timing) is implied by this sequence diagram.</p> <blockquote> <p>2) call synchronous message .(please clarify it a bit in your answer)</p> </blockquote> <p>It is similar to calling a method on an object - the flow of control in a synchronous message passes immediately from the sender to receiver, and on receipt of a synchronous message the receiver is shown as the active object. </p> <blockquote> <p>The pink line represent the duration of time that the "lifeline1" needs to reach the "lifeline2" to execute</p> </blockquote> <p>No timing is implied by this sequence diagram - it shows the sequence of operations only. Duration timings are shown using the Duration Constraint and Duration Observation notation shown in table 14.1 of the OMG&dagger; <a href="http://www.omg.org/spec/UML/2.4.1/" rel="nofollow">UML2 Superstructure</a>. </p> <blockquote> <p>3)The blue rectangle represent the duration of message execution</p> </blockquote> <p>At the stage in the process represented by the blue rectangle, the 'lifeline2' object is executing in response to the message. Saying 'message execution' is a bit vague - the object is executing, not the message.</p> <blockquote> <p>4)And the dotted line represents the duration of time that the "lifeline2" needs to reach the "lifeline2" to resume its execution.</p> </blockquote> <p>The dotted line is a return message. Because the message sent from "lifeline1" is synchronous, it does not continue execution until the return message is received.</p> <blockquote> <p>5)the black rectangle represent the period when a lifeline1 was idle</p> </blockquote> <p>Execution specification blocks on UML sequence diagrams represent when the object is actively executing. A object which is idle waiting for a synchronous message to return should be represented without the execution specification block on its lifeline. (the colours have no significance in UML - they are there in this diagram to let you talk about a particular block, they do not to show whether the object is active or not)</p> <p>I would take a slightly more formal approach and say that the execution should not be shown where it is, but should start after the return message is received to represent that lifeline1 continues - just like the a method which calls a method in another object does not receive flow of control back until the call returns. </p> <p>However, it very common to show an object which is waiting for a return message with an execution specification as it is sort of still controlling the sequence of operations. </p> <p>(Unfortunately the UML is so big and fluffy, almost any example anyone comes up with can be read in a different manner. As I have used UML to generate executable models for checking processes for deadlocks using pi-calculus, I tend to have a more rigid interpretation than someone who is, say, writing one on a white board to explain something to a co-developer)</p> <p>&dagger; the Object Management Group who maintain the UML standards, not as in 'OMG it's huge', though that also applies.</p>

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