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Unified Modeling Language Reference Manual, The, Second Edition by Grady Booch, Ivar Jacobson, James Rumbaugh

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Sequence Diagram

A sequence diagram displays an interaction as a two-dimensional chart. The vertical dimension is the time axis; time proceeds down the page. The horizontal dimension shows the roles that represent individual objects in the collaboration. Each role is represented by a vertical column containing a head symbol and a vertical line—a lifeline. During the time an object exists, it is shown by a dashed line. During the time an execution specification of a procedure on the object is active, the lifeline is drawn as a double line.

In general, a sequence diagram shows only sequences of messages and not exact time intervals. A timing diagram can be used when metric time is important, but for understanding the semantics of interactions, ...

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