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sequence diagrams

A sequence diagram shows, as parallel vertical lines (lifelines), different processes or objects that live simultaneously, and, as horizontal arrows, the messages exchanged between them, in the order in which they occur. This allows the specification of simple runtime scenarios in a graphical manner.

Sequence diagrams are typically used to model:
  1. Usage scenarios.  A usage scenario is a description of a potential way your system is used. The logic of a usage scenario may be part of a use case, perhaps an alternate course. It may also be one entire pass through a use case, such as the logic described by the basic course of action or a portion of the basic course of action, plus one or more alternate scenarios. The logic of a usage scenario may also be a pass through the logic contained in several use cases. For example, a student enrolls in the university, and then immediately enrolls in three seminars.
  2. The logic of methods.   Sequence diagrams can be used to explore the logic of a complex operation, function, or procedure.  One way to think of sequence diagrams, particularly highly detailed diagrams, is as visual object code.
  3. The logic of services.  A service is effectively a high-level method, often one that can be invoked by a wide variety of clients.  This includes web-services as well as business transactions implemented by a variety of technologies such as CICS/COBOL or CORBA-compliant object request brokers (ORBs).
  4. UML sequence diagrams model the flow of logic within your system in a visual manner, enabling you both to document and validate your logic, and are commonly used for both analysis and design purposes.  Sequence diagrams are the most popular UML artifact for dynamic modeling, which focuses on identifying the behavior within your system.

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