Chapter 10: System Sequence Diagrams

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Chapter 10 System Sequence Diagrams

• 10.1 Introduction
• A system sequence diagram (SSD) is a fast and
  easily created artifact that illustrates input
  and output events related to the systems under
  discussion. They are input to operation contracts
  and, most importantly, object design.
• We can relate SSDs to the other models we have
  seen so far see Figure 10.1

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Figure 10.1 SSDs within the UP
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• 10.2 Examples

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• An SSD shows, for a particular course of events
  within a use case, the external actors that
  interact directly with the system, the system (as
  a black box), and the system events that the
  actors generate. Time proceeds downward, and the
  ordering of events should follow their order in
  the scenario.
• See Figure 10.2 for the SSD of main success
  scenario for POS
• As analised in the use cases, actors generate
  system events requesting some system operations
  to handle the events.
• We can draw an SSD for a main success scenario of
  each use case, and frequent or complex
  alternative scenarios.
• SSDs help to validate, clarify and refine use
  cases.

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Figure 10.2 POS success scenario
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• Interaction frames are used to show loops (and
  other constructs) in sequence diagrams common
  interaction frames operators include
• alt Alternative multiple fragments only the
  one whose condition is true will execute
• opt Optional the fragment executes only if the
  supplied condition is true.
• loop The fragment may execute multiple times,
  and the guard indicates the basis of iteration.
• See figure 10.3 for an example of these

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Figure 10.3 Illustrating Various Interaction
Frames
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• 10.3 Relationship Between SSDs and Use Cases

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• 10.4 Conclusions
• Don't create SSDs for all scenarios, rather draw
  them only for the scenarios chosen for the next
  iteration. And, they shouldn't take long to
  sketch, perhaps a few minutes or a half hour.
• SSDs are also very useful when you want to
  understand the interface and collaborations of
  existing systems, or to document the overall
  architecture of the application.
• SSDs represent visually the essential aspects of
  a use case.
• For each event that the system receives from an
  actor, the system is expected to implement an
  operation, of the same name, to perform something
  of value to the actors.