System Sequence Diagrams

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Chapter 10

• System Sequence Diagrams

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System Sequence Diagram

• Shows
• the external actors that interact directly with
  the system
• the system as a black box
• system events that the actors generate
• one scenario of a use case
• Events depicted in the SSD are those implied or
  suggested by the use case text.
• actor generates system events
• a system operation handles the event

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Fig. 10.2 SSD for a Process Sale scenario
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System Events

• External input events
• From actors
• Humans
• Other systems (see Fig. 23.1)
• Timer events
• Faults or exceptions

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Fig. 10.3 Relationship between SSDs and Use Cases
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Fig. 10.4 Naming System Events and Operations

• Express abstract intention, not at the physical
  level of the input device.
• Start with a verb.

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Fig. 10.5 A sketched SSD for Play Monopoly Game
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SSDs within the Unified Process

• SSDs not explicitly mentioned in the UP, but
  popularized by the Fusion method.
• Part of the Use-Case Model.
• Visualization of interactions implied in the
  scenarios of use cases.
• Most SSDs created during Elaboration.
• Draw them only for scenarios chosen for the next
  iteration.