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Visualizing System Events: System Sequence Diagrams

Learn how System Sequence Diagrams illustrate input and output events in software systems, helping validate use cases and identify processing gaps. Understand the importance of abstract event naming and chronological event sequencing.

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Visualizing System Events: System Sequence Diagrams

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  1. PRJ566 System Sequence Diagrams

  2. System Sequence Diagrams • A system sequence diagram …. Illustrates input and output events related to the system under discussion. • Larman, APPLYING UML AND PATTERNS, p. 173

  3. System Sequence Diagrams • The use case text and its implied system events are input to a SSD (system sequence diagram). • Larman, APPLYING UML AND PATTERNS, p. 17

  4. System Sequence Diagrams • Use cases describe how external actors interact with the software system… • An actor generates system events to a system, requesting some system operation to handle the event. • The use case text implies the event…the SSD makes it concrete and explicit. • Larman, APPLYING UML AND PATTERNS, p. 176

  5. System Sequence Diagrams • A system sequence diagram is a picture that shows, for one particular scenario of a usecase, the events that external actors generate, their order and the inter-system events. • All systems are treated as a black box. • Larman, APPLYING UML AND PATTERNS, p. 176

  6. Why Draw System Sequence Diagrams? • The external input events—the system events are an important part of analyzing system behavior. • System behavior is a description of what a system does, without explaining how it does it. • Larman, APPLYING UML AND PATTERNS, p. 173

  7. System Sequence Diagrams • System events should be expressed at the abstract level of intention rather than in terms of the physical input device. • Larman, APPLYING UML AND PATTERNS, p. 178

  8. System Sequence Diagrams • How to name system events and operations • describe at a high level i.e. enterItem instead of scanItem • ‘enter’ captures the intent of the operations without specifying a design choice (i.e. scanning)

  9. System Sequence Diagram • Shows the external actors that interact directly with the system • Shows the ‘system’ • Shows the system events that the actors generate • Done in a chronological sequence (from first to last event over time) • Order of events should follow the order of the scenario

  10. System Sequence Diagrams • Validates use cases • helps identify processing that has not been covered by the process described in the use case • Helps us identify objects and classes

  11. Browse Catalog SSD

  12. How Many System Sequence Diagrams? One for each of the HD scenarios in the use case specification • Larman, APPLYING UML AND PATTERNS, p. 176

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