PRJ566 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

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

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

System Sequence Diagrams  A system sequence diagram is a picture that shows, for one particular scenario of a use case, 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

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

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

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)

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

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

Browse Catalog SSD Actor System Selects the Browse Catalog option on the Online Auction Main Page. Retrieves the list of categories and displays the Category Page with a drop down list of categories and an Exit Button. Selects an option from the drop down list Retrieves all the items for that category and displays an Item List Page showing a table with item names with a Display Button and an Exit Button. Selects an item and clicks the Display Button. Retrieves the product name, it’s picture and a description and displays the information on the Item Information Page with a Place Bid Button and an Exit Button. Clicks the Place Bid Button.Use case ends and starts the Place Bid use case.

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