SSD

[jimin-kiim]

SSD

• system sequence diagram
• illustrates input and output events related to the system
• most SSDs are crated during elaboration
  • supporting estimation
  • writing system operation contracts
  • identifying the details of the system events to clarify what major operations which the system must be designed to handle
• used for deriving operation contracts
• used for object design
• UML; sequence diagram
• 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)
  • the system events that the actors generate
• depict the system behavior in terms of what the system does, not how it does (black box)
  • system behavior is also captured by use cases and operation contracts
• advantage
  • useful to know precisely what are the external input event- the system events to which the system should react
  • external events from actors
  • timer events
  • faults or exceptions

[jimin-kiim]

Operation Contracts

• written in Elaboration step
  • used in Domain Model as contracts of high-level system operations
  • used in Design Model for object methods
• use a pre and post condition form to describe detailed changes to objects in a domain model as the result of system operation
  • system operation:
  • operation that the system as a black box component offers in its public interface
  • can be identified while sketching SSDs
  • shows system events
    • events or I/O messages relative to the system
  • system interfaces: the entire set of system operations across all use cases
• construction
  • operation name : name of operation and parameters
  • cross references: use cases this operation can occur with
  • preconditions: noteworthy assumptions about the state of the system or objects in the Domain Model before execution of the operation. non trivial assumptions the reader should be told
  • postcondition
  • the most important section.
  • changes in the state of objects in the domain model after completion of the operation
    • instances created/deleted
    • associations formed/broken
    • attributes changed
  • necessary when the outcome of a system operation is not clear from the use case description
    • when more detail and precision is useful and required
  • expressed in the past tense

    example

• Operation: enterItem(itemID: ItemID, quantity: integer)
• Cross References: Use Cases: Process Sale
• Precondition: There is a sale underway
• Postcondition
  • A SalesLineItem instance sli was created (instance creation).
  • sli was associated with the current Sale (association formed).
  • sli.quantity became quantity (attribute modification).
  • sli was associated with a ProductDescription, based on itemID match (association formed).