Software Construction: System Behavior and Diagrams

System Behavior

• System behavior describes what a system does, without explaining how it does it.
• A system sequence diagram (SSD) is a fast and easily created artifact that illustrates input and output events related to the systems under discussion.

System Sequence Diagrams (SSDs)

• A SSD is a picture that shows, for a particular scenario of a use case, the events that external actors generate, their order, and inter-system events.
• SSDs are a type of sequence diagram in UML, following the Unified Modeling Language style and notation.
• SSDs show sequences for specific use cases, unlike standard sequence diagrams that show progression of events over time.

SSD vs. Sequence and Use Case Diagrams

• Standard sequence diagrams show progression of events over time.
• Use case diagrams represent a user's interaction with the system.
• SSDs are part of the Use-Case Model, visualizing interactions implied in the use cases.

SSD Notations

• Objects are represented by a box shape with an underlined title, modeling the system as a black box.
• Actors are represented by stick figures, showing entities that interact with the system.
• Events are represented by dashed lines (lifelines), showing system events generated by actors.

SSD Example

• SSD treats all systems as a black box, emphasizing events that cross the system boundary from actors to systems.

Operation Contracts

• Contracts for operations help define system behavior, describing the outcome of executing a system operation in terms of state changes to domain objects.
• Contracts describe detailed system behavior in terms of state changes to objects in the Domain Model, after a system operation has executed.

Operation Contracts Components

• Pre-conditions are conditions that the state of the system must satisfy before the execution of the operation.
• Post-conditions are conditions that the system state has to satisfy after the execution of the operation.

Operation Contracts Schema

• Operation: name of operation (parameters)
• Cross Reference: the Use Cases in which the OC occurs
• Preconditions: notable assumptions about the state of the system or objects in DM before execution
• Postconditions: state of objects in DM after execution of operation