UML Sequence Diagrams

During the requirements analysis phase, the system can be treated as a single "black box", which means that we can look at the system's behavior (what it does) without explaining how it does it. Read this section to see an example of a simplified trace diagram that shows only system input events. This is called a system sequence diagram.

11. System Operations Contracts

Use cases describe system behavior which is usually sufficient. But, sometimes a more detailed description of system behavior is needed. Again, a system sequence diagram does not describe the effect of the execution of an operation invoked. It is missing the details necessary to understand the system response – the system behaviour. Part of the understanding of the system behaviour is to understand the system state changes carried out by system operations. A system state is a snapshot of the system at a particular moment of time which describes the objects of classes currently existing, current values of attributes of these objects, and current links between the objects at that time. The execution of a system operation changes the system state into another state:

Old objects may be removed,
New objects and links may be created, and
Values for attributes of objects may be modified.

To describe detailed system behavior in terms of state changes to objects in the Domain Model, after a system operation has executed, contracts are being used. We need to know how to write a contract for a system operation. As explained by Liu, the contract of an operation is defined mainly in terms of its pre-conditions and post-conditions.

Pre-conditions are the conditions that the state of the system is assumed to satisfy or must hold true before the execution of the operation
Post-conditions are the conditions that the system state has to satisfy when the execution operation has finished, or the constraint that must hold true after the completion of an operation.