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.

7. System Behaviour: System Sequence Diagrams and Operations


We need to identify the operations that the system needs to perform and in what order the system need to perform these operations to carry out a use case, and the effect of such an operation on the system, i.e. on the objects of the systems.

A use case defines a class of conversations between the actors and the system, and an individual conversation of this class is a realization of the use case. Obviously, there may be many realizations for a use case. A scenario of a use case is a particular instance or realized path through the use case, i.e. a particular realization of the use case. 

This activity is intending to introduce system sequence diagrams which are used to find system events and system operations. Latter we will see how to create a contract of system operations. 

System operations are the operations that system needs to perform to carry out a use case and the effects of these operations on the system. In this case, use cases – use case diagram and use case descriptions are inputs of this activity.