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.

10. Recording system operations

The set of all required system operations are determined by identifying the system input events. The name of the input event and the name of the operation are identical; the distinction is that the input event is the named stimulus, the operation is the response. 

For example, from the inspection of the use case Buy Items with Cash, we can identify the system operations to be:

  • enterItem(upc, quantity) 
  • endSale() 
  • makePayment()

The system operations can be grouped as operations of a type named System. The parameter may optionally be ignored. Figure 3.15 shows a system concept showing identified system operation.


Figure 3.15: System Concept Showing System Operations for a "But Items with Cash" Use Case

The system operations for "Make Phone Calls" use case is shown in Figure 3.16.


Figure 3.16: System Concept Showing System Operations for a "Make Phone Calls" Use Case