UML Sequence Diagrams

8. System Input Events and System Operations

When an actor interacts with the system for a certain use case, events are being generated to a system. This event is requesting the system to perform some operations in response. Liu, indicate that events generated by actors are very tightly related to operations that the system can perform. This implies that we identify system's operations by identifying events that actors generate.

By definition: A system input event is an external input generated by an actor to a system. A system input event initiates a responding operation while a system operation is an operation that the system executes in response to a system input event.

Some system operations also generate output events to the actors to prompt the next system event that an actor can perform; and A system event is either an input event or an output event.

Therefore, a system input event triggers a system operation, and a system operation response to a system input event.

As an example given by Liu, we can formally define a scenario of a use case as a sequence of system events that occur during a realization of the use case. Consider the use case of Make Phone Calls for a telephone system, which involves two actors, Caller (initiator) and Callee. The following sequence of events is a scenario of this use case:

Input Events	Output Events
1. Caller lifts a receiver	2. Telephone starts dial tone
3. Caller dials a number	4. Telephone rings to Callee
	5. Telephone is ringing tone to Caller
6. Callee answers the ringing phone	7. Telephone ringing tone stops
	8. Telephone ringing tone stops
	R9. Telephones connectedW6
10. Callee hangs up by returning the telephone receiver	11. connection broken
12. Caller hangs up by returning the telephone receiver

We can represent the set sequence of events is a scenario using event tracer diagram as shown in Figure 3.13:

Figure 3.13: Telephone system event trace diagram for the "Make Phone Calls"