8 Interaction Diagrams
FIGURE 3.68
Entry and exit points.
stm State X
passive
active
Cancel
End
stm Entry and Exit points
State X State Y
End
Continue
Activate
quickStart
stm State X
passive
active
quickStart
204 CHAPTER 3 UML??”Unifi ed Modeling Language
Sequence diagrams are based on the interaction model that describes the interaction
between elements of the system. It answers the question: ??? When does who
call whom and how? ??? In contrast, the activity model answers the question: ??? What
happens in which sequence? ??? , and the state model: ??? How does an object respond
to events in a specifi c state? ???
The other UML interaction diagrams that are not used by SysML are views on
the same interaction model. This means that you don ??™ t lose any modeling potential,
but only different options of representing the model.
3.8.1 Interaction
An interaction describes a communication between lifelines. This communication
is based on the exchange of messages in the form of operation calls or signals.
An interaction describes a behavior of the system, just like an activity or a
state machine.
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