Chapter 8. Focusing on Interaction Links: Communication Diagrams

The main purpose of sequence diagrams is to show the order of events between the parts of your system that are involved in a particular interaction. Communication diagrams add another perspective to an interaction by focusing on the links between the participants .

Communication diagrams are especially good at showing which links are needed between participants to pass an interaction's messages. With a quick glance at a communication diagram, you can tell which participants need to be connected for an interaction can take place.

On a sequence diagram, the links between participants are implied by the fact that a message is passed between them. Communication diagrams provide an intuitive way to show the links between participants that are required for the events that make up an interaction. On a communication diagram, the order of the events involved in an interaction is almost a secondary piece of information.

Sequence and communication diagrams are so similar that most UML tools can automatically convert from one diagram type to the other. The difference between the two approaches is largely personal preference. If you're happier looking at interactions from a link perspective, then communication diagrams are likely to be for you; however, if you prefer to see the order of the interactions as clearly as possible, then you're likely to be in the sequence diagram camp.