A participant in an interaction diagram may be a complex element in and of itself. UML allows you to link interaction diagrams by creating a part decomposition reference from a participant to a separate diagram. For example, you may have a Purchase Item interaction diagram that has a participant execute a credit card authorization. The actual details of the authorization process are probably not of interest to the readers of your Purchase Item diagram; however, they are vitally important to the developers responsible for the authorization subsystem. To help reduce clutter on your diagrams, you can create a separate diagram showing how the authorization subsystem validates credit cards and place a decomposition reference to that on the Purchase Item diagram.

To create a part decomposition reference, simply place ref interaction_diagram_name after the instance name in the head of your lifeline. Figure 10-31 shows how the Purchase Item and authorization diagrams can be modeled.

Example of a decomposition diagram

Figure 10-31. Example of a decomposition diagram

Messages that come into or out of the decomposed lifeline are treated as gates that must be matched by corresponding gates on the decomposition. A gate represents a point where a message crosses the boundary between the immediate interaction fragment and the outside environment. A gate has no symbol of its own; you simply show a ...

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