Sequence diagrams with localities

After drawing a locality view, the next step is to analyze how the operations on the various logical elements will be deployed at these places. To do this we construct a new sequence diagram, similar to the ones we have already done, but instead of the logical elements and actors, we use the localities and the actors as the lifelines. We create such a locality interaction (sequence) diagram for each operation at the level above which we are doing our locality analysis. Thus there will be a locality interaction diagram for each white-box sequence diagram at this level.

To determine the messages between the elements on our locality interaction diagram, we simply copy the messages from the white-box sequence diagram one-for-one onto the new diagram. The messages are the same; the difference is the elements to which the messages go:

￿In the white-box sequence diagrams, messages are requests of some logical system element to perform some operation.

￿In the locality interaction diagram, the same messages indicate where the operation is to be implemented.

We can think of it as a request being made of a distribution location, where part of the system is implemented. Notice that it is quite common to have numerous reflexive messages (messages that go from an element back to itself), because this means that a number of operations happen consecutively at one place.

Figure 5-3shows how the initiate new sale operation from an earlier illustration is distributed across the locations in the retail system.

Chapter 5. Understanding distribution of responsibility

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IBM SG24-7368-00 manual Sequence diagrams with localities