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72 Model Driven Systems Development with Rational Products
Figure 4-1 Level 1 context diagram showing both human and non-human actors
Operation analysis
For each operation, the question we next need to answer is, how is this operation
accomplished (or realized) as a collaboration of elements at the next level of
decomposition? To answer this, we first write out the flow of events for the
operation. Normally, when we write a flow of events, say for a use case, we keep
to a black-box perspective exclusively. To realize an operation as a collaboration
of lower level elements, clearly we need both black- and white-box perspectives.
In a way, we already have the black-box perspective of each operation. Look at
the black-box sequence diagram of any use case that uses this operation. You
will see a series of messages (requests) beginning with the one that invokes the
operation. Follow this series of messages until you hit the next operation on the
same element, or the end of the use case, whichever comes first and stop. What
you have traced is the set of black-box interactions that accomplish this
operation.