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76 Model Driven Systems Development with Rational Products
Figure 4-4 White-box sequence diagram for operation Compute Online Sale
In the same way as described for black-box sequence diagrams, operations are
identified for system elements by looking for arrows pointing in towards each
logical element. Note that with the black-box sequence diagram, we identify only
operations on a single system element (the enterprise) while with the white-box
sequence diagram we identify operations on all the elements at the next level.
Thus we only use the black-box sequence diagram to get started (for example,
when analyzing the enterprise use cases), and can use white-box sequence
diagrams at every level below that.
As the operations for each element at this level are identified by realizing each
operation from the level above, they are refactored and consolidated in the same
way we described previously.
Flowdown to further levels
To continue the flowdown to levels below level n, the same process is used.
Each operation of each level n logical element is realized using an operation
specification and white-box sequence diagram, thus identifying operations on
elements at the next level.