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Chapter 4. White-box thinking: Understanding collaboration 75
The heart of an operation specification is the flow of events. The columns to the
left, system actor action and black-box step, allow the modeler to show the
black-box flow of events for the operation. This can be useful as the white-box
flow is being developed. Because this black-box flow also appears identically in
the use case specification in which this operation is used, in practice we
sometimes delete this black-box information after the operation’s white-box
sequence has solidified. The white-box steps incorporate all of the behavior
specified in the black-box steps, described at this lower level of abstraction.
In the white-box sequence, notice that we do not use the term
system nor do we
use the name of the enterprise. Anytime the black-box flow named something
that the system or the enterprise does, we must translate that into what the
elements of the system or enterprise do. Main flows are thus expanded, followed
by any alternate flows as shown in the example.
The table also contains columns for process and locality, which are not
completed initially, but will be used later to express joint realization of the
operations.
With the flow of events created, we now draw a white-box sequence diagram to
allow us to determine the operations that the elements at this level must perform
to realize the operation from the level above. White-box sequence diagrams are
quite similar to the black-box sequence diagrams. The difference is that instead
of a single UML classifier (or SysML block) to represent the system, we instead
use multiple UML classifiers (or SysML blocks) representing the logical elements
at this decomposition level.
We then translate the white-box expansion flow of events developed before, into
requests made between these logical system elements and the actors. In the
example of Figure 4-4, the actors In Store Customer and Bank Credit Card
System interact with the Sales Clerk (modeled here as an element, but could
have been shown as a worker (if we do not plan to further decompose) and six
logical system elements.