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.
Chapter 4. White-box thinking: Understanding collaboration | 75 |
