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Chapter 4. White-box thinking: Understanding collaboration 73
In Figure 4-2 the operation reject trade, from a black-box perspective, would
encompass the Enterprise Application’s request of the Quality Officer to update
trade status, as well as Enterprise Application’s request of the Quality Engineer
to update trade status.
Note that some operations consist of only one black-box interaction, the one that
invokes that operation. This is the case with Provide Energy or AS Trade Details
in the same diagram.
Figure 4-2 Black-box sequence diagram
It helps to keep this black-box sequence in mind as we proceed to the work of
creating the white-box expansion of the operation. To create this expansion, first
we will write an operation specification for each operation. This operation
specification, like a use case specification, describes a sequence of events to
accomplish a goal. To write one for an operation, we work our way through the
black-box description of the operation, and elaborate the black-box actions into
white box, by explaining how the elements at the next decomposition level
collaborate to accomplish the operation.
In the accompanying example, we show how the operation initiate new sale is
realized by a collaboration of the point of sale and order processing elements
(here called subsystems).