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Chapter 3. Black-box thinking: Defining the system context 51
Figure 3-3 Retail use case diagram
An enterprise use case diagram can show all use cases for the enterprise or a
subset of them as just noted for clarity. What is important is that all the use cases
shown are at the same level of decomposition, that is, the enterprise level, or
level 0. Actors shown are enterprise actors—the same ones shown on the
enterprise context diagram if one has been developed. Because the enterprise is
treated as a black box, no workers are shown. Workers (people inside the
system) will likely become actors at lower levels of abstraction.
Actor involvement in use cases
One of the most common omissions made in use case modeling for MDSD is to
overlook some actor interaction. It is easy enough to identify the primary, or
initiating actor associated with a use case, but it is easy to overlook other actors
who have a supporting role in the carrying out of the use case. In MDSD, this is a
particularly serious omission, because the actor interactions allow the
identification of the operations the system must perform to realize the use case.
This will be seen in later steps as the operations analysis proceeds, but for now,
understand that all actor interactions must be captured. Such omissions can, of
course, be discovered and provided later, but the recommendation here is to try
to identify all of the system interactions—do not skip any for the sake of brevity or
speed.