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Chapter 3. Black-box thinking: Defining the system context 59
I/O entities are another way to abstract information, and can also represent
physical items as well. Information entities can also be I/O entities if they are sent
or received outside the system. Both I/O entities and information entities can be
used to create a domain model, or even multiple domain models at various levels
of abstraction in the model. A domain model is a UML class or SysML block
diagram showing the entities and their relationships, such as multiplicity
(one-to-one, one-to-many) and generalization/inheritance.
Message naming: A quiz
Because this topic is so important, let us review the principles covered so far with
a little exercise. Which of the messages in the following diagram seem to be
correct, and which seem to contain an error? It should be noted that this is
merely a grouping of independent messages for presentation purposes, no
sequence is implied (Figure 3-6).
Figure 3-6 Which messages are correct?
The best way to do this quiz is to read each message in its full plan language
form using the term
requests:
The first message would be read: The human resources system requests the
payroll system to send the payroll record. If this sounds like a correct
statement of the behavior of the system, then this message is well-named—it
does and it is. It means that the payroll system must be capable of sending a
payroll record, which seems sensible.