IBM SG24-7368-00 manual

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.

Chapter 3. Black-box thinking: Defining the system context 59