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4.3 Communicate Associations
Figure 4-1 shows actors associated with the project
management system and Figure 4-3 shows use cases
associated with the project management system, but how are actors and
use cases related? A specialized type
of association, called a
communicate association, addresses the question
of how actors and use cases are related and which actors participate
in or initiate use cases. (Associations are discussed in Chapter 3.)As discussed in Chapter 2, a communicate
association between an actor and
a use case indicates that the actor uses the use case; that is, it
indicates that the actor communicates with the system and
participates in the use case. A use case may have associations with
multiple actors, and an actor may have associations with multiple use
cases. A communicate association is shown as a solid-line between an
actor and a use case.Figure 4-4 shows that a project manager
participates in managing projects, a resource manager participates in
managing resources, and a system administrator and backup system
participates in administering the system.
Figure 4-4. Actors and use cases
A navigation arrow on an association pointing toward a use case
indicates that the actor initiates the interaction with the system.
Figure 4-4 shows that a project manager initiates
the interaction with the project management system to manage
projects, and a resource manager initiates the interaction with the
project management system to manage resources.A navigation arrow on an association pointing toward an actor
indicates that the system initiates the interaction with the actor.
Figure 4-4 shows that the project management system
initiates the interaction with the backup system to back up project
management data.Rather than use two arrows when either the system or the actor may
initiate an interaction, navigation arrows on both ends of such an
association are dropped. Figure 4-4 shows that
either a system administrator or the system may initiate an
interaction to administer the system. The system administrator might
initiate an interaction with the system to back up the data, or, for
example, the system might initiate an interaction with the system
administrator informing the actor that system resources are low.Be aware, however, that a lack of navigation arrows may simply result
from a modeler choosing not to specify anything about the initiation
of an interaction. Thus, with respect to Figure 4-4, you can't be absolutely
certain that either actor can initiate a system administration
interaction. It could be that the system administrator only can
initiate the interaction, and the UML modeler simply chose not to
specify initiation in this one case. It simply depends on the
modeling guidelines the modeler is using.
