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We proceed in the same way if the actor is an external system. Only this time
we take the view of the external system.
If the actor ??™ s end from where the information fl ow starts is a user system we
take the view of the user behind the user system and proceed similarly to user
actors. From this modeling view, the user system itself does not have an interest,
apart from the data transfer between a user and the system.
Similar to user systems, we describe the use cases for boundary systems from
the view of the external system that communicates over the boundary system.
Actors representing environmental effects don ??™ t have to be considered as we
look for use cases. They are the only actors that are not necessarily participating
in use cases.
If the actor is an actuator, then it participates in some part of a fl ow, but it is
not the trigger of a use case. A situation where an actuator sends domain-relevant
information to the system is rather unusual; and it would be even more unusual if
FIGURE 2-32
Describing use cases for services.


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