Guideline : Representing
Interfaces to External Systems
If the system communicates with another system, there will be one or more
boundary classes identified in Activity:
Use Case Analysis to describe the communication protocol. Another system may
be anything from software to hardware units that the current system will use,
such as printers, terminals, alarm devices, and sensors. In each case, a
boundary class will be identified whose purpose is to mediate the communication
with the other system.
Example
An Automated Teller Machine (ATM) must communicate with the
ATM Network to ascertain whether a customer's bank number and PIN are correct,
and whether they have sufficient funds in their account to effect a withdrawal.
Since the ATM Network is an external system (from the perspective of the ATM),
we would use a boundary class to represent it in Use Case
Analysis.
If the interface(s) to the system are simple and well-defined, a single class
may be sufficient to represent the external system. Often, however, these
interfaces are too complex to be represented using a single class; they often
require complex collaborations of many classes. Moreover, interfaces between
systems are often highly reusable across applications. As a result, in many
cases, a subsystem more appropriately models the system interfaces.
The use of a subsystem allows the interface to the external system to be
defined and stabilized, while leaving the design details of the system interface
to remain hidden while its definition evolves.
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