If the system communicates with another system, there will be one or more boundary classes identified in Design the Solution to describe the communication protocol. An external 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 external 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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