OpenUP/OpenUP_baseline_EN/library/openup_basic/guidances/guidelines/repres_interfaces_to_ext_systems.xmi

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  <mainDescription>&lt;p&gt;
    Interfaces with external systems should be consistently handled throughout the system, so markers need to be identified
    in the architecture to make sure that the team develop the coherant software. The architecture need not include a
    specific, detailed design for each system interface. It is often enough to simply identify the existence of the
    interfacre as a significant part of the architecture and create a subsystem to encapsulate the detail, so that it can
    be developed later.
&lt;/p&gt;
&lt;p&gt;
    The &lt;a class=&quot;elementLink&quot;
    href=&quot;./../../../openup_basic/guidances/concepts/entity_control_boundary_pattern,_uF-QYEAhEdq_UJTvM1DM2Q.html&quot;
    guid=&quot;_uF-QYEAhEdq_UJTvM1DM2Q&quot;&gt;Entity-Control-Boundary Pattern&lt;/a&gt;&amp;nbsp;provides the basis for a useful technique to
    support this.
&lt;/p&gt;
&lt;p&gt;
    If the system communicates with another system, define one or more boundary classes 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 that mediates the communication with
    the external system will be identified.
&lt;/p&gt;
&lt;p&gt;
    Example:
&lt;/p&gt;
&lt;blockquote&gt;
    &lt;p&gt;
        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 the customer has sufficient funds to withdrawal the requested amount. The
        ATM network is an external system (from the perspective of the ATM); therefore, you would use a
        &lt;strong&gt;boundary&lt;/strong&gt; class to represent it in a use-case analysis.
    &lt;/p&gt;
&lt;/blockquote&gt;
&lt;p&gt;
    If the interfaces with 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 by 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 models the system interfaces more appropriately.
&lt;/p&gt;
&lt;p&gt;
    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 hidden as the system evolves.&lt;br /&gt;
&lt;/p&gt;</mainDescription>
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