服务提供者目录是服务发现进程中的一个可选的部件。
The service provider catalog is an optional part of the service discovery process.
在基本模式中代理是可选的但是在企业系统中经常是关键性部件。
The broker is optional in the base pattern but is often a critical piece in enterprise systems.
也就是说，如果适当的先决条件成立，可以使用可选功能部件来创建层结构。
In other words, attempt to use optional features as way of creating layers that would be available if the appropriate prerequisites are present.
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