服务不仅是可复用代码的组件，更是运行程序的一部分，客户端可以不必合并它自己的代码直接调用该程序。
More than just a component of reusable code, a service is part of a running program that can be invoked by a client without having to incorporate the code itself.
包装程序的另一部分（即对遗留应用程序本身进行调用并返回的那一部分）通常必须手工编码。
The other side of the wrapper -- the one that invokes and returns from the legacy application itself -- typically must be hand coded.
不过，这样的应用程序必须十分小心，保证它总是发出多个重叠发送调用，而不是等待某个重叠发送结束了才发出下一个。
Such an application must, however, take great care to ensure that it posts multiple overlapped sends, instead of waiting for one overlapped send to complete before posting another.
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