它们可以有多种形式，但是主要归为三类：用户界面服务、实现服务和业务规则服务。
They can take many forms, but fall into three categories: user interface services, implementation services and business rules services.
所有这些操作都可以通过一个自助服务用户界面控制。
All of this can be controlled through a self service user interface.
非阻塞调用在设计用户界面时以及服务调用非常费时的情况下很有用。
Nonblocking invocations are useful in designing user interfaces and when service invocations take a lot of time.
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