A service invocation is almost always a two-wayoperation, meaning that a request is sent from the consumer to the provider and a response is sent back in the other direction.
服务调用几乎始终是双向操作，这意味着请求是从使用者发送到提供者的，而响应是按照相反的方向返回的。
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The process contains a receive activity followed by an invoke activity, and a reply activity that is needed only if the operation offered by the process is a two-wayoperation.