The Call mediator is used to send messages out of the ESB to an endpoint. The Call mediator invokes the service in an asynchronous (non-blocking) manner. Then, the underlying worker thread returns without waiting for the response. Mediation will be paused from that point. When the response is received, the mediation flow resumes from the next mediator in the sequence.
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For another example, see Sample 500: Call Mediator for Non-Blocking Synchronous Service Invocation.