A proxy service defines virtual services hosted on the ESB that can accept requests, mediate them, and deliver them to an actual service. A proxy service can perform transport or interface switching and expose different semantics from the actual service: WSDL, policies, and QoS aspects like WS-RM, WS-Security, etc. It can mediate the messages before they are delivered to the actual endpoint and mediate the responses before they reach the client. You can also list a combination of tasks to be performed on the messages received by the proxy service and terminate the flow or send a message back to the client even without sending it to the actual service.
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