Message handling in an enterprise service bus

A data processing system comprises a service bus (12) connected between a client (10) and a server (14), the service bus comprising one or more applications (24) arranged to mediate message flow between the client and. the server. A method of operating the data processing system comprises the steps of receiving a message from the client at the service bus, and mediating the message at an application of the service bus, the mediation comprising adding a header to the message. The header defines a source and a condition under which a target can respond directly to the source, the source comprising either the client or an application of the service bus and the target comprising either an application of the service bus or the server. The method further comprises receiving the mediated message at the target, detecting that the condition within the header of the mediated message is satisfied, and transmitting a response directly to the source as defined in the header of the mediated message.