1. An application integration system, comprising: a publishing application configured to generate and send a message, the message being sendable in accordance with a user-defined composite policy comprising at least first and second policy layers; first and second broker clusters, each said broker cluster comprising a plurality of brokers, each said broker being configured to relay messages from the publishing application to at least one subscribing application; a composite cluster connection associated with the publishing application; and a plurality of cluster connections associated with the composite cluster connection; wherein the composite cluster connection is configured to route the message to at least one said cluster connection in dependence on the first policy layer, and wherein each said cluster connection is configured to route the message to at least one broker in dependence on the second policy layer.