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, wherein at least one said policy layer is based on a multi-send policy that sends messages to multiple brokers, wherein the multi-send policy is a “guaranteed” multi-send policy that is considered successful only if N out of M brokers receive the message, where N is a user-defined constant and where M is the number of brokers in the associated cluster, and wherein each said cluster connection is configured to route the message to at least one broker in dependence on the second policy layer.