Abstract: A publish/subscribe data processing broker apparatus has: a means for receiving from a publisher application data messages published on a subject-specific stream by the publisher application; and a means for distributing the received published data messages to a subscriber application which has requested to receive messages on the stream upon which the published messages were published, the means for distributing including a plurality of connected distribution agent data processing units; wherein one of the streams is reserved for messages published by a publishing distribution agent data processing unit and intended for receipt by a subscribing distribution agent data processing unit located adjacent the publishing distribution agent data processing unit or intended for receipt by a subscriber application located adjacent the publishing distribution agent data processing unit.