1. In a publish/subscribe data processing broker network having a plurality of broker computer systems communicating with each other via the network each of which has an input for receiving published messages directly from a publisher application and/or receiving subscription data from a subscriber application, a first broker computer system comprising: means for receiving via said input subscription data from a subscriber application; means for providing an acknowledgement to said subscriber application to acknowledge that the subscription data has been received in response to having received the subscription data from the subscriber application; means for propagating said subscription data to a group of other broker computer systems; means for determining whether said means for propagating has propagated said subscription data to the group of other broker computer systems; and means for receiving via said input an indication from said subscriber application that said acknowledgement should not be sent to the subscriber application until the means for determining has determined that said means for propagating has propagated said subscription data to the group of other broker computer systems.