Message processing control in a publish/subscribe system

Messages in a publish/subscribe system can be processed after they leave the original publisher and before they are delivered to an ultimate subscriber. Each published message has an associated topic string and a PubLevel value initially assigned by the original publisher. Subscribers are assigned different SubLevel values. These subscribers register their subscriptions with a broker. When a published message having the appropriate topic string is received, the broker performs a subscriber selection process using the message topic string, the current message PubLevel value and the assigned SubLevel values of subscribers registered for the topic string. When a subscriber is selected, the broker changes the message PubLevel value and transmits the message only to the selected subscriber. The subscriber may be an intercepting subscriber that processes the published message and then re-publishes it back to the broker. The broker may repeat the subscriber selection process multiple times with the published message being sent to multiple intercepting systems, each of which processes and republishes the message before the processed message is finally sent by the broker to an ultimate subscriber.