1. A publisher-subscriber system stored on a computer-readable storage medium, comprising: a broker configured to receive and distribute at least one data stream from publishers to subscribers in accordance with subscriptions, wherein distributing the at least one data stream to subscribers includes determining subscribers which are to receive the at least one data stream, at least in part, by using a stream-level attribute which has been appended to the at least one data stream to filter a set of subscriptions associated with the subscribers; and a classifier configured to extend a data stream with the stream-level attribute creating an annotated data stream wherein the annotated data stream applies the stream-level attribute to all messages therein, wherein subscribers can subscribe to the annotated data stream created by the classifier.