We consider in this paper a class of Publish-Subscribe (pub-sub) systems called topic-based systems, where users subscribe to topics and are notified on events that belong to those subscribed topics. With the recent flourishing of RSS news syndication, these systems are regaining popularity and are raising new challenging problems. In most of the modern topics-based systems, the events in each topic are delivered to the subscribers via a supporting, distributed, data structure (typically a multicast tree). Since peers in the network may come and go frequently, this supporting structure must be continuously maintained so that "holes" do not disrupt the events delivery. The dissemination of events in each topic thus incurs two main costs: (1) the actual transmission cost for the topic events, and (2) the maintenance cost for its supporting structure. This maintenance overhead becomes particularly dominating when a pub-sub system supports a large number of topics with moderate ...