Large-scale information dissemination systems for selective information distribution are gaining increasing importance. This is due to the fast proliferation of different communication infrastructures, e.g., Internet and wireless networks, and the increase in mobile computing potential. However, targeted information dissemination will only be effective if the distributed information is pertinent for the user. In this context the publish and subscribe paradigm is a good interaction model: This interaction model enables publishers to publish information through the publish and subscribe system and provides subscribers with the ability to express their interest within the system in order to be notified afterwards of any publications matching their registered interests. In the context of information dissemination, requirements for a publish and subscribe (pub/sub) system are threefold: First, the system must achieve interoperability between different devices and infrastructures; One solution is to use extensible Markup Language (XML) as the common data presentation format for publications. Second the system must support a (very) expressive filtering language that enables subscribers to accurately define their interests over publi- cations. Finally, it has to be able to manage millions of users, manage high volatility of users' interests, and filter large quantities of content per unit of time.
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