The increase in the amount of data on the Internet has led to the development of a new generation of applications based on selective information dissemination where, data is distributed only to interested clients. Such applications require a new middleware architecture that can efficiently match user interests with available information. Middleware that can satisfy this requirement include event-based architectures such as publish-subscribe systems (hereafter referred to as pub/sub systems). The pub/sub paradigm has recently gained a significant interest in the database community for the support of information dissemination applications for which other models turned out to be inadequate. In pub/sub systems, clients are autonomous components that exchange information by publishing events and by subscribing to the classes of events they are interested in. In these systems, publishers produce information, while subscribers consume it. A component usually generates a message when it wants the external world to know that a certain event has occurred. All components that have previously expressed their interest in receiving such events will be notified about it. The central component of this architecture is the event dispatcher. This component records all subscriptions in the system. When a certain event is published, the event dispatcher matches it against all subscriptions in the system. When the incoming event verifies a subscription, the event dispatcher sends a notification to the corresponding subscriber.
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