ToPSS - The Toronto Publish/Subscribe System Family

The publish/subscribe paradigm is a simple interaction style with wide applicability and use. Clients in a publish/subscribe system are autonomous components that exchange information, coordinate activities, or interact by publishing content and by subscribing to content of interest. Content generically refers to the kinds of messages available in the system. Content maybe comprised of any kind of text messages, XML documents, RDF documents, or set of attribute-value pairs. Clients that produce information are referred to as publishers, while clients that consume information are referred to as subscribers. A client can be both a publisher and a subscriber. A publisher generates messages (i.e., publications) to inform the external world that a certain event has occurred. A subscriber, expresses its interest in events by means of subscriptions. A subscriber is notified of the occurrence of these events through publications. The central component of a publish/subscribe system is the broker (a.k.a. content-based router). The broker persists all subscriptions in the system. When a publication is submitted, the broker matches it against all subscriptions. On a match, the broker notifies the corresponding subscriber(s). Note, a subscriber maybe another broker; similarly a publisher maybe a broker. It is important to note that messages from publishers (publications) do not contain any address; instead, they are routed through the system solely based on their content. This is where the notion of a content-based network and content-based routing is founded. A network of brokers can be formed by federating brokers who, by acting as clients to each other, publish and subscribe to each others’ messages.