XL peer-to-peer Pub/Sub systems

Increasingly, one of the most prominent ways to disseminate information on the Web is through “notifications” (also known as alerts), and as such they are at the core of many large-scale applications. For instance, users are notified of articles in which they are interested through RSS feeds, of posts from their friends through social networks, or of recommendation generated by various sites. Event notification usually relies on the so-called Publish-Subscribe (Pub/Sub) communication paradigm. In Pub/Sub systems, subscribers sign up for events or classes of events in order to be asynchronously notified afterward by the system. The size of such systems (with respect to events and subscriptions) keeps growing, and providing scalable implementations of Pub/Sub systems is extremely challenging. Although there exist popular examples of centralized Pub/Sub systems that currently support a large number of subscribers, such as online social networks, they periodically face formidable challenges due to peak loads and do not always offer a support for fine-grain subscriptions. In fact, providing scalability along with expressiveness in the subscription patterns calls for distributed implementations of Pub/Sub systems. In parallel, peer-to-peer (P2P) overlay networks have emerged, providing a sound and highly scalable network foundation upon which to build distributed applications including Pub/Sub systems. In this article, we focus on fully decentralized (P2P), highly scalable, Pub/Sub systems. More specifically, we investigate how Pub/Sub and P2P research can be integrated. We define the design space and explore it in a systematic way. We expose an understanding of available design choices; provide a comprehensive classification and understanding of prominent P2P Pub/Sub systems, positioning them against the design dimensions; and highlight correlations between and implications, benefits, and shortcomings of design alternatives