Hybrid Publish-Subscribe: A Compromise Approach for Large-Scale

The purpose of this article is to propose a scalable, topologically and traffic-wise self-adapting approach to a publish/subscribe paradigm for supporting event-based applications on a wide-area network. Large event systems requiring an efficient group communication as well as anonymity of particular peers tend to be fully decoupled. The publish/subscribe paradigm provides this requirement which enlarges a range of dimensions involving such system. There are two main approaches, namely topic-based and content-based publish/subscribe. The advantages and disadvantages of the approaches are often complementary one to another. This article presents a method of improving efficiency of large-scale systems while preserving scalability by involving cooperation of topic-based and content-based techniques. 1 The Publish/Subscribe Paradigm The publish/subscribe paradigm is a loose communication scheme for modelling the autonomous interaction among participants in event systems and introduces a powerful mechanism for distributed and mobile architectures. On the contrary to the classic interaction models like request/reply, publish/subscribe provides three levels of decoupling: time, space and data flow. The publish/subscribe interaction scheme is represented by three types of participants: a publisher, a subscriber and a broker. Whereas the first two are notification providers or consumers (clients), the last one is a mediator (server) assuring data storage and delivery. A typical publish/subscribe system consists of a number of brokers linked together into a general graph. Publishers register their interest to notify about events (by sending advertisements) to their own broker as well as subscribers register their interest (by sending subscriptions) to be notified about events. It is upon particular broker to effectively disseminate appropriate notifications across the whole network of brokers on the basis of received subscriptions/advertisements. The basic system model for publish/subscribe interaction is illustrated in Figure 1. For detail description we refer to [1]. Subscribers usually register their interest in particular events, rather than in all events. The different approaches to handle the events with respect to subscriptions/advertisements have led to define two characteristic event schemes,