Efficient publish/subscribe through a self-organizing broker overlay and its application to SIENA

Recently many scalable and efficient solutions for event dissemination in publish/subscribe (pub/sub) systems have appeared in the literature. This dissemination is usually done over an overlay network of brokers and its cost can be measured as the number of messages sent over the overlay to allow the event to reach all intended subscribers. Efficient solutions to this problem are often obtained through smart dissemination algorithms that avoid flooding events on the overlay. In this paper we propose a complementary approach, that is obtaining efficient event dissemination by reorganizing the overlay network topology. More specifically, this reorganization is done through a self-organizing algorithm executed by brokers whose aim is to directly connect, through overlay links, pairs of brokers matching same events. In this way, on average, the number of brokers involved in an event dissemination decreases, thus, reducing its cost. Even though the paradigm of the self-organizing algorithm is general and then applicable to any overlay-based pub/sub system, its concrete implementation depends on the specific system. As a consequence we studied the effect of the introduction of the self-organizing algorithm in the context of a specific system implementing a tree-based routing strategy, namely SIENA, showing the actual performance benefits through an extensive simulation study. In particular performance results point out the capacity of the algorithm to converge to an overlay topology accommodating efficient event dissemination with respect to a specific scenario. Moreover, the algorithm shows a significant capacity to adapt the overlay network topology to continuously changing scenarios while keeping an efficient behavior with respect to event dissemination.