Distributed, Expressive Top-k Subscription Filtering using Covering in Publish/Subscribe Systems

Top-k filtering is an effective way of reducing the amount of data sent to subscribers in pub/sub applications. In this paper, we investigate top-k subscription filtering, where a publication is delivered only to the k best ranked subscribers. The naive approach to perform filtering early at the publisher edge broker works only if complete knowledge of the subscriptions is available, which is not compatible with the wellestablished covering optimization in publish/subscribe systems. We propose an efficient rank-cover technique to reconcile top-k subscription filtering with covering. We extend the covering model to support top-k and describe a novel algorithm for forwarding subscriptions to publishers while maintaining correctness. We also establish a framework for supporting different types of ranking semantics, such as fairness and diversity. Finally, we conduct an experiential evaluation and perform sensitivity analysis to demonstrate that our optimized rank-cover algorithm retains both covering and fairness while achieving properties advantageous to our targeted workloads.