CoQUOS: Lightweight Support for Continuous Queries in Unstructured Overlays

The utility and the effectiveness of peer-to-peer (P2P) content distribution systems can be greatly enhanced by augmenting their ad-hoc content discovery mechanisms with two capabilities, namely a mechanism to enable the peers to register their queries and receive notifications when corresponding data-items are added to the network and a means for the peers to advertise their new content. While P2P-based publish-sub scribe systems can infuse these capabilities, developing full-fledged publish-subscribe systems on top of unstructured P2P networks requires complex techniques, and it is often an overkill for many P2P applications. For these applications, we study the alternate continuous query paradigm, which is functionally similar to publish-subscribe systems, but provides best-effort notification guarantees. This paper presents CoQUOS - a scalable and lightweight middleware to support continuous queries in unstructured P2P networks. A key strength of the CoQUOS system is that it can be implemented on any unstructured overlay network. Moreover, CoQUOS preserves the simplicity and flexibility of the overlay network. Central to our design of the CoQUOS middleware is a completely decentralized scheme to register a query at different regions of the P2P network. This mechanism includes two novel components, namely cluster resilient random walk algorithm for propagating query to various regions of the network and dynamic probability-based query registration technique for ensuring that the registrations are well distributed. Our experiments show that the proposed techniques are highly effective and their overheads are low.