Two-tier cooperation: a scalable protocol for Web cache sharing

The benefits of Web caching can be improved by systems of cooperative cache servers that share their cached documents. The increasing number of Web cache servers over the Internet makes the scalability of the cooperation protocol a major issue to be addressed. In this paper, we propose the Two-Tier Cooperation (2TC) protocol, which is specifically designed for systems of dozens or hundreds of cache servers with no centralized control. 2TC embeds two classical cooperation approaches for distributed Web caching systems, namely informed cooperation (IC) and query cooperation (QC), that are applied within different subsets of cache servers in the system. IC is applied within subsets of close servers and lets them cooperate through mutual exchange of state information related to their cache content. QC lets more distant cache servers cooperate through query/reply messages to locate documents within the global cache. Thanks to the use of IC among close cache servers, QC can explore the cache content of several cache servers through a single query message. High scalability arises as few queries explore the cache content of many cache servers and state information is exchanged within small groups of close cache servers. We report experimental results based on real traces that compare a prototype implementation of 2TC with classical protocols of the informed and query classes. The results point out a strong reduction (up to 50%) of the amount of transferred information to manage cooperation. This overhead reduction is achieved with no performance degradation in terms of latency and cache hit rate.