Online result cache invalidation for real-time web search

Caches of results are critical components of modern Web search engines, since they enable lower response time to frequent queries and reduce the load to the search engine backend. Results in long-lived cache entries may become stale, however, as search engines continuously update their index to incorporate changes to the Web. Consequently, it is important to provide mechanisms that control the degree of staleness of cached results, ideally enabling the search engine to always return fresh results. In this paper, we present a new mechanism that identifies and invalidates query results that have become stale in the cache online. The basic idea is to evaluate at query time and against recent changes if cache hits have had their results have changed. For enhancing invalidation efficiency, the generation time of cached queries and their chronological order with respect to the latest index update are used to early prune unaffected queries. We evaluate the proposed approach using documents that change over time and query logs of the Yahoo! search engine. We show that the proposed approach ensures good query results (50% fewer stale results) and high invalidation accuracy (90% fewer unnecessary invalidations) compared to a baseline approach that makes invalidation decisions off-line. More importantly, the proposed approach induces less processing overhead, ensuring an average throughput 73% higher than that of the baseline approach.