5Tuning the Capacity of Search Engines: Load-Driven Routing and Incremental Caching to Reduce and Balance the Load

This article introduces an architecture for a document-partitioned search engine, based on a novel approach combining collection selection and load balancing, called load-driven routing. By exploit-ing the query-vector document model, and the incremental caching technique, our architecture can compute very high quality results for any query, with only a fraction of the computational load used in a typical document-partitioned architecture. By trading off a small fraction of the results, our technique allows us to strongly reduce the computing pressure to a search engine back-end; we are able to retrieve more than 2/3 of the top-5 results for a given query with only 10 % the computing load needed by a configuration where the query is processed by each index partition. Alternatively, we can slightly increase the load up to 25 % to improve precision and get more than 80 % of the top-5 results. In fact, the flexibility of our system allows a wide range of different configurations, so as to easily respond to different needs in result quality or restrictions in computing power. More important, the system configuration can be adjusted dynamically in order to fit unexpected query peaks or unpredictable failures. This article wraps up some recent works by the authors, showing the results obtained by tests conducted on 6 million documents, 2,800,000 queries and real quer