Currently, the most popular file-sharing applications have used either centralized or flood-based search algorithms. Napster has been successful in providing a centralized index with presumably perfect query recall. Succeeding, more distributed protocols like Gnutella and Kazaa have used flood-based search procedures in which a query is propagated through an unstructured network. Query result quality in such networks suffers as queries for items that are rare have high probabilities of not being found as the entire corpus is not covered during a search. In this paper, we present a new and improved implementation of a distributed file-sharing system yielding (1) query result quality better than flooding and close to a centralized index, and (2) low-maintenance network overhead. These improvements result from our optimized approaches to (a) high churn rates (clients and servers frequently entering and leaving the system) and (b) skewed workloads (high variation in access frequencies vs. key). High churn rates are addressed by keeping all data in soft state, which is periodically refreshed, such that the loss of a server or client is quickly reflected in the indexes; higher refresh rates imply fewer false positives. Skewed workloads are load balanced with the use of a layer of indirection for placing and locating data, such that data is partitioned and distributed based on the frequency of use. A trace-driven prototype evaluation based on Gnutella system traces shows that our prototype implementation achieves a low network bandwidth, attains max-average load ratios within a factor of three across all servers, and has positive recall values for over 90% of all queries, despite a high churn rate; the recall would be 100% absent churn.





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