Controlled replication for hash routing-based web caching

Abstract

This paper studies controlled local replication for hash routing, such as CARP, among an array of cooperating proxy web caches. Hashing partitions the entire URL space among the cooperating web caches, creating a single logical cache. Each partition is assigned to a cache server. Duplication of cache contents is eliminated and total incoming traffic to the cooperating web caches is minimized. When hashing is implemented on the caching servers, client requests for non-assigned-partition objects are forwarded to sibling caches. Thus, response times of client requests can increase substantially due to increased inter-cache traffic and CPU overhead. We propose a controlled local replication of non-assigned-partition objects in each cache server to effectively reduce the inter-cache traffic due to request forwarding. Trace-driven simulations are conducted to study the performance impact of local replication, as replication reduces the effective size of the logical cache. The results show that (1) regardless of cache sizes, with a controlled local replication, the average response time, inter-cache traffic and CPU overhead can be effectively reduced without noticeable increases in incoming traffic; (2) for very large cache sizes, a larger amount of local replication can be allowed to greatly reduce inter-cache traffic without increasing incoming traffic; and (3) local replication is effective even if clients are dynamically assigned to different cache servers.