Controlled Local Replication for Hash Routing among Proxy Web Caches

This paper studies controlled local replication for hash routing, such as CARP, among a collection of loosely-coupled proxy web cache servers. Hash routing partitions the entire URL space among the shared web caches, creating a single logical cache. Each partition is assigned to a cache server. Client requests for non-assigned-partition objects are forwarded to sibling caches. Duplication of cache contents is eliminated and total incoming traffic to the shared web caches is minimized. However, the 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. Trace-driven simulations are conducted to study the performance impact of local replication. The results show that (1) regardless of cache sizes, with a controlled 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 reduce inter-cache traffic without increasing incoming traffic; and (3) local replication is effective even if clients are dynamically assigned to different cache servers.