Local replication for proxy web caches with hash routing

Abstract

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. Duplication of cache contents is eliminated and total incoming traffic to the shared web caches is minimized. Client requests for non-assigned-partition objects are forwarded to sibling caches. However, request forwarding increases not only inter-cache traffic but also cpu utilization, thus slows the client response time. We propose a controlled local replication of non-assigned-partition objects in each cache server to effectively reduce the inter-cache traffic. We use a multiple-exit LRU to implement controlled local replication. 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 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 reduce inter-cache traffic without increasing incoming traffic; and (3) local replication is effective even if clients are dynamically assigned to different cache servers.