Replication for load balancing and hot-spot relief on proxy web caches with hash routing

Hash routing is an emerging approach to coordinating a collection of collaborative proxy caches. Hash routing partitions the entire URL space among the proxy caches. Each partition is assigned to a cache server. Duplication of cache contents is eliminated. Client requests to a cache server for non-assigned-partition objects are forwarded to proper sibling caches. In the presence of access skew, the load level of the cache servers can be quite unbalanced, limiting the benefits of hash routing. We examine an adaptable controlled replication (ACR) of non-assigned-partition objects in each cache server to reduce the load imbalance and relieve the problem of hot-spot references. Trace-driven simulations are conducted to study the effectiveness of ACR. The results show that (1) access skew exists, and the load of the cache servers tends to be unbalanced in hash routing; (2) with a relatively small amount of ACR, say 10% of the cache size, significant improvements in load balance can be achieved; (3) ACR provides a very effective remedy for load imbalance due to hot-spot references; and (4) increasing the cache size does not improve load balance unless replication is allowed.