Efficient state estimators for load control policies in scalable Web server clusters

Replication of information across a server cluster provides a promising way to support popular Web sites. However a Web server cluster requires some mechanism for directing requests to the best server. One common approach is to use the Domain Name Server (DNS) as a centralized schedule. However address caching mechanisms and the non-uniformity of the load from different client domains complicate the load balancing issue and make existing scheduling algorithms for traditional distributed systems not applicable to Web server clusters. We consider the theoretical DNS policies that require some system state information. We extend them to realistic situations where state information needs to be estimated with low computation and communication overhead. We show that by incorporating these estimators into the DNS policies, load balancing improves substantially, even if the DNS control is limited to a small portion of client requests.