Abstract
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 the scheduling of requests to the most available server. One common approach is to use the local Domain Name Server (DNS) as a centralized dispatcher. The main problem is that WWW address caching mechanisms (although reducing network traffic) only let this DNS dispatcher control a very small fraction of the requests reaching the Web-server cluster. The non-uniformity of the load from different client domains, and the high variability of real Web workload introduce additional degrees of complexity to the load balancing issue. These characteristics make existing scheduling algorithms for traditional distributed systems not applicable to control the load of Web-server clusters and motivate the research on entirely new DNS policies that require some system state information. We analyze various DNS dispatching policies under realistic situations where state information needs to be estimated with low computation and communication overhead so as to be applicable to a Web cluster architecture. In a model of realistic scenarios for the Web cluster, a large set of simulation experiments shows that, by incorporating the proposed state estimators into the dispatching policies, the effectiveness of the DNS scheduling algorithms can improve substantially, in particular if compared to the results of DNS algorithms not using adequate state information.