Geographic Load Balancing for Scalable Distributed Web Systems

Users of highly popular Web sites may experience long delays when accessing information. Upgrading content site infrastructure from a single node to a locally distributed Web cluster composed by multiple server nodes provides a limited relief, because the cluster wide-area connectivity may become the bottleneck. A better solution is to distribute Web clusters over the Internet by placing content nodes in strategic locations. A geographically distributed architecture where the Domain Name System (DNS) servers evaluate network proximity and users are served from the closest cluster reduces network impact on response time. On the other hand, serving closest requests only may cause un-balanced servers and may increase system impact on response time. To achieve a scalable Web system, we propose to integrate DNS proximity scheduling with an HTTP request redirection mechanism that any Web server can activate. We demonstrate through simulation experiments that this further dispatching mechanism augments the percentage of requests with guaranteed response time, thereby enhancing the Quality of Service of geographically distributed Web sites. However, HTTP request redirection should be used selectively because the additional round-trip increases network impact on latency time experienced by users. As a further contribution, this paper proposes and compares various mechanisms to limit reassignments with no negative consequences on load balancing.