Document replication and distribution in extensible geographically distributed web servers

A geographically distributed web server (GDWS) system consists of multiple server nodes interconnected by a metropolitan area network (MAN) or a wide area network (WAN). It can achieve better eÆciency in handling ever-increasing web requests than centralized web servers because its throughput will not be limited by available bandwidth connecting to a central server. The key research issue in the design of GDWS is how to replicate and distribute the documents of a website among the server nodes. This paper proposes a density-based replication scheme and applies it to our proposed Extensible GDWS architecture. We adopted a partial duplication scheme where document replication targets only at hot objects in a website. To distribute the replicas generated via the density-based replication scheme, we propose four di erent document distribution algorithms: Greedy-cost, Maximal-density, Greedy-penalty, and Proximity-aware. A proximity-based routing mechanism is designed to incorporate these algorithms for achieving better web server performance in a WAN environment. Simulation results show that the Greedy-penalty algorithm yields most stable load balancing performance, and the Greedy-cost algorithm causes least internal traÆc. Our scheme can achieve 80% of the performance of full-replication, with half the disk space.