An Adaptive Approach for Downloading Replicated Web Resources

The success of a Web service is largely dependent on its responsiveness (i.e., its availability and timeliness) in the delivery of the information its users (clients) require. A practical approach to the provision of responsive Web services is based on introducing redundancy in the service by replicating the service itself across a number of servers geographically distributed over the Internet. Provided that the replica servers be maintained mutually consistent, service responsiveness can be guaranteed by dynamically binding the client to the most convenient replica (e.g., the nearest, lightly loaded, available replica; the available replica with the least congested connection to the client). Based on this approach, we have developed a software mechanism [GHI01] that meets effectively the responsiveness requirement mentioned above. In essence, this mechanism, rather than binding a client to its most convenient replica server, engages all the available replicas in supplying a fragment of the Web document that client requires. The size of the fragment a replica is requested to supply is dynamically evaluated on the basis of the response time that replica can provide its client with. In addition, the proposed mechanism can dynamically adapt to changes in both the network and the replica servers status, thus tolerating possible replica or communication failures that may occur at runtime. Our mechanism can be implemented either as part of the browser software or as part of a Proxy server. In this paper, we describe the design, the development, and the performance evaluation of both these implementations of our mechanism. The performance results we have obtained from our evaluation exercise illustrate the adequacy of the mechanism we propose, in order to provide responsive Web services.