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Most of the techniques used for increasing the availability of Web services do not provide fault tolerance for requests being processed at the time of server failure. Other schemes require deterministic servers or changes to the Web client. These limitations are unacceptable for many current and future applications of the Web. We have developed an efficient implementation of a client-transparent mechanism for providing fault-tolerant Web service that does not have the limitations mentioned above. The scheme is based on a hot standby backup server that maintains logs of requests and replies. The implementation includes modifications to the Linux kernel and to the Apache Web server, using their respective module mechanisms. We describe the implementation and present an evaluation of the impact of the backup scheme in terms of throughput, latency, and CPU processing cycles overhead.
Date of Conference: 14-16 Oct. 2002