This paper describes a highly available distributed video ondemand (VoD) service which is inherently fault tolerant. The VoD serviceis provided by multiple servers that reside at different sites. Newservers may be brought up “on the fly” to alleviate the loadon other servers. When a server crashes it is replaced by another serverin a transparent way; the clients are unaware of the change of serviceprovider. In test runs of our VoD service prototype, such transitionsare not noticeable to a human observer who uses the service. Our VoDservice uses a sophisticated flow control mechanism and supportsadjustment of the video quality to client capabilities. It does notassume any proprietary network technology: it uses commodity hardwareand publicly available network technologies (e.g., TCP/IP, ATM). Ourservice may run on any machine connected to the Internet. The serviceexploits a group communication system as a building block for highavailability. The utilization of group communication greatly simplifiesthe service design
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