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Linux Journal: Understudy [Review]

Jul 03, 2000, 22:36 (0 Talkback[s])
(Other stories by Daniel Allen)

"Very interesting efforts are underway to offer completely Linux-based server clustering solutions. These include the open-source Linux Virtual Server, and other work being done by the High Availability Linux Project. These projects show great promise, and they may be the right answer for sites wishing to be close to the bleeding edge. However, small businesses need fully supported solutions that do not require substantial modification to their existing, possibly heterogeneous, networks. This is the gap which Polyserve hopes to fill with Understudy. As you will see below, I think it does the job nicely."

"Understudy is a software-based server clustering utility that implements load balancing and failover protection for Linux (Red Hat, Debian and Slackware), Solaris, Cobalt, FreeBSD and Windows NT. It supports between two and ten heterogeneous servers in a cluster, all of which must be located on the same DNS subnet. Polyserve hopes to release a newer version soon that circumvents the single subnet requirement. A cluster of servers can provide any service, including web, mail, news or file sharing."

"When a server goes down, it is marked inactive within the cluster and another server takes its place in seconds. When the server comes back up, it is immediately reintegrated into the cluster. By using Understudy in conjunction with a load-rotation scheme called ``round robin DNS'', a site can also provide simple load balancing. Load balancing requires one additional IP address for each server in the cluster. Simple failover requires only one IP address for a ``virtual host'', which is how users see the cluster."

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