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Linux Journal: Efficiently Updating Web Sites on Clusters

Jan 15, 2003, 13:00 (0 Talkback[s])
(Other stories by M. David Minnigerode)

"Management said the requirements for the web site infrastructure were simple. We could expect thousands of visitors to our site at any time over the next few months, as the marketing effort took hold. At any given time we would need to support up to a few hundred concurrent downloads of our desktop product demo. A noticeable performance drop in our web site was not acceptable.

"At the time, our web site ran on a 2.2.x Linux distribution on a dual Dell 2450. Its performance was rock solid, but we were uneasy relying on a single machine for our entire business. We determined that the answer was to replace the single server with a cluster. For less than the cost of the Dell, we built a cluster of four 1U one-processor machines. The cluster's performance was excellent. By stripping down Apache, we could support at least 400 downloads over HTTP and still have a responsive site. This left one problem: we needed to be able to update the site often without affecting the performance.

"Typical strategies for doing these frequent updates were not satisfactory. Either the site would be down for more than a few moments when the update occurred, or the site would be in an inconsistent state during the update. Worst of all, the site could be left in an inconsistent state if the update failed part-way through the process. To overcome these drawbacks I applied a little cross-discipline creativity. By applying the page flipping technique from the graphics world, I was able to achieve a quick and non-intrusive method of updating the clustered web site..."

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