Linux Journal: Efficiently Updating Web Sites on Clusters

“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…”

Complete
Story