Consistent Hashing. Beautifully clear explanation of consistent hashing, a simple technique that allows you to add new caching servers to a cluster without re-hashing your keys and hence invalidating all of your caches.
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That article by Tom Kleinpeter inspired me to write Flexihash, a consistent hashing implementation for PHP, open source under the MIT licence. Just in case anybody else building scalable web apps is still using PHP ;)