Rails and Scaling with Multiple Databases. Ryan Tomayko explains how his team spreads a high traffic Rails application across five separate PostgreSQL databases by giving each client their own schema—similar to how WordPress MU scales.
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I should clarify that the site is probably not best described as being "high-traffic". We have a smallish number of users (muuuch smaller than twitter) but we're crunching a lot of data on the back-end - sometimes tens of thousands of records per request. The pattern of load this creates is undoubtedly much different than a high traffic site but still interesting I hope.
By the way, do you have a pointer to literature on the WordPress MU scheme? I'd love to take a look at it.
Here's all I could find: http://photomatt.net/2006/03/01/wordpress-and-lyce um/
You can also grab the code from http://mu.wordpress.org/ and test install it somewhere - installation is pretty painless.