I hacked up a session data handler to optionally keep login session data in memcached rather than the local filesystem. Combined with a change to the cookie settings, the English Wikipedia login sessions can now be shared between en.wikipedia.org on larousse and en2.wikipedia.org on pliny, without having to separately log in on each server.

This makes it not too painful to use a rewrite rule for simple load balancing, bouncing some portion of the page space to en and some portion to en2. Yay!

There may still be some issues dealing with uploaded files, but disabling uploads on en2, ensuring that Special:Upload is redirected to en, and that all /upload/* files get loaded off en, should keep things pretty consistent for the meantime.

Potential problems: * Memcached is designed to be 'lossy'; if it fills up it throws away the least recently used data to make room for new data. This shouldn't be a problem for session handling, hopefully; they're set to expire after an hour of non-usage anyway.

* Expiration times haven't been thoroughly tested yet.

* There's currently no failsafe; if memcached dies, the session data falls into a black hole and no one can log in. (Except perhaps if they click 'remember my password'.) It should be possible to check that it couldn't contact the server and fall back to local storage, or the database, or something.

-- brion vibber (brion @ pobox.com)