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Lets assume I have a tumblr account:

http://sheehan.tumblr.com

Tumblr allows it so that it can point to www.sheehan.com if I enter an A-Record.

How does Tumblr scale this to millions of users? How is it possible to assign a single A-Record to multiple subdomains?

Sheehan Alam
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basically there is an apache/whatever header directive that redirects it to your site. It will be a very small amount of data ( check out 301/ moved permanently headers for more ).

For scaling it to millions, this kind of data ( username (key) -> custom domain(value) ) you could store the data for all the users in a memory mapped hash and then put a squid ( or similar ) proxy to cache.

Rohan Monga
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