How do massively scalable sites like Facebook and Google implement sessions?

Does anyo开发者_开发技巧ne have any insight into their system architecture? Do they use Memcache? Surely every time I click on Facebook my HTTP requests aren't being channeled to the same server where my session is in memory?

A basic solution would be to store the session identifier & associated state in a database (or equivalent). This allows any application node in a cluster to access the session.

In practice, such a solution is slow and read-through caching (e.g. with Memcache), session replication, etc. are used to improve performance.