How many servers are used to host a commercial website like StackOverflow? Are there caching appliances involved?
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This is the server configuration, IIRC:
There's also a backup appliance for regular database backups. Caching is done on the individual web servers (not shared) so one of the goals for HAproxy is to attempt to route users to the same server for repeated requests through IP hashing.
Email traffic is hosted with Google Apps.