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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:

  • web (
  • web (
  • web ( / careers)
  • web (
  • web (meta / / HAproxy load balancer for in a VM / cacti monitoring / etc
  • database (all sites)

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.

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Where is sstatic hosted? – Ólafur Waage Nov 4 '09 at 23:05
Wait, am I understanding this correctly? There are just 2 front end web servers for serving all of Stack Overflow's traffic? – orokusaki Jan 29 '10 at 4:20
@orokusaki: there are now three - the third was added just a few weeks ago. the real miracle is that all the sites run off of one database server. – Kyle Cronin Jan 29 '10 at 5:59
That is incredible. I've always wondered what it takes to run such a large website. That makes Digg look like even more of an epic fail considering that years ago when their traffic was just a fraction of what it is now, they were running 17 servers and barely making it. It's very exciting, for my application that I'm working on slowly, that one DB server could handle all that. Do you know how much Ram / Procs / Other hardware specs there are? – orokusaki Jan 29 '10 at 16:32
@orokusaki this should answer your questions… . it mentions two database servers, but one is a backup in case of failure in the primary one. – Kyle Cronin Jan 29 '10 at 22:08

Here is an update, from

Network Diagram

For more information, see the Blog post.

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I thought the HAProxy box was a virtualized UNIX server, so I don't know it would count as an additional "box". – JMP Nov 4 '09 at 23:03
Well, it is still involved. I'll clarify. – John Gietzen Nov 4 '09 at 23:07

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