The website has 0.56 billion PVS per month, but only 25 servers are used.

The website has 0.56 billion PVS per month, but only 25 servers are used.
The Q & A community network StackExchange consists of more than 100 websites, including StackOverflow, which is ranked 54th by Alexa. StackExchang has 4 million users, with a monthly PV of 0.56 billion million,However, only 25 servers are used.And the CPU load is not high. It does not use cloud computing, because cloud computing may slow down, making it more difficult to optimize and eliminate system faults. StackOverflow still uses Microsoft's architecture. It is very practical. Microsoft's infrastructure can work effectively and is cheap enough. There is no convincing reason to make changes. But this does not mean that it does not use Linux. It uses Linux in a meaningful place. Its Windows server runs Windows 2012 R2 and Centos 6.4 on Linux server. The size of website database ms SQL is 2 TB, all stored on SSD. It has 11 Web servers running IIS and 2 load balancing servers running HAProxy, two cache servers running Redis.

Detailed technical solution please move: http://highscalability.com/blog/2014/7/21/stackoverflow-update-560m-pageviews-a-month-25-servers-and-i.html