StackOverflow: 560 million impressions per month, 25 servers

StackOverflow has published traffic statistics for all StackExchange sites. StackExchange currently ranks 54th on the Internet in terms of audience. The site has registered 4 million users, published 8 million questions and 40 million answers. Attendance is 560 million views per month.

Surprisingly, only 25 servers are enough for StackOverflow to work. But since the CPU load is about 10-15%, 5 servers could do all the work.

Statistics

• 100% annual attendance growth
• 2 TB of SQL data on SSDs
• Each server has two 320 GB SSDs in RAID 1
• Read-write ratio 40:60
• On database servers, the average CPU load is 10%
• 11 web servers on IIS
• 2 servers for load balancers, 1 active, HAProxy installed
• 4 active database nodes, MS SQL installed
• 3 application servers with a tag search engine
• 3 servers with the ElasticSearch search engine
• 2 machines with distributed cache under Redis
• 2 network hubs (Nexus 5596 + Fabric Extenders)
• 2 Cisco 5525-X ASA Firewalls
• 2 Cisco 3945 Routers
• 2 read-only SQL servers for use with the Stack Exchange API
• Virtual machines for various functions