Their service has a typical small scale structure:
- PHP's Zend Framework 2
- Two m1.medium for web servers
- ELB to split the load
- master/slave MySQL database
- Siege for load testing
The very sensible tips that can handle 30,000 concurrent users per web server:
- Use PHP's APC feature. APC is opcode cache that is "really a requirement in order for a website to have a chance at performing well."
- Put everything that's not a .php request on a CDN. Don't serve static files from your web server. They put everything on S3 and use CloudFront as their CDN. Recent CloudFront problems have caused them to serve directly from S3.
- Don't make connections to other servers in your PHP code. Making connections to other servers blocks the server and slows down processing. Use the APC key/value store for data storage and Varnish for caching full pages.
- Use Varnish. Using Varnish mage the single biggest difference to their performance under a load test.
- Use a c1.xlarge. The c1.xlarge has 8 CPUs which really helps under load. The m1.medium has only 1 CPU for processing requests.
Pretty straightforward, yet good advice. The original article has many more details and is well worth reading.