What does it take to run a web app with 5K – 10K users?
The most difficult thing is going to be getting to 10K active users :)These days RAM is cheap and SSD storage is also widely available. For a very long time, one of my side projects with 50K users was hosted in a EC2 small instance. With that out of the way, here are a few things you will need to take care of:* Security (especially passwords) - Rails should take care of most of this for you, but you should ensure that you patch vulnerabilities when they are discovered. Also, stuff like having only key-based login to your servers etc.* Backups - Take regular backups of all user data. It's also VERY important that you actually try restoring the data as well, as it's quite possible that backups are not occurring properly.* One click deployment - Use Capistrano or Fabric to automate your deployments.* A good feedback/support system - this could even be email to begin with (depending on the volume you expect), but it should be accessible.* Unit tests - as your app grows in complexity, you will never be able to test all the features manually. I'm not a big fan of test driven development, but really, start writing unit tests as soon as you have validated your product idea.* Alerts, monitoring and handling downtime - Downtimes are inevitable. Your host or DNS could go down, you might run out of disk space, etc. Use something like Pingdom to alert you of such failures.* Logging, logging, logging - I can't stress on this enough. When things break, logging is crucial in piecing together what happened. Use log rotation to archive old logs so they don't hog the disk space.