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.
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.