EC2 instances are not reliable: they degrade all the time. The solution, recommended by Amazon, is to have redundancy. So, plan on having at least two of every

EC2 instances are not reliable: they degrade all the time. The solution, recommended by Amazon, is to have redundancy. So, plan on having at least two of every type of server or using their services like RDS. With EC2 my approach has always been that if you've got enough traffic to justify it, you should move rather quickly to a load-balanced solution. This really forces you to stop relying on what's stored on any particular server, and focus your energy on making sure you can boot new ones and get them seamlessly into the rotation. Obviously this doesn't work for stuff like databases (for which I definitely recommend RDS or SimpleDB if they suit your needs), but even there, if you're hosting your own and make sure that the data goes on to EBS volumes instead of the instance stores, you can set things up so that you snapshot those volumes regularly and can boot up new database servers without trouble. Services like Rightscale can help manage stuff like this pretty easily, too, though they're probably a bit expensive for startups (and you've got to be willing to put in a little setup time, I've always found that their templates needed pretty substantial customization to suit my needs); in any case, Amazon is starting to pull more of that type of functionality into AWS itself, with stuff like CloudFormation and Elastic Beanstalk, so if I was starting now, I'd probably give those a closer look.