What is the recommended server layout for a production Rails application that is expected to receive medium (not heavy, but not light either) traffic? I'm thinking of a setup running Apache and Passenger (mod_rails). How many application servers, web servers, and database servers make sense? How should they be set up to provide redundancy, failover, reliability, etc.?
I know this is a broad question that depends on a lot of factors, but I'm just looking for general advice and suggestions for how this should be approached.
Passenger and Apache or Nginx are great starting points.
The rest basically depends on your budget and requirements and are essentially impossible to give a canned response on.
A single server is a good place to start, and you will probably be surprised by the volume of traffic a single decently spec'd box with the right tuning can turn over.
If you have budget for two servers, a splitting the web server and database is the next logical step.