production.rb

set :aws_servers, `/usr/bin/curl -s 'http://url.aws/show'`.split(', ')
set :rails_env, "production"
set :user,   "deploy"
set :runner, "deploy"
set :branch, "master"

aws_servers.each do |aws_server|
  role :app, aws_server
  role :web, aws_server
end

role :db, aws_servers.sample, primary: true