Detecting what server Rails is running under

  # Detects what server your Rails app is running under.
  # Either passenger, fastcgi, mongrel, webrick or thin

  def running_under
    # For passenger, IN_PHUSION_PASSENGER is key
    is_passenger = (defined?(IN_PHUSION_PASSENGER) and IN_PHUSION_PASSENGER)
    
    # Not sure about fastcgi, but this should do the trick
    is_fastcgi = (Kernel.const_get('RailsFCGIHandler') if defined?(FCGI)) rescue false
    
    # Webrick is require'd only as a last resort
    is_webrick = !defined?(WEBrick).nil?
    
    # For mongrel and thin, seems best to look for instances
    is_mongrel = false
    ObjectSpace.each_object(Mongrel::HttpHandler) { is_mongrel = true; break; } if defined?(Mongrel)
    is_thin = false
    ObjectSpace.each_object(Thin::Runner) { is_thin = true; break; } if defined?(Thin)
    
    # Earlier versions of passenger don't have IN_PHUSION_PASSENGER, so here's a workaround
    unless is_fastcgi or is_mongrel or is_webrick or is_thin
      is_passenger = defined?(Passenger) and Passenger.class == Module
    end
    
    if is_passenger
      "Passenger"
    elsif is_fastcgi
      "FastCGI"
    elsif is_mongrel
      "Mongrel"
    elsif is_webrick
      "WEBrick"
    elsif is_thin
      "Thin"
    else
      "Unknown"
    end
  end