Application Error

h2. Errors and Environments Rails has the concept of [[Environments|environment]] to represent the stages of an application's lifecycle (test, development, and production are the defaults). Errors are handled as appropriate for the deployment target. The *test and development environments provide developer-relevant error handling whereas the production environment returns an error page with HTTP status 500* (Application Error). "ActionController::Rescue":http://rails.rubyonrails.com/classes/ActionController/Rescue.html is the module responsible for error handling. h2. Error Pages in your Application It is possible to configure the error page returned by your Rails application, in either a global or controller specfifc way. By combining these approaches you can have a global default and override it in individual controllers. h3. Globally Navigate to your apps /controller directory and edit application.rb:


  def rescue_action_in_public(exception)
    render_text "There was a global error processing your request.
"
  end
  def local_request?
    false
  end

h3. Controller specific Edit the controller you would like to have respond to errors differently to include it own implementation of rescue_action_in_public:


  def rescue_action_in_public(exception)
    render_text "There was a controller specfic error processing your request.
"
  end

These examples show simple text rendering for error pages, but there are many other possibilities(such as email, logging, redirects, etc.) in the @rescue_action_in_public@ method(see HowtoSendEmailWhenRailsThrowsAnException, for example). h3. Pre-controller Some errors can occur before any controller is loaded. In those cases, nothing defined in ApplicationController will help, since the controller isn't around to deal with the exception. To deal with this you can redefine @rescue_action_in_public@ in @ActionController::Base@ in a seperate file and pull it in through @/config/environment.rb@. */config/environment.rb*


# Include your app's configuration here:
require 'error_handler_basic' # defines AC::Base#rescue_action_in_public

*/lib/error_handler_basic.rb*


class ActionController::Base
  def rescue_action_in_public(exception)
    render_text <


...

TOKEN
  end  
end

h3. Pre-Pre-controller The above advice doesn't always help, since in many cases where an error happens too early to be handled by your controller's @rescue_action_*@ method, it will also be too early to be handled by @ActionController::Base#rescue_action_in_public@. An example of such an error is if an error occurs while recreating the session object. Rails doesn't seem to provide any clean way to handle these errors, they just result in an exception escaping the @Dispatcher.dispatch@ method, which is handled in a different way depending what web server you're running under. I override the @Dispatcher.dispatch@ method with a version which has an extra level of error handling to catch more errors and handle them in a web-server independant way. Add this to your environment.rb, and modify to your taste:

class << Dispatcher
  def dispatch(cgi = CGI.new,
               session_options = ActionController::CgiRequest::DEFAULT_SESSION_OPTIONS)
    begin
      request, response =
        ActionController::CgiRequest.new(cgi, session_options),
        ActionController::CgiResponse.new(cgi)
      prepare_application
      ActionController::Routing::Routes.recognize!(request).process(request, response).out
    rescue Object => exception
      begin
        ActionController::Base.process_with_exception(request, response, exception).out
      rescue
        # The rescue action above failed also, probably for the same reason
        # the original action failed.  Do something simple which is unlikely
        # to fail.  You might want to redirect to a static page instead of this.
        e = exception
        cgi.header("type" => "text/html")
        cgi.out('cookie' => '') do
          <<-RESPONSE
		
		  Application Error
		  
		    Application Error
		    #{e.class}: #{e.message}

		    #{e.backtrace.join("\n")}
		  
		
	      RESPONSE
        end
      end
    ensure
      reset_application
    end
  end
end

h2. Notes h3. Set @consider_all_requests_local@ to @false@ When working on this functionality locally you will need to make sure that your @/config/environments/development.rb@ has the following set in it:


ActionController::Base.consider_all_requests_local = false

h3. Request the real IP Also, despite defining @local_request?@ as above, you may have to change the address used to make the request. Neither @localhost@, not @127.0.0.1@ work, instead use the actual IP address of your machine.

Question: Does this mean then that there is no simple way to define the default error page for a controller, a way that works whether in development or production or trying it out on localhost? Not-really-an-answer: While the above instructions are not painless, they aren't difficult either, and they will allow you to see your custom error page durring development, if you like.

h2. See also * DynamicErrorPagesWish category: Howto