Get response status after every controller is executed in Ruby on Rails

Question

I have a RoR Rest API and I want to emit a metric with the status of each response of my API. I managed to do that for all the cases except from those where the controller is crashing.

For example with the following code in the ApplicationController:

require 'statsd-ruby'

class ApplicationController < ActionController::API
  after_action :push_status_metric

  def push_status_metric
    statsd = Statsd.new ENV['STATSD_LOCATION'], ENV['STATSD_PORT']
    puts normalize_status_metric(response.status)

    statsd.increment('ds.status.' + normalize_status_metric(response.status).to_s + '.int') unless request.fullpath == '/health'
  end

  private

  def normalize_status_metric(status)
    return 100 if status >= 100 && status < 200
    return 200 if status >= 200 && status < 300
    return 300 if status >= 300 && status < 400
    return 400 if status >= 400 && status < 500
    return 500 if status >= 500 && status < 600
    0
  end
end

But this solution doesn't capture errors such as ActionController::RoutingError and ActiveRecord::RecordNotFound.

I tried the following code:

  rescue_from StandardError do |exception|
    statsd = Statsd.new ENV['STATSD_LOCATION'], ENV['STATSD_PORT']
    statsd.increment('ds.status.' + normalize_status_metric(response.status).to_s + '.int') unless request.fullpath == '/health'

    raise exception
  end

But when this callback is executed, the response.status value is always 200 (seems like it's not set yet by the framework up to this point).

Answer 1

Knowing the the rails logger manages to do this, we can take a look at its class, ActionController::LogSubscriber and that process_action method. So, the status in that event could be nil, and we can see how they then convert an exception to a status, if an exception exists:

status = payload[:status]
if status.nil? && payload[:exception].present?
  exception_class_name = payload[:exception].first
  status = ActionDispatch::ExceptionWrapper.status_code_for_exception(exception_class_name)
end

So now, we can do something similar by subscribing to this event on our own, with Active Support Instrumentation, by creating an initializer:

ActiveSupport::Notifications.subscribe 'process_action.action_controller' do |*args|
  event = ActiveSupport::Notifications::Event.new(*args)

  # opening a file here is probably a performance nightmare, but you'd be doing something with statsd, not a file, anyway
  open('metrics.txt', 'a') do |f|
    # get the action status, this code is from the ActionController::LogSubscriber
    status = event.payload[:status]

    if status.nil? && event.payload[:exception].present?
      exception_class_name = event.payload[:exception].first

      status = ActionDispatch::ExceptionWrapper.status_code_for_exception(exception_class_name)
    end

    f.puts "process_action.action_controller controller: #{event.payload[:controller]} - action: #{event.payload[:action]} - path: #{event.payload[:path]} - status: #{status}"
  end
end

and hitting it a few times with a simple controller:

class HomeController < ApplicationController
  def non_standard_status
    render html: 'This is not fine', status: :forbidden
  end

  def error
    raise ActiveRecord::RecordNotFound, 'No Records Found'
  end

  def another_error
    raise ArgumentError, 'Some Argument is wrong'
  end

  def this_is_fine
    render html: 'This is fine'
  end
end

yields a file:

process_action.action_controller controller: HomeController - action: non_standard_status - path: /forbidden - status: 403
process_action.action_controller controller: HomeController - action: error - path: /error - status: 404
process_action.action_controller controller: HomeController - action: another_error - path: /error2 - status: 500
process_action.action_controller controller: HomeController - action: this_is_fine - path: /fine - status: 200