What I learned setting up delayed_paperclip and Sidekiq

A recent project required an admin user to be able to upload images. Using paperclip we got the uploading going in no time. But, having the uploads going through the app server was very slow (~1.5s requests) so we moved the uploading into a background job. Having never messed with background jobs before I chose to use delayed_paperclip as an easy way to wade into the topic.

Using delayed_paperclip is appealing because it will solve our problem and teach us about background jobs all without having to write custom code. We have to just figure out how to get everything installed.

Railscasts has a great walkthrough of setting up vanilla Sidekiq and Sidekiq’s docs are pretty helpful if you have specific questions.

So, jumping into the code…

The model

We’re working with the Item model, which allows the admin to upload images. Adding

process_in_background :image

is all we need to get things going from the model’s perspective.

# app/models/image.rb
class Item < ActiveRecord::Base
  # other code goes here (validations, relationships, etc)

  has_attached_file :image, style: {
    thumb: '100x100>',
    square: '200x200#'

  process_in_background :image

  # other code goes here

Additions to Gemfile

We’ll need these gems.

# Gemfile
# other gems...
gem 'sidekiq'
gem 'delayed_paperclip'
gem 'sinatra', require: false
gem 'slim'

Testing for user access

We want only admins to be able to upload images so using RSpec, you can test that non admin users receive a RoutingError upon trying to access the Sidekiq page.

expect{visit '/sidekiq'}.to raise_error(ActionController::RoutingError)

Some more specifics can be found on the Sidekiq monitoring wiki

Updated routes

To implement the Sidekiq monitoring page and limit access to it we do the following:

# config/routes.rb
require 'sidekiq/web'
require 'admin_constraint'

YourApp::Application.routes.draw do
  # other routes go here

  mount Sidekiq::Web, at: '/sidekiq', :constraints => AdminConstraint.new


Limiting access

We define the below class to act as the constraint on the ‘/sidekiq’ route. In this way the server responds with a routing error (like we tested for) instead of a not authorized redirect, which stops the server from divulging that the /sidekiq routes exists to those that should not be able to access it.

# lib/admin_constraint.rb
class AdminConstraint

  def mathces?(request)
    return false unless request.session[:user_id]
    user = User.find request.session[:user_id]
    user && user.admin?


Getting it all to work

If your prod app is going to run off a Procfile it’s best to develop using a Procfile as well. Heroku Toolbelt makes this task really simple. You can run your app locally and have it boot up with a Procfile by running

foreman start

Learn more about Heroku and foreman here. More on the Procfile settings in just a bit.

Locally, I still had to fire up a redis server manually. So, if you see

Error connecting to Redis on (ECONNREFUSED)

Check to make sure redis is running (by default, on port 6379).

At this point you should be good to go with image uploading happening in a background job locally.

Deployment checklist

We’re going to deploy to Heroku as they have some nicities that make getting our background jobs off the ground easier so we can see our work in action faster.

  1. Create or update your Procfile to fire up Sidekiq upon launch. Mine looks like:
  web: bundle exec rails server -p $PORT
  worker: bundle exec sidekiq -c 5 -v -q paperclip

Notice that I’m taking a shortcut and creating a new queue (-q paperclip) for Sidekiq instead of using the default queue, which is named ‘default’. I’m sure there are headaches coming down the pipe later on managing multiple queues but in the name of getting things going, I’d recommend naming your queue ‘paperclip’.

  1. I had some issues getting Heroku to automatically start my worker so I had to manually start it using heroku ps:scale worker+1. After a couple dyno restarts, the worker started picking up automatically. So, if your worker isn’t running (you can check by running heroku ps) you can manually start it up.

  2. Heroku provides a plug and play Redis instance that you can install by running

  Heroku addson:add redistogo

Next level resources

I look forward to learning more about advanced background job setups and how best to test these sorts of systems. For when you write your own code to utilize threads check out another great Railscast on the topic.