Rails Asset Pipeline on Heroku Cedar

Last Updated: 25 August 2014

cedar rails ruby

Table of Contents

Rails applications running on the Heroku Cedar stack can have the asset pipeline compiled locally, at deploy-time, or at run time. For new users, we recommend reading Getting Started with Rails 3.x on Heroku before proceeding further.

Heroku recommends using the asset pipeline with a CDN to increase end user experience and decrease load on your Heroku app. If you have questions about Ruby on Heroku, consider discussing it in the Ruby on Heroku forums.

The Rails 4 Asset Pipeline

If you are using Rails 4 and the asset pipeline it is recommended to familiarize yourself with the document below. Much of the behavior between Rails 3 and Rails 4 is similar. Once finished please read the Rails 4 Asset Pipeline on Heroku which covers the differences between the two experiences.

The Rails 3 Asset Pipeline

The Rails 3 asset pipeline is supported on Heroku’s Cedar stack. The new pipeline makes assets a first class citizen in the Rails stack. By default, Rails uses CoffeeScript for JavaScript and SCSS for CSS. DHH has a great introduction during his keynote for RailsConf.

The Rails asset pipeline provides an assets:precompile rake task to allow assets to be compiled and cached up front rather than compiled every time the app boots.

There are two ways you can use the asset pipeline on Heroku.

  1. Compiling assets locally.
  2. Compiling assets during slug compilation.

Compiling assets locally

If a public/assets/manifest.yml is detected in your app, Heroku will assume you are handling asset compilation yourself and will not attempt to compile your assets. Rails 4 uses a file called public/assets/manifest-<md5 hash>.json instead. On both versions you can generate this file by running $ rake assets:precompile locally and checking the resultant files into Git.

To compile your assets locally, run the assets:precompile task locally on your app. Make sure to use the production environment so that the production version of your assets are generated.

RAILS_ENV=production bundle exec rake assets:precompile

A public/assets directory will be created. Inside this directory you’ll find a manifest.yml which includes the md5sums of the compiled assets in Rails 3. In Rails 4 the file will be manifest-<md5 hash>.json. Adding public/assets to your git repository will make it available to Heroku.

git add public/assets
git commit -m "vendor compiled assets"

Now when pushing, the output should show that your locally compiled assets were detected:

-----> Preparing Rails asset pipeline
       Detected manifest.yml, assuming assets were compiled locally

Compiling assets during slug compilation

If you have not compiled assets locally, we will attempt to run the assets:precompile task during slug compilation. Your push output will show:

-----> Preparing Rails asset pipeline
       Running: rake assets:precompile

Please see the Troubleshooting section below on explanations of how the rake task works during our slug compilation process.

Failed assets:precompile

If the assets:precompile task fails, the output will be displayed and the build will exit.

Asset caching

Caching of static assets can be implemented in-application using the Rack::Cache middleware or in a more distributed fashion with a CDN. Serving assets from your application does require dyno-resources so please consider an appropriate asset caching strategy for your needs.


Failures in the assets:precompile task

In Rails 3.x, you can prevent initializing your application and connecting to the database by ensuring that the following line is in your config/application.rb:

config.assets.initialize_on_precompile = false

Do not forget to commit to git after changing this setting.

In Rails 4.x this option has been removed and is no longer needed.

When deploying if you see something similar to the following:

could not connect to server: Connection refused
Is the server running on host "" and accepting
TCP/IP connections on port xxxx?

This means that your app is attempting to connect to the database as part of rake assets:precompile. Because your database may not be available, the command may fail. You should be able to see the same error by running this command:

       PATH=$PATH \
       DATABASE_URL=postgres://user:pass@ \
       /bin/sh -c 'bundle exec rake --trace  assets:precompile'

The above command uses env -i which runs everything following it without any environment variables. We then manually pass in GEM_PATH, PATH and a nonexistent DATABASE_URL. Finally we run the command using bin/sh -c this tells our shell to execute bundle exec rake --trace assets:precompile.

Note If you are using another database you will need to replace postgres with the database adapter you are using as detected from your Gemfile

If rake assets:precompile is still not working, you can debug this locally by configuring a nonexistent database in your local config/database.yml and attempting to run rake assets:precompile. Ideally you should be able to run this command without connecting to the database.


If you were previously using therubyracer or therubyracer-heroku, these gems are no longer required and strongly discouraged as these gems use a very large amount of memory.

A version of Node is installed by the Ruby buildpack that will be used to compile your assets.

Updating PATH

If you need to compile assets at runtime, you must add bin to your PATH to access the JavaScript runtime. Check your current configuration using heroku config:

$ heroku config
PATH => vendor/bundle/ruby/1.9.1/bin:/usr/local/bin:/usr/bin:/bin

If your PATH variable does not include bin on its own, update it by running:

$ heroku config:set PATH=bin:vendor/bundle/ruby/1.9.1/bin:/usr/local/bin:/usr/bin:/bin
Adding config vars:
  PATH => vendor/bundle/ru...usr/bin:/bin:bin
Restarting app... done, v7.

No debug output at all

If you see no debug output and your asset:precompile task is not run, ensure that rake is in your Gemfile and properly committed.