Last updated December 15, 2020

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

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 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

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.

Troubleshooting

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.

Before you can compile your assets on Heroku, you’ll need to be able to compile them locally, run this command to debug your assets:

$ RAILS_ENV=production bundle exec rake assets:precompile

This should complete with no errors. Do NOT check in the assets into git after running this command.

therubyracer

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.

Compile Set to True in Production

If you have enabled your application to config.assets.compile = true in production, your application might be very slow. This was best described in a stack overflow post:

When you have compile on, this is what happens: Every request for a file in /assets is passed to Sprockets. On the first request for each and every asset it is compiled and cached in whatever Rails is using for cache (usually the filesystem). On subsequent requests Sprockets receives the request and has to look up the fingerprinted filename, check that the file (image) or files(css and js) that make up the asset were not modified, and then if there is a cached version serve that.

This setting is also known to cause other run-time instabilities and is generally not recommended. Instead we recommend either precompiling all of your assets on deploy (which is the default) or if that is not possible compiling assets locally.