Synchronises Assets between Rails and S3
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Asset Sync

Synchronises Assets between Rails and S3.

Asset Sync is built to run with the new Rails Asset Pipeline feature introduced in Rails 3.1. After you run bundle exec rake assets:precompile your assets will be synchronised to your S3 bucket, optionally deleting unused files and only uploading the files it needs to.

This was initially built and is intended to work on Heroku but can work on any platform.


If you are upgrading from a version of asset_sync < 0.2.0 (i.e. 0.1.x). All of the references to config variables have changed to reference those used in Fog. Ensure to backup your asset\_sync.rb or asset\_sync.yml files and re-run the generator. You may also then need to update your ENV configuration variables (or you can change the ones that are referenced).


We are currently trying to talk with Heroku to iron these out.

  1. Will not work on heroku on an application with a RAILS_ENV configured as anything other than production
  2. Will not work on heroku using ENV variables with the configuration as described below, you must hardcode all variables


When you see rake assets:precompile during deployment. Heroku is actually running something like

env RAILS_ENV=production DATABASE_URL=scheme://user:pass@ bundle exec rake assets:precompile 2>&1

This means the RAILS_ENV you have set via heroku:config is not used.

Workaround: you could have just one S3 bucket dedicated to assets and ensure to set keep the existing remote files

AssetSync.configure do |config|
  config.fog_directory = 'app-assets'
  config.existing_remote_files = "keep"

2. ENV varables not available

Currently when heroku runs rake assets:precompile during deployment. It does not load your Rails application's environment config. This means using any ENV variables you could normally depend on are not available. For now you can just run heroku run rake assets:precompile after deploy.

Workaround: you could just hardcode your AWS credentials in the initializer or yml

AssetSync.configure do |config|
  config.aws_access_key_id = 'xxx'
  config.aws_secret_access_key = 'xxx'
  config.fog_directory = 'mybucket'


Add the gem to your Gemfile

gem "asset_sync"

The following steps are now optional as of version 0.1.7 there is a built-in initializer lib/engine.rb

Generate the rake task and config file

rails g asset_sync:install

If you would like to use a YAML file for configuration instead of the default (Rails Initializer) then

rails g asset_sync:install --use-yml

The default provider is AWS but you can pick which one you need.

rails g asset_sync:install --provider=Rackspace
rails g asset_sync:install --provider=AWS


Configure config/environments/production.rb to use Amazon S3 as the asset host and ensure precompiling is enabled.

# config/environments/production.rb
config.action_controller.asset_host = do |source, request|
  request.ssl? ? "https://#{ENV['FOG_DIRECTORY']}" : "http://#{ENV['FOG_DIRECTORY']}"

We support two methods of configuration.

  • Rails Initializer
  • A YAML config file

Using an Initializer is the default method and is best used with environment variables. It's the recommended approach for deployments on Heroku.

Using a YAML config file is a traditional strategy for Capistrano deployments. If you are using Moonshine (which we would recommend) then it is best used with shared configuration files.

The recommend way to configure asset_sync is by using environment variables however it's up to you, it will work fine if you hard code them too. The main reason is that then your access keys are not checked into version control.

Initializer (config/initializers/asset_sync.rb)

The generator will create a Rails initializer at config/initializers/asset_sync.rb.

AssetSync.configure do |config|
  config.fog_provider = 'AWS'
  config.fog_directory = ENV['FOG_DIRECTORY']
  config.aws_access_key_id = ENV['AWS_ACCESS_KEY_ID']
  config.aws_secret_access_key = ENV['AWS_SECRET_ACCESS_KEY']

  # Don't delete files from the store
  # config.existing_remote_files = "keep"
  # Increase upload performance by configuring your region
  # config.fog_region = 'eu-west-1'
  # Automatically replace files with their equivalent gzip compressed version
  # config.gzip_compression = true
  # Use the Rails generated 'manifest.yml' file to produce the list of files to 
  # upload instead of searching the assets directory.
  # config.manifest = true
  # Fail silently.  Useful for environments such as Heroku
  # config.fail_silently = true

YAML (config/asset_sync.yml)

If you used the --use-yml flag, the generator will create a YAML file at config/asset_sync.yml.

defaults: &defaults
  fog_provider: "AWS"
  fog_directory: "rails-app-assets"
  aws_access_key_id: "<%= ENV['AWS_ACCESS_KEY_ID'] %>"
  aws_secret_access_key: "<%= ENV['AWS_SECRET_ACCESS_KEY'] %>"
  # You may need to specify what region your storage bucket is in
  # fog_region: "eu-west-1"
  existing_remote_files: keep # Existing pre-compiled assets on S3 will be kept
  # To delete existing remote files.
  # existing_remote_files: delete
  # Automatically replace files with their equivalent gzip compressed version
  # gzip_compression: true
  # Fail silently.  Useful for environments such as Heroku
  # fail_silently = true

  <<: *defaults

  <<: *defaults

  <<: *defaults

Environment Variables

Add your Amazon S3 configuration details to heroku

heroku config:add AWS_ACCESS_KEY_ID=xxxx
heroku config:add AWS_SECRET_ACCESS_KEY=xxxx
heroku config:add FOG_DIRECTORY=xxxx

Or add to a traditional unix system

export AWS_ACCESS_KEY_ID=xxxx
export FOG_DIRECTORY=xxxx

Available Configuration Options


  • existing_remote_files: what to do with previously precompiled files, options are keep or delete
  • gzip_compression: when enabled, will automatically replace files that have a gzip compressed equivalent with the compressed version.
  • manifest: when enabled, will use the manifest.yml generated by Rails to get the list of local files to upload. experimental

Required (Fog)

  • fog_provider: your storage provider AWS (S3) or Rackspace (Cloud Files)
  • fog_directory: your bucket name


  • fog_region: the region your storage bucket is in e.g. eu-west-1


  • aws_access_key_id: your Amazon S3 access key
  • aws_secret_access_key: your Amazon S3 access secret


  • rackspace_username: your Rackspace username
  • rackspace_api_key: your Rackspace API Key.

Rackspace Optional

  • rackspace_auth_url: Rackspace auth URL, for Rackspace London use:

Amazon S3 Multiple Region Support

If you are using anything other than the US buckets with S3 then you'll want to set the region. For example with an EU bucket you could set the following with YAML.

  # ...
  aws_region: 'eu-west-1'

Or via the initializer

AssetSync.configure do |config|
  # ...
  config.fog_region = 'eu-west-1'

Automatic gzip compression

With the gzip_compression option enabled, when uploading your assets. If a file has a gzip compressed equivalent we will replace that asset with the compressed version and sets the correct headers for S3 to serve it. For example, if you have a file master.css and it was compressed to master.css.gz we will upload the .gz file to S3 in place of the uncompressed file.

If the compressed file is actually larger than the uncompressed file we will ignore this rule and upload the standard uncompressed version.


With Rails 3.1 on the Heroku cedar stack, the deployment process automatically runs rake assets:precompile. If you are using ENV variable style configuration. Due to the methods with which Heroku compile slugs, there will be an error raised by asset_sync as the environment is not available. This causes heroku to install the rails31_enable_runtime_asset_compilation plugin which is not necessary when using asset_sync and also massively slows down the first incoming requests to your app.

To prevent this part of the deploy from failing (asset_sync raising a config error), but carry on as normal set fail_silently to true in your configuration and ensure to run heroku run rake assets:precompile after deploy.

Rake Task

A rake task is included in asset_sync to enhance the rails precompile task by automatically running after it:

# asset_sync/lib/tasks/asset_sync.rake
Rake::Task["assets:precompile"].enhance do


  1. Add some before and after filters for deleting and uploading
  2. Support more cloud storage providers
  3. Better test coverage


Have borrowed ideas from:


MIT License. Copyright 2011 Rumble Labs Ltd.