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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).

Heroku Labs (BETA)

Previously there were several issues with using asset_sync on Heroku as described in our Heroku dev centre article.

Now to get everything working smoothly with using ENV variables to configure asset_sync we just need to enable the user_env_compile functionality. In short:

heroku labs:enable user_env_compile -a myapp

Hopefully this will make it's way into the platform as standard.


Add the gem to your Gemfile

gem "asset_sync"

If you want, you can put it within your :assets group in your Gemfile.

group :assets do
  gem 'sass-rails',   '~> 3.2.3'
  gem 'coffee-rails', '~> 3.2.1'
  gem 'uglifier', '>= 1.0.3'
  gem "asset_sync"

This is good practice when pre-compiling your assets as it will reduce load time and server memory in production. The only caveat being, you may not be able to use a custom initializer, without perhaps wrapping it with.

if defined?(AssetSync)



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

  config.action_controller.asset_host = "//#{ENV['FOG_DIRECTORY']}"

On HTTPS: the exclusion of any protocol in the asset host declaration above will allow browsers to choose the transport mechanism on the fly. So if your application is available under both HTTP and HTTPS the assets will be served to match.

The only caveat with this is that your S3 bucket name must not contain any periods so, for example would not work under HTTPS as SSL certificates from Amazon would interpret our bucket name as not a subdomain of, but a multi level subdomain. To avoid this don't use a period in your subdomain or switch to the other style of S3 URL.

  config.action_controller.asset_host = "//{ENV['FOG_DIRECTORY']}" 

Also, ensure the following are defined (in production.rb or application.rb)

  • config.assets.digest is set to true.
  • config.assets.enabled is set to true.


AssetSync supports the following methods of configuration.

  • Built-in Initializer (configured through environment variables)
  • Rails Initializer
  • A YAML config file

Using the Built-in Initializer is the default method and is supposed to be used with environment variables. It's the recommended approach for deployments on Heroku.

If you need more control over configuration you will want to use a custom rails initializer.

Configuration using a YAML file (a common strategy for Capistrano deployments) is also suppored.

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.

Built-in Initializer (Environment Variables)

The Built-in Initializer will configure AssetSync based on the contents of your environment variables.

Add your 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
heroku config:add FOG_PROVIDER=AWS
# and optionally:
heroku config:add FOG_REGION=eu-west-1
heroku config:add ASSET_SYNC_GZIP_COMPRESSION=true
heroku config:add ASSET_SYNC_MANIFEST=true

Or add to a traditional unix system

export AWS_ACCESS_KEY_ID=xxxx
export FOG_DIRECTORY=xxxx

Rackspace configuration is also supported

heroku config:add RACKSPACE_USERNAME=xxxx
heroku config:add RACKSPACE_API_KEY=xxxx
heroku config:add FOG_DIRECTORY=xxxx
heroku config:add FOG_PROVIDER=Rackspace

The Built-in Initializer also sets the AssetSync default for existing_remote_files to keep.

Custom Rails Initializer (config/initializers/asset_sync.rb)

If you want to enable some of the advanced configuration options you will want to create your own initializer.

Run the included Rake task to generate a starting point.

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

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)

Run the included Rake task to generate a starting point.

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

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
  # To ignore existing remote files and overwrite.
  # existing_remote_files: ignore
  # Automatically replace files with their equivalent gzip compressed version
  # gzip_compression: true
  # Fail silently.  Useful for environments such as Heroku
  # fail_silently = true
  # Always upload. Useful if you want to overwrite specific remote assets regardless of their existence
  #  eg: Static files in public often reference non-fingerprinted application.css
  #  note: You will still need to expire them from the CDN's edge cache locations
  # always_upload: ['application.js', 'application.css']
  # Ignored files. Useful if there are some files that are created dynamically on the server and you don't want to upload on deploy.
  # ignored_files: ['ignore_me.js', !ruby/regexp '/ignore_some/\d{32}\.css/']

  <<: *defaults

  <<: *defaults

  <<: *defaults

Available Configuration Options

All AssetSync configuration can be modified directly using environment variables with the Built-in initializer. e.g.

AssetSync.config.fog_provider == ENV['FOG_PROVIDER']

Simply upcase the ruby attribute names to get the equivalent environment variable to set. The only exception to that rule are the internal AssetSync config variables, they must be prepended with ASSET_SYNC_* e.g.

AssetSync.config.gzip_compression == ENV['ASSET_SYNC_GZIP_COMPRESSION']

AssetSync (optional)

  • existing_remote_files: ('keep', 'delete', 'ignore') what to do with previously precompiled files. default: 'keep'
  • gzip_compression: (true, false) when enabled, will automatically replace files that have a gzip compressed equivalent with the compressed version. default: 'false'
  • manifest: (true, false) when enabled, will use the manifest.yml generated by Rails to get the list of local files to upload. experimental. default: 'false'
  • enabled: (true, false) when false, will disable asset sync. default: 'true' (enabled)
  • ignored_files: an array of files to ignore e.g. ['ignore_me.js', %r(ignore_some/\d{32}\.css)] Useful if there are some files that are created dynamically on the server and you don't want to upload on deploy default: []

Fog (Required)

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

Fog (Optional)

  • 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.

Google Storage

  • google_storage_access_key_id: your Google Storage access key
  • google_storage_secret_access_key: your Google Storage access secret

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 environment variable.

heroku config:add FOG_REGION=eu-west-1

Or via a custom initializer

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

Or via YAML

  # ...
  aws_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.

Fail Silently

With the fail_silently option enabled, when running rake assets:precompile AssetSync will never throw an error due to missing configuration variables.

With the new user_env_compile feature of Heroku (see above), this is no longer required or recommended. Yet was added for the following reasons:

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 within the asset_sync gem to enhance the rails precompile task by automatically running after it.

  # asset_sync/lib/tasks/asset_sync.rake
  if Rake::Task.task_defined?("assets:precompile:nondigest")
    Rake::Task["assets:precompile:nondigest"].enhance do
    Rake::Task["assets:precompile"].enhance do

Sinatra/Rack Support

You can use the gem with any Rack application, but you must specify two additional options; prefix and public_path.

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']
  config.prefix = "assets"
  config.public_path = Pathname("./public")

Then manually call AssetSync.sync at the end of your asset precompilation task.

namespace :assets do
  desc "Precompile assets"
  task :precompile do
    target = Pathname('./public/assets')
    manifest =, "./public/assets/manifest.json")

    sprockets.each_logical_path do |logical_path|
      if (!File.extname(logical_path).in?(['.js', '.css']) || logical_path =~ /application\.(css|js)$/) && asset = sprockets.find_asset(logical_path)
        filename = target.join(logical_path)
        puts "Write asset: #{filename}"



  1. Add some before and after filters for deleting and uploading
  2. Support more cloud storage providers
  3. Better test coverage
  4. Add rake tasks to clean old assets from a bucket


Inspired by:


MIT License. Copyright 2011 Rumble Labs Ltd.

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