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Asset Pipeline

This guide covers the asset pipeline introduced in Rails 3.1.
By referring to this guide you will be able to:

  • Understand what the asset pipeline is and what it does
  • Properly organize your application assets
  • Understand the benefits of the asset pipeline
  • Adding a pre-processor to the pipeline
  • Package assets with a gem


What is the Asset Pipeline?

The asset pipeline provides a framework to concatenate and minify or compress JavaScript and CSS assets. It also adds the ability to write these assets in other languages such as CoffeeScript, SCSS and ERB.

Prior to Rails 3.1 these features were added through third-party Ruby libraries such as Jammit and Sprockets. Rails 3.1 is integrated with Sprockets through ActionPack which depends on the sprockets gem, by default.

By having this as a core feature of Rails, all developers can benefit from the power of having their assets pre-processed, compressed and minified by one central library, Sprockets. This is part of Rails’ “Fast by default” strategy as outlined by DHH in his 2011 keynote at Railsconf.

In new Rails 3.1 application the asset pipeline is enabled by default. It can be disabled in application.rb by putting this line inside the Application class definition:

config.assets.enabled = false

It is recommended that you use the defaults for all new apps.

Main Features

The first feature of the pipeline is to concatenate assets. This is important in a production environment, as it reduces the number of requests that a browser must make to render a web page. While Rails already has a feature to concatenate these types of assets — by placing :cache => true at the end of tags such as javascript_include_tag and stylesheet_link_tag — many people do not use it.

The default behavior in Rails 3.1 and onward is to concatenate all files into one master file each for JS and CSS. However, you can separate files or groups of files if required (see below). In production, an MD5 fingerprint is inserted into each filename so that the file is cached by the web browser but can be invalidated if the fingerprint is altered.

The second feature is to minify or compress assets. For CSS, this usually involves removing whitespace and comments. For JavaScript, more complex processes can be applied. You can choose from a set of built in options or specify your own.

The third feature is the ability to code these assets using another language, or language extension. These include SCSS or Sass for CSS, CoffeeScript for JavaScript, and ERB for both.

What is Fingerprinting and Why Should I Care?

Fingerprinting is a technique whereby the filenames of content that is static or infrequently updated is altered to be unique to the content contained in the file.

When a filename is unique and based on its content, HTTP headers can be set to encourage caches everywhere (at ISPs, in browsers) to keep their own copy of the content. When the content is updated, the fingerprint will change and the remote clients will request the new file. This is generally known as cachebusting.

The most effective technique is to insert a hash of the content into the name, usually at the end. For example a CSS file global.css is hashed and the filename is updated to incorporate the hash.

global.css => global-908e25f4bf641868d8683022a5b62f54.css

This is the strategy adopted by the Rails asset pipeline.

Rails’ old strategy was to append a query string to every asset linked with a built-in helper. In the source the generated code looked like this:


This has several disadvantages:

  1. Not all caches will cache content with a query string
    Steve Souders recommends, “…avoiding a querystring for cacheable resources”. He found that in these case 5-20% of requests will not be cached.
  2. The file name can change between nodes in multi-server environments.
    The query string in Rails is based on the modification time of the files. When assets are deployed to a cluster, there is no guarantee that the timestamps will be the same, resulting in different values being used depending on which server handles the request.

The other problem is that when static assets are deployed with each new release of code, the mtime of all these files changes, forcing all remote clients to fetch them again, even when the content of those assets has not changed.

Fingerprinting avoids all these problems by ensuring filenames are consistent based on their content.

Fingerprinting is enabled by default for production and disabled for all the others environments. You can enable or disable it in your configuration through the config.assets.digest option.

More reading:

How to Use the Asset Pipeline

In previous versions of Rails, all assets were located in subdirectories of public such as images, javascripts and stylesheets. With the asset pipeline, the preferred location for these assets is now the app/assets directory. Files in this directory are served by the Sprockets middleware included in the sprockets gem.

This is not to say that assets can (or should) no longer be placed in public; they still can be and will be served as static files by the application or web server. You would only use app/assets if you wish your files to undergo some pre-processing before they are served.

When a scaffold or controller is generated for the application, Rails also generates a JavaScript file (or CoffeeScript file if the coffee-script gem is in the Gemfile) and a Cascading Style Sheet file (or SCSS file if sass-rails is in the Gemfile) for that controller.

For example, if a ProjectsController is generated, there will be a new file at app/assets/javascripts/ and another at app/assets/stylesheets/projects.css.scss. You should put any JavaScript or CSS unique to a controller inside their respective asset files, as these files can then be loaded just for these controllers with lines such as <%= javascript_include_tag params[:controller] %> or <%= stylesheet_link_tag params[:controller] %>.

Asset Organization

Assets can be placed inside an application in one of three locations: app/assets, lib/assets or vendor/assets.

app/assets is for assets that are owned by the application, such as custom images, JavaScript files or stylesheets.

lib/assets is for your own libraries’ code that doesn’t really fit into the scope of the application or those libraries which are shared across applications.

vendor/assets is for assets that are owned by outside entities, such as code for JavaScript plugins.

All subdirectories that exist within these three locations are added to the search path for Sprockets (visible by calling Rails.application.config.assets.paths in a console). When an asset is requested, these paths are looked through to see if they contain an asset matching the name specified. Once an asset has been found, it’s processed by Sprockets and served.

Coding Links to Assets

To access assets, you use the same tags that you are generally familiar with:

Sprockets does not add any new methods to require your assets, you still use the familiar javascript_include_tag and stylesheet_link_tag.

<%= stylesheet_link_tag “application” >
<= javascript_include_tag “application” %>

In regular views you can access images in the assets/images directory like this:

<%= image_tag “rails.png” %>

Images can be organized into directories if required, and they can be accessed by specifying the directory’s name in the tag:

<%= image_tag “icons/rails.png” %>

Providing that assets are enabled within your application (config.assets.enabled in the current environment’s file is not set to false), this file is served by Sprockets unless a file at public/assets/rails.png exists, in which case that file is served.

Alternatively, a file with an MD5 hash after its name such as public/assets/rails-af27b6a414e6da00003503148be9b409.png is also picked up by Sprockets. How these hashes are generated is covered in the Production Assets section later on in this guide.

Otherwise, Sprockets looks through the available paths until it finds a file that matches the name and then serves it, first looking in the application’s assets directories and then falling back to the various engines of the application.

If you want to use a css data URI — a method of embedding the image data directly into the CSS file — you can use the asset_data_uri helper.

#logo { background: url(<%= asset_data_uri ‘logo.png’ %>) }

This inserts a correctly-formatted data URI into the CSS source.


If you add an erb extension to a CSS asset, making it something such as application.css.erb, then you can use the asset_path helper in your CSS rules:

.class { background-image: <%= asset_path ‘image.png’ %> }

This writes the path to the particular asset being referenced. In this example, it would make sense to have an image in one of the asset load paths, such as app/assets/images/image.png, which would be referenced here. If this image is already available in public/assets as a fingerprinted file, then that path is referenced.

Note that the closing tag cannot be of the style -%>.


When using the asset pipeline, paths to assets must be re-written and sass-rails provides _url and _path helpers for the following asset classes: image, font, video, audio, javascript, stylesheet.

  • image_url(“rails.png”) becomes url(/assets/rails.png)
  • image_path(“rails.png”) becomes “/assets/rails.png”.

The more generic form can also be used but the asset path and class must both be specified:

  • asset_url(“rails.png”, “image”) becomes url(/assets/rails.png)
  • asset_path(“rails.png”, “image”) becomes “/assets/rails.png”

Manifest Files and Directives

Sprockets uses manifest files to determine which assets to include and serve. These manifest files contain directives — instructions that tell Sprockets which files to require in order to build a single CSS or JavaScript file. With these directives, Sprockets loads the files specified, processes them if necessary, concatenates them into one single file and then compresses them (if Rails.application.config.assets.compress is set to true). By serving one file rather than many, the load time of pages are greatly reduced as there are fewer requests to make.

For example, in the default Rails application there’s a app/assets/javascripts/application.js file which contains the following lines:

//= require jquery
//= require jquery_ujs
//= require_tree .

In JavaScript files, the directives begin with //=. In this case, the file is using the require and the require_tree directives. The require directive is used to tell Sprockets the files that you wish to require. Here, you are requiring the files jquery.js and jquery_ujs.js that are available somewhere in the search path for Sprockets. You need not supply the extensions explicitly. Sprockets assumes you are requiring a .js file when done from within a .js file.

NOTE. In Rails 3.1, the jquery.js and jquery_ujs.js files are located inside the vendor/assets/javascripts directory contained within the jquery-rails gem.

The require_tree . directive tells Sprockets to include all JavaScript files in this directory into the output. Only a path relative to the file can be specified. There is also a require_directory directive which includes all JavaScript files only in the directory specified (no nesting).

There’s also a default app/assets/stylesheets/application.css file which contains these lines:

/* …

  • require_self
  • require_tree .

The directives that work in the JavaScript files also work in stylesheets, obviously including stylesheets rather than JavaScript files. The require_tree directive here works the same way as the JavaScript one, requiring all stylesheets from the current directory.

In this example require_self is used. This puts the CSS contained within the file (if any) at the top of any other CSS in this file unless require_self is specified after another require directive.

You can have as many manifest files as you need. For example the admin.css and admin.js manifest could contain the JS and CSS files that are used for the admin section of an application.

For some assets (like CSS) the compiled order is important. You can specify individual files and they are compiled in the order specified:

/* …

  • require reset
  • require layout
  • require chrome


The file extensions used on an asset determine what preprocessing is applied. When a controller or a scaffold is generated with the default Rails gemset, a CoffeeScript file and a SCSS file are generated in place of a regular JavaScript and CSS file. The example used before was a controller called “projects”, which generated an app/assets/javascripts/ and a app/assets/stylesheets/projects.css.scss file.

When these files are requested, they are processed by the processors provided by the coffee-script and sass-rails gems and then sent back to the browser as JavaScript and CSS respectively.

Additional layers of pre-processing can be requested by adding other extensions, where each extension is processed in a right-to-left manner. These should be used in the order the processing should be applied. For example, a stylesheet called app/assets/stylesheets/projects.css.scss.erb is first processed as ERB, then SCSS and finally served as CSS. The same applies to a JavaScript file — app/assets/javascripts/ is processed as ERB, CoffeeScript and served as JavaScript.

Keep in mind that the order of these pre-processors is important. For example, if you called your JavaScript file app/assets/javascripts/ then it is processed with the CoffeeScript interpreter first, which wouldn’t understand ERB and therefore you would run into problems.

In Development

In the development environment assets are compiled and cached on the first request after the server is started. Sprockets sets a must-validate Cache-Control HTTP header to reduce request overhead on subsequent requests – on these the browser gets a 304 (not-modified) response.

If any of the files in the manifest have changed between requests, the server responds with a new compiled file.

Debugging Assets

You can put ?debug_assets=true or ?debug_assets=1 at the end of a URL or set config.assets.debug and Sprockets expands the lines which load the assets. For example, if you had an app/assets/javascripts/application.js file containing these lines:

//= require “projects”
//= require “tickets”

By default, this only renders this line when used with <%= javascript_include_tag “application” %> in a view or layout:

When the debug_assets parameter is set, this line is expanded out into three separate lines, separating out the combined file into their parts.

This allows the individual parts of an asset to be rendered and debugged separately.

NOTE. Assets debugging is turned on by default in development and test environments.

In Production

In the production environment, assets are served slightly differently.

On the first request the assets are compiled and cached as described above, however the manifest names are altered to include an MD5 hash. Files names typically look like these:


The MD5 is generated from the contents of the compiled files, and is included in the HTTP Content-MD5 header.

Sprockets also sets the Cache-Control HTTP header to max-age=31536000. This signals all caches between your server and the client browser that this content (the file served) can be cached for 1 year. The effect of this is to reduce the number of requests for this asset from your server; the asset has a good chance of being in the local browser cache or some intermediate cache.

This behavior is controlled by the setting of config.assets.digest setting in Rails (which is true for production, false for everything else).

Precompiling Assets

Even though assets are served by Rack::Cache with far-future headers, in high traffic sites this may not be fast enough.

Rails comes bundled with a rake task to compile the manifests to files on disc. These are located in the public/assets directory where they are served by your web server instead of the Rails application.

The rake task is:

bundle exec rake assets:precompile

Capistrano (v2.8.0+) has a recipe to handle this in deployment. Add the following line to Capfile:

load ‘deploy/assets’

This links the folder specified in config.assets.prefix to shared/assets. If you already use this folder you’ll need to write your own deployment task.

It is important that this folder is shared between deployments so that remotely cached pages that reference the old compiled assets still work for the life of the cached page.

The default matcher for compiling files includes application.js, application.css and all files that do not end in js or css:

[ /\w+\.(?!js|css).+/, /application.(css|js)$/ ]

If you have other manifests or individual stylesheets and JavaScript files to include, you can append them to the precompile array:

config.assets.precompile += [‘admin.js’, ‘admin.css’, ‘swfObject.js’]

The rake task also generates a manifest.yml that contains a list with all your assets and their fingerprints, using this manifest file the assets helpers avoid hitting to Sprockets to recalculate MD5 hashes for files. Manifest file typically look like this:

rails.png: rails-bd9ad5a560b5a3a7be0808c5cd76a798.png
jquery-ui.min.js: jquery-ui-7e33882a28fc84ad0e0e47e46cbf901c.min.js
jquery.min.js: jquery-8a50feed8d29566738ad005e19fe1c2d.min.js
application.js: application-3fdab497b8fb70d20cfc5495239dfc29.js
application.css: application-8af74128f904600e41a6e39241464e03.css

The manifest file is generated by default in same folder of your precompiled assets, you can change the location of the file setting the config.assets.manifest option with the full path of the folder where you want save it.

Precompiled assets exist on the filesystem and are served directly by your webserver. They do not have far-future headers by default, so to get the benefit of fingerprinting you’ll have to update your server configuration to add them.

For Apache:

<LocationMatch “^/assets/.*$”>

  1. Some browsers still send conditional-GET requests if there’s a
  2. Last-Modified header or an ETag header even if they haven’t
  3. reached the expiry date sent in the Expires header.
    Header unset Last-Modified
    Header unset ETag
    FileETag None
  4. RFC says only cache for 1 year
    ExpiresActive On
    ExpiresDefault “access plus 1 year”

TODO: nginx instructions

When files are precompiled, Sprockets also creates a Gzip (.gz) version of your assets. This avoids the server having to do this for any requests; it can simply read the compressed files from disc. You must configure your server to use gzip compression and serve the compressed assets that will be stored in the public/assets folder. The following configuration options can be used:

TODO: Apache instructions

TODO: nginx instructions

By default Rails assumes that you have your files precompiled in the production environment, if you want use live compiling (compile your assets during runtime) in production you must set the config.assets.compile to true. You can use this option to fallback to Sprockets when you are using precompiled assets but there are any missing precompiled files. If config.assets.compile option is set to false and there are missing precompiled files you will get an “AssetNoPrecompiledError” indicating the name of the missing file.

Customizing the Pipeline

CSS Compression

There is currently one option for compressing CSS – YUI. This Gem extends the CSS syntax and offers minification.

The following line enables YUI compression, and requires the yui-compressor gem.

config.assets.css_compressor = :yui

The config.assets.compress must be set to true to enable CSS compression


Possible options for JavaScript compression are :closure, :uglifier and :yui. These require the use of the closure-compiler, uglifier or yui-compressor gems respectively.

The default Gemfile includes uglifier. This gem wraps UglifierJS (written for NodeJS) in Ruby. It compresses your code by removing white space and other magical things like changing your if and else statements to ternary operators where possible.

The following line invokes uglifier for JavaScript compression.

config.assets.js_compressor = :uglifier

The config.assets.compress must be set to true to enable JavaScript compression

Using Your Own Compressor

The compressor config settings for CSS and JavaScript also take any Object. This object must have a compress method that takes a string as the sole argument and it must return a string.

class Transformer
def compress(string)

To enable this, pass a new Object to the config option in application.rb:

config.assets.css_compressor =

Changing the assets Path

The public path that Sprockets uses by default is /assets.

This can be changed to something else:

config.assets.prefix = “/some_other_path”

This is a handy option if you have any existing project (pre Rails 3.1) that already uses this path or you wish to use this path for a new resource.

X-Sendfile Headers

The X-Sendfile header is a directive to the server to ignore the response from the application, and instead serve the file specified in the headers. This option is off by default, but can be enabled if your server supports it. When enabled, this passes responsibility for serving the file to the web server, which is faster.

Apache and nginx support this option which is enabled in config/environments/production.rb.

  1. config.action_dispatch.x_sendfile_header = “X-Sendfile” # for apache
  2. config.action_dispatch.x_sendfile_header = ‘X-Accel-Redirect’ # for nginx

WARNING: If you are upgrading an existing application and intend to use this option, take care to paste this configuration option only into production.rb (and not application.rb) and any other environment you define with production behavior.

How Caching Works

Sprockets uses the default rails cache store to cache assets in dev and production. The only difference is file names are fingerprinted and get far-future headers in production.

TODO: Add more about changing the default store.

Adding Assets to Your Gems

Assets can also come from external sources in the form of gems.

A good example of this is the jquery-rails gem which comes with Rails as the standard JavaScript library gem. This gem contains an engine class which inherits from Rails::Engine. By doing this, Rails is informed that the directory for this gem may contain assets and the app/assets, lib/assets and vendor/assets directories of this engine are added to the search path of Sprockets.

Making Your Library or Gem a Pre-Processor

TODO: Registering gems on Tilt enabling Sprockets to find them.

Upgrading from Old Versions of Rails

There are two issues when upgrading. The first is moving the files to the new locations. See the section above for guidance on the correct locations for different file types.

The second is updating the various environment files with the correct default options. The following changes reflect the defaults in version 3.1.0.

In application.rb:

  1. Enable the asset pipeline
    config.assets.enabled = true
  1. Version of your assets, change this if you want to expire all your assets
    config.assets.version = ‘1.0’

In development.rb:

  1. Do not compress assets
    config.assets.compress = false
  1. Expands the lines which load the assets
    config.assets.debug = true

And in production.rb:

  1. Compress JavaScripts and CSS
    config.assets.compress = true
  1. Don’t fallback to assets pipeline if a precompiled asset is missed
    config.assets.compile = false
  1. Generate digests for assets URLs
    config.assets.digest = true
  1. Defaults to Rails.root.join(“public/assets”)
  2. config.assets.manifest = YOUR_PATH
  1. Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
  2. config.assets.precompile += %w( search.js )

There are no changes to test.rb.

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