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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

endprologue.

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, Sass 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 Action Pack 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 keynote at RailsConf 2011.

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

config.assets.enabled = false

You can also disable it while creating a new application by passing the —skip-sprockets option.

rails new appname —skip-sprockets

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 -, it has a series of limitations. For example, it cannot generate the caches in advance, and it is not able to transparently include assets provided by third-party libraries.

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 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 are 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 cache busting.

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 digest, for example becoming:

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:

/stylesheets/global.css?1309495796

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 this case 5-20% of requests will not be cached. Query strings in particular do not work at all with some CDNs for cache invalidation.
  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 fixes these problems by avoiding query strings, and 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.

In production, the default is to precompile these files to public/assets so that they can be more efficiently delivered by the web server.

When a scaffold or controller is generated for the application, Rails also generates a JavaScript file (or CoffeeScript file if the coffee-rails 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/projects.js.coffee 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] %>.

NOTE: You will need a ExecJS supported runtime in order to use CoffeeScript. If you are using Mac OS X or Windows you have a JavaScript runtime installed in your operating system. Check ExecJS documentation to know all supported JavaScript runtimes.

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 traversed to see if they contain an asset matching the name specified. Once an asset has been found, it’s processed by Sprockets and served.

You can add additional (fully qualified) paths to the pipeline in config/application.rb. For example:

config.assets.paths << “#{Rails.root}/app/assets/flash”

Coding Links to Assets

Sprockets does not add any new methods to access 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” %>

Provided that the pipeline is enabled within your application (and not disabled in the current environment context), this file is served by Sprockets. If a file exists at public/assets/rails.png it is served by the webserver.

Alternatively, a request for a file with an MD5 hash such as public/assets/rails-af27b6a414e6da00003503148be9b409.png is treated the same way. How these hashes are generated is covered in the In Production section later on in this guide.

Sprockets will also look through the paths specified in config.assets.paths which includes the standard application paths and any path added by Rails engines.

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

<%= image_tag “icons/rails.png” %>

CSS and ERB

If you add an erb extension to a CSS asset, making it something such as application.css.erb, then helpers like asset_path are available in your CSS rules:

.class { background-image: url(<%= 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.

If you want to use a 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.

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

CSS and Sass

When using the asset pipeline, paths to assets must be re-written and sass-rails provides -url and -path helpers (hyphenated in Sass, underscored in Ruby) for the following asset classes: image, font, video, audio, JavaScript and 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”
JavaScript/CoffeeScript and ERB

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

$(‘#logo’).attr({
src: “<%= asset_path(‘logo.png’) %>”
});

This writes the path to the particular asset being referenced.

Similarly, you can use the asset_path helper in CoffeeScript files with erb extension (eg. application.js.coffee.erb):

$(‘#logo’).attr src: “<%= asset_path(‘logo.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 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-rails gem provides the jquery.js and jquery_ujs.js files via the asset pipeline. You won’t see them in the application tree.

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

Directives are processed top to bottom, but the order in which files are included by require_tree is unspecified. You should not rely on any particular order among those. If you need to ensure some particular JavaScript ends up above some other, require it before in the manifest. Note that the family of require directives prevents files from being included twice in the output.

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.

The same remarks about ordering made above apply. In particular, you can specify individual files and they are compiled in the order specified:

/* …

  • require reset
  • require layout
  • require chrome
    */

Preprocessing

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/projects.js.coffee and an 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 preprocessing 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/projects.js.coffee.erb is processed as ERB, CoffeeScript, and served as JavaScript.

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

In Development

In development mode assets are served as separate files in the order they are specified in the manifest file.

This manifest app/assets/javascripts/application.js:

//= require core
//= require projects
//= require tickets

would generate this HTML:

The body param is required by Sprockets.

Turning Debugging off

You can turn off debug mode by updating config/environments/development.rb to include:

config.assets.debug = false

When debug mode is off Sprockets concatenates and runs the necessary preprocessors on all files. With debug mode turned off the manifest above would generate instead:

Assets are compiled and cached on the first request after the server is started. Sprockets sets a must-revalidate 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.

Debug mode can also be enabled in the Rails helper methods:

<%= stylesheet_link_tag “application”, :debug => true >
<= javascript_include_tag “application”, :debug => true %>

The :debug option is redundant if debug mode is on.

You could potentially also enable compression in development mode as a sanity check, and disable it on-demand as required for debugging.

In Production

In the production environment Rails uses the fingerprinting scheme outlined above. By default it is assumed that assets have been precompiled and will be served as static assets by your web server.

During the precompilation phase an MD5 is generated from the contents of the compiled files, and inserted into the filenames as they are written to disc. These fingerprinted names are used by the Rails helpers in place of the manifest name.

For example this:

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

generates something like this:

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

NOTE: Under normal circumstances the default option should not be changed. If there are no digests in the filenames, and far-future headers are set, remote clients will never know to refetch the files when their content changes.

Precompiling Assets

Rails comes bundled with a rake task to compile the asset manifests and other files in the pipeline to the disk.

Compiled assets are written to the location specified in config.assets.prefix. The default setting will use the public/assets directory.

You must use this task either during deployment or locally if you do not have write access to your production filesystem.

The rake task is:

bundle exec rake assets:precompile

Asset templates in general have access to application objects. You may prevent application
initialization in precompilation setting config.assets.initialize_on_precompile to false.

WARNING: Application objects may still be in scope in asset templates in develoment mode.
If config.assets.initialize_on_precompile is set to false it is your responsability not
to use them at all.

Capistrano (v2.8.0 and above) 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 shared 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.

NOTE. If you are precompiling your assets locally, you can use bundle install —without assets on the server to avoid installing the assets gems (the gems in the assets group in the Gemfile).

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 add 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 respective fingerprints. This is used by the Rails helper methods and avoids handing the mapping requests back to Sprockets. A typical manifest file looks like:


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 default location for the manifest is the root of the location specified in config.assets.prefix (‘/assets’ by default).

This can be changed with the config.assets.manifest option. A fully specified path is required:

config.assets.manifest = ‘/path/to/some/other/location’

NOTE: If there are missing precompiled files in production you will get an AssetNoPrecompiledError exception indicating the name of the missing file(s).

Server Configuration

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”

For nginx:

location ~ ^/assets/ {
expires 1y;
add_header Cache-Control public;

  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.
    add_header Last-Modified "";
    add_header ETag "";
    break;
    }

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 disk. 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:

For Apache:

<LocationMatch “^/assets/.*$”>

  1. 2 lines to serve pre-gzipped version
    RewriteCond %{REQUEST_FILENAME}.gz -s
    RewriteRule ^(.+) $1.gz [L]
  1. without these, Content-Type will be “application/x-gzip”
    <FilesMatch “^/assets/.*\.css.gz$”>
    ForceType text/css

<FilesMatch “^/assets/.*\.js.gz$”>
ForceType text/javascript

For nginx:

location ~ ^/(assets)/ {
root /path/to/public;
gzip_static on; # to serve pre-gzipped version
expires max;
add_header Cache-Control public;
}

Live Compilation

In some circumstances you may wish to use live compilation. In this mode all requests for assets in the pipeline are handled by Sprockets directly.

To enable this option set:

config.assets.compile = true

On the first request the assets are compiled and cached as outlined in development above, and the manifest names used in the helpers are altered to include the MD5 hash.

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 mode uses more memory, performs poorer than the default and is not recommended.

When deploying a production application to a system without any pre-existing JavaScript runtimes, you may want to add one to your Gemfile:

group :production do
gem ‘therubyracer’
end

Customizing the Pipeline

CSS Compression

There is currently one option for compressing CSS, YUI. The YUI CSS compressor provides 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.

JavaScript 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

NOTE: You will need a ExecJS supported runtime in order to use uglifier. If you are using Mac OS X or Windows you have installed a JavaScript runtime in your operating system. Check ExecJS documentation to know all supported JavaScript runtimes.

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)
do_something_returning_a_string(string)
end
end

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

config.assets.css_compressor = Transformer.new

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 development and 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’
  1. Change the path that assets are served from
  2. config.assets.prefix = “/assets”

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. Choose the compressors to use
  2. config.assets.js_compressor = :uglifier
  3. config.assets.css_compressor = :yui
  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. The defaults in the test environment are: config.assets.compile is true and config.assets.compress, config.assets.debug and config.assets.digest are false.

The following should also be added to Gemfile:

  1. Gems used only for assets and not required
  2. in production environments by default.
    group :assets do
    gem ‘sass-rails’, “~> 3.1.0”
    gem ‘coffee-rails’, “~> 3.1.0”
    gem ‘uglifier’
    end

If you use the assets group with Bundler, please make sure that your config/application.rb has the following Bundler require statement.

if defined?(Bundler)

  1. If you precompile assets before deploying to production, use this line
    Bundler.require *Rails.groups(:assets => %w(development test))
  2. If you want your assets lazily compiled in production, use this line
  3. Bundler.require(:default, :assets, Rails.env)
    end

Instead of the old Rails 3.0 one

  1. If you have a Gemfile, require the gems listed there, including any gems
  2. you’ve limited to :test, :development, or :production.
    Bundler.require(:default, Rails.env) if defined?(Bundler)
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