asset_pipeline.md

The Asset Pipeline
==================

This guide covers the asset pipeline.

After reading this guide, you will know:

* What the asset pipeline is and what it does.
* How to properly organize your application assets.
* The benefits of the asset pipeline.
* How to add a pre-processor to the pipeline.
* How to 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 and pre-processors such as CoffeeScript, Sass and ERB.

The asset pipeline is technically no longer a core feature of Rails 4, it has
been extracted out of the framework into the
[sprockets-rails](https://github.com/rails/sprockets-rails) gem.

The asset pipeline is enabled by default.

You can disable the asset pipeline while creating a new application by 
passing the `--skip-sprockets` option.

```bash
rails new appname --skip-sprockets
```

Rails 4 automatically adds the `sass-rails`, `coffee-rails` and `uglifier` 
gems to your Gemfile, which are used by Sprockets for asset compression:

```ruby
gem 'sass-rails'
gem 'uglifier'
gem 'coffee-rails'
```

Using the `--skip-sprockets` option will prevent Rails 4 from adding 
`sass-rails` and `uglifier` to Gemfile, so if you later want to enable
the asset pipeline you will have to add those gems to your Gemfile. Also,
creating an application with the `--skip-sprockets` option will generate
a slightly different `config/application.rb` file, with a require statement
for the sprockets railtie that is commented-out. You will have to remove
the comment operator on that line to later enable the asset pipeline:

```ruby
# require "sprockets/railtie"
```

To set asset compression methods, set the appropriate configuration options 
in `production.rb` - `config.assets.css_compressor` for your CSS and
`config.assets.js_compressor` for your Javascript:

```ruby
config.assets.css_compressor = :yui
config.assets.js_compressor = :uglify
```

NOTE: The `sass-rails` gem is automatically used for CSS compression if included
in Gemfile and no `config.assets.css_compressor` option is set.


### Main Features

The first feature of the pipeline is to concatenate assets, which can reduce the
number of requests that a browser makes to render a web page. Web browsers are
limited in the number of requests that they can make in parallel, so fewer
requests can mean faster loading for your application.

Sprockets concatenates all JavaScript files into one master `.js` file and all
CSS files into one master `.css` file. As you'll learn later in this guide, you
can customize this strategy to group files any way you like. In production,
Rails inserts an MD5 fingerprint into each filename so that the file is cached
by the web browser. You can invalidate the cache by altering this fingerprint,
which happens automatically whenever you change the file contents.

The second feature of the asset pipeline is asset minification or compression.
For CSS files, this is done by 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 of the asset pipeline is it allows coding assets via a
higher-level language, with precompilation down to the actual assets. Supported
languages include Sass for CSS, CoffeeScript for JavaScript, and ERB for both by
default.

### What is Fingerprinting and Why Should I Care?

Fingerprinting is a technique that makes the name of a file dependent on the
contents of the file. When the file contents change, the filename is also
changed. For content that is static or infrequently changed, this provides an
easy way to tell whether two versions of a file are identical, even across
different servers or deployment dates.

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

The technique sprockets uses for fingerprinting is to insert a hash of the
content into the name, usually at the end. For example a CSS file `global.css`

```
global-908e25f4bf641868d8683022a5b62f54.css
```

This is the strategy adopted by the Rails asset pipeline.

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

```
/stylesheets/global.css?1309495796
```

The query string strategy has several disadvantages:

1. **Not all caches will reliably cache content where the filename only differs by
query parameters**<br>
    [Steve Souders recommends](http://www.stevesouders.com/blog/2008/08/23/revving-filenames-dont-use-querystring/),
 "...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.**<br>
    The default query string in Rails 2.x 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.

3. **Too much cache invalidation**<br>
    When static assets are deployed with each new release of code, the mtime
(time of last modification) 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
that filenames are consistent based on their content.

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

More reading:

* [Optimize caching](http://code.google.com/speed/page-speed/docs/caching.html)
* [Revving Filenames: don’t use
* querystring](http://www.stevesouders.com/blog/2008/08/23/revving-filenames-dont-use-querystring/)


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.

Assets can still be placed in the `public` hierarchy. Any assets under `public`
will be served as static files by the application or web server. You should use
`app/assets` for files that must undergo some pre-processing before they are
served.

In production, Rails precompiles these files to `public/assets` by default. The
precompiled copies are then served as static assets by the web server. The files
in `app/assets` are never served directly in production.

### Controller Specific Assets

When you generate a scaffold or a controller, 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. Additionally, when generating a scaffold, Rails generates
the file scaffolds.css (or scaffolds.css.scss if `sass-rails` is in the
`Gemfile`.)

For example, if you generate a `ProjectsController`, Rails will also add a new
file at `app/assets/javascripts/projects.js.coffee` and another at
`app/assets/stylesheets/projects.css.scss`. By default these files will be ready
to use by your application immediately using the `require_tree` directive. See
[Manifest Files and Directives](#manifest-files-and-directives) for more details
on require_tree.

You can also opt to include controller specific stylesheets and JavaScript files
only in their respective controllers using the following:

`<%= javascript_include_tag params[:controller] %>` or `<%= stylesheet_link_tag
params[:controller] %>`

When doing this, ensure you are not using the `require_tree` directive, as that
will result in your assets being included more than once.

WARNING: When using asset precompilation, you will need to ensure that your
controller assets will be precompiled when loading them on a per page basis. By
default .coffee and .scss files will not be precompiled on their own. This will
result in false positives during development as these files will work just fine
since assets are compiled on the fly in development mode. When running in
production, however, you will see 500 errors since live compilation is turned
off by default. See [Precompiling Assets](#precompiling-assets) for more
information on how precompiling works.

NOTE: You must have an 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](https://github.com/sstephenson/execjs#readme) documentation to know all
supported JavaScript runtimes.

You can also disable generation of controller specific asset files by adding the
following to your `config/application.rb` configuration:

```ruby
  config.generators do |g|
    g.assets false
  end
```

### Asset Organization

Pipeline 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 and CSS frameworks.

#### Search Paths

When a file is referenced from a manifest or a helper, Sprockets searches the
three default asset locations for it.

The default locations are: the `images`, `javascripts` and `stylesheets`
directories under the `apps/assets` folder, but these subdirectories
are not special - any path under `assets/*` will be searched.

For example, these files:

```
app/assets/javascripts/home.js
lib/assets/javascripts/moovinator.js
vendor/assets/javascripts/slider.js
vendor/assets/somepackage/phonebox.js
```

would be referenced in a manifest like this:

```js
//= require home
//= require moovinator
//= require slider
//= require phonebox
```

Assets inside subdirectories can also be accessed.

```
app/assets/javascripts/sub/something.js
```

is referenced as:

```js
//= require sub/something
```

You can view the search path by inspecting
`Rails.application.config.assets.paths` in the Rails console.

Besides the standard `assets/*` paths, additional (fully qualified) paths can be
added to the pipeline in `config/application.rb`. For example:

```ruby
config.assets.paths << Rails.root.join("lib", "videoplayer", "flash")
```

Paths are traversed in the order they occur in the search path. By default,
this means the files in `app/assets` take precedence, and will mask
corresponding paths in `lib` and `vendor`.

It is important to note that files you want to reference outside a manifest must
be added to the precompile array or they will not be available in the production
environment.

#### Using Index Files

Sprockets uses files named `index` (with the relevant extensions) for a special
purpose.

For example, if you have a jQuery library with many modules, which is stored in
`lib/assets/library_name`, the file `lib/assets/library_name/index.js` serves as
the manifest for all files in this library. This file could include a list of
all the required files in order, or a simple `require_tree` directive.

The library as a whole can be accessed in the application manifest like so:

```js
//= require library_name
```

This simplifies maintenance and keeps things clean by allowing related code to
be grouped before inclusion elsewhere.

### 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`:

```erb
<%= stylesheet_link_tag "application", media: "all" %>
<%= javascript_include_tag "application" %>
```

If using the turbolinks gem, which is included by default in Rails 4, then
include the 'data-turbolinks-track' option which causes turbolinks to check if
an asset has been updated and if so loads it into the page:

```erb
<%= stylesheet_link_tag "application", media: "all", "data-turbolinks-track" => true %>
<%= javascript_include_tag "application", "data-turbolinks-track" => true %>
```

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

```erb
<%= 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 web server.

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](#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 paths added by Rails 
engines.

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

```erb
<%= image_tag "icons/rails.png" %>
```

WARNING: If you're precompiling your assets (see [In Production](#in-production)
below), linking to an asset that does not exist will raise an exception in the
calling page. This includes linking to a blank string. As such, be careful using
`image_tag` and the other helpers with user-supplied data.

#### CSS and ERB

The asset pipeline automatically evaluates ERB. This means if you add an
`erb` extension to a CSS asset (for example, `application.css.erb`), then
helpers like `asset_path` are available in your CSS rules:

```css
.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](http://en.wikipedia.org/wiki/Data_URI_scheme) —
a method of embedding the image data directly into the CSS file — you can use
the `asset_data_uri` helper.

```css
#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`, you can then use the `asset_path` helper in your
JavaScript code:

```js
$('#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 (e.g., `application.js.coffee.erb`):

```js
$('#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 can be greatly reduced because the browser
makes fewer requests. Compression also reduces file size, enabling the
browser to download them faster.


For example, a new Rails 4 application includes a default
`app/assets/javascripts/application.js` file containing the following lines:

```js
// ...
//= require jquery
//= require jquery_ujs
//= require_tree .
```

In JavaScript files, Sprockets directives begin with `//=`. In the above case,
the file is using the `require` and the `require_tree` directives. The `require`
directive is used to tell Sprockets the files 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.

The `require_tree` directive tells Sprockets to recursively include _all_
JavaScript files in the specified directory into the output. These paths must be
specified relative to the manifest file. You can also use the
`require_directory` directive which includes all JavaScript files only in the
directory specified, without recursion.

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 in the concatenated file, require the prerequisite file first
in the manifest. Note that the family of `require` directives prevents files
from being included twice in the output.

Rails also creates a default `app/assets/stylesheets/application.css` file
which contains these lines:

```css
/* ...
*= require_self
*= require_tree .
*/
```

Rails 4 creates both `app/assets/javascripts/application.js` and
`app/assets/stylesheets/application.css` regardless of whether the
--skip-sprockets option is used when creating a new rails application. This is
so you can easily add asset pipelining later if you like.

The directives that work in JavaScript files also work in stylesheets
(though obviously including stylesheets rather than JavaScript files). The
`require_tree` directive in a CSS manifest 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 precise location of the `require_self` call. If
`require_self` is called more than once, only the last call is respected.

NOTE. If you want to use multiple Sass files, you should generally use the [Sass
`@import`
rule](http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#import) instead
of these Sprockets directives. Using Sprockets directives all Sass files exist
within their own scope, making variables or mixins only available within the
document they were defined in.

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. For example, you
might concatenate three CSS files together this way:

```js
/* ...
*= 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.

In development mode, or if the asset pipeline is disabled, when these files are 
requested they are processed by the processors provided by the `coffee-script`
and `sass` gems and then sent back to the browser as JavaScript and CSS
respectively. When asset pipelining is enabled, these files are preprocessed and
placed in the `public/assets` directory for serving by either the Rails app or
web server.

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, then
CoffeeScript, and served as JavaScript.

Keep in mind 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`:

```js
//= require core
//= require projects
//= require tickets
```

would generate this HTML:

```html
<script src="/assets/core.js?body=1"></script>
<script src="/assets/projects.js?body=1"></script>
<script src="/assets/tickets.js?body=1"></script>
```

The `body` param is required by Sprockets.

### Turning Debugging Off

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

```ruby
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:

```html
<script src="/assets/application.js"></script>
```

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 Rails helper methods:

```erb
<%= stylesheet_link_tag "application", debug: true %>
<%= javascript_include_tag "application", debug: true %>
```

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

You can 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 Sprockets uses the fingerprinting scheme outlined
above. By default Rails assumes 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:

```erb
<%= javascript_include_tag "application" %>
<%= stylesheet_link_tag "application" %>
```

generates something like this:

```html
<script src="/assets/application-908e25f4bf641868d8683022a5b62f54.js"></script>
<link href="/assets/application-4dd5b109ee3439da54f5bdfd78a80473.css" media="screen"
rel="stylesheet" />
```

Note: with the Asset Pipeline the :cache and :concat options aren't used
anymore, delete these options from the `javascript_include_tag` and
`stylesheet_link_tag`.

The fingerprinting behavior is controlled by the `config.assets.digest`
initialization option (which defaults to `true` for production and `false` for
everything else).

NOTE: Under normal circumstances the default `config.assets.digest` 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.

Compiled assets are written to the location specified in `config.assets.prefix`.
By default, this is the `/assets` directory.

You can call this task on the server during deployment to create compiled
versions of your assets directly on the server. See the next section for
information on compiling locally.

The rake task is:

```bash
$ RAILS_ENV=production bundle exec rake assets:precompile
```

Capistrano (v2.15.1 and above) includes a recipe to handle this in deployment.
Add the following line to `Capfile`:

```ruby
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 referencing 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 non-JS/CSS files (this will include all image assets
automatically) from `app/assets` folders including your gems:

```ruby
[ Proc.new { |path, fn| fn =~ /app\/assets/ && !%w(.js .css).include?(File.extname(path)) },
/application.(css|js)$/ ]
```

NOTE: The matcher (and other members of the precompile array; see below) is
applied to final compiled file names. This means anything that compiles to
JS/CSS is excluded, as well as raw JS/CSS files; for example, `.coffee` and
`.scss` files are **not** automatically included as they compile to JS/CSS.

If you have other manifests or individual stylesheets and JavaScript files to
include, you can add them to the `precompile` array in `config/application.rb`:

```ruby
config.assets.precompile += ['admin.js', 'admin.css', 'swfObject.js']
```

Or, you can opt to precompile all assets with something like this:

```ruby
# config/application.rb
config.assets.precompile << Proc.new do |path|
  if path =~ /\.(css|js)\z/
    full_path = Rails.application.assets.resolve(path).to_path
    app_assets_path = Rails.root.join('app', 'assets').to_path
    if full_path.starts_with? app_assets_path
      puts "including asset: " + full_path
      true
    else
      puts "excluding asset: " + full_path
      false
    end
  else
    false
  end
end
```

NOTE. Always specify an expected compiled filename that ends with .js or .css,
even if you want to add Sass or CoffeeScript files to the precompile array.

The rake task also generates a `manifest-md5hash.json` that contains a list with
all your assets and their respective fingerprints. This is used by the Rails
helper methods to avoid handing the mapping requests back to Sprockets. A
typical manifest file looks like:

```ruby
{"files":{"application-723d1be6cc741a3aabb1cec24276d681.js":{"logical_path":"application.js","mtime":"2013-07-26T22:55:03-07:00","size":302506,
"digest":"723d1be6cc741a3aabb1cec24276d681"},"application-12b3c7dd74d2e9df37e7cbb1efa76a6d.css":{"logical_path":"application.css","mtime":"2013-07-26T22:54:54-07:00","size":1560,
"digest":"12b3c7dd74d2e9df37e7cbb1efa76a6d"},"application-1c5752789588ac18d7e1a50b1f0fd4c2.css":{"logical_path":"application.css","mtime":"2013-07-26T22:56:17-07:00","size":1591,
"digest":"1c5752789588ac18d7e1a50b1f0fd4c2"},"favicon-a9c641bf2b81f0476e876f7c5e375969.ico":{"logical_path":"favicon.ico","mtime":"2013-07-26T23:00:10-07:00","size":1406,
"digest":"a9c641bf2b81f0476e876f7c5e375969"},"my_image-231a680f23887d9dd70710ea5efd3c62.png":{"logical_path":"my_image.png","mtime":"2013-07-26T23:00:27-07:00","size":6646,
"digest":"231a680f23887d9dd70710ea5efd3c62"}},"assets"{"application.js":
"application-723d1be6cc741a3aabb1cec24276d681.js","application.css":
"application-1c5752789588ac18d7e1a50b1f0fd4c2.css",
"favicon.ico":"favicona9c641bf2b81f0476e876f7c5e375969.ico","my_image.png":
"my_image-231a680f23887d9dd70710ea5efd3c62.png"}}
```

The default location for the manifest is the root of the location specified in
`config.assets.prefix` ('/assets' by default).

NOTE: If there are missing precompiled files in production you will get an
`Sprockets::Helpers::RailsHelper::AssetPaths::AssetNotPrecompiledError`
exception indicating the name of the missing file(s).

#### Far-future Expires Header

Precompiled assets exist on the filesystem and are served directly by your web
server. 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 those
headers.

For Apache:

```apache
# The Expires* directives requires the Apache module `mod_expires` to be
# enabled.
<Location /assets/>
  # Use of ETag is discouraged when Last-Modified is present
  Header unset ETag FileETag None
  # RFC says only cache for 1 year
  ExpiresActive On ExpiresDefault "access plus 1 year"
</Location>
```

For nginx:

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

  add_header ETag "";
  break;
}
```

#### GZip Compression

When files are precompiled, Sprockets also creates a
[gzipped](http://en.wikipedia.org/wiki/Gzip) (.gz) version of your assets. Web
servers are typically configured to use a moderate compression ratio as a
compromise, but since precompilation happens once, Sprockets uses the maximum
compression ratio, thus reducing the size of the data transfer to the minimum.
On the other hand, web servers can be configured to serve compressed content
directly from disk, rather than deflating non-compressed files themselves.

Nginx is able to do this automatically enabling `gzip_static`:

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

This directive is available if the core module that provides this feature was
compiled with the web server. Ubuntu/Debian packages, even `nginx-light`, have
the module compiled. Otherwise, you may need to perform a manual compilation:

```bash
./configure --with-http_gzip_static_module
```

If you're compiling nginx with Phusion Passenger you'll need to pass that option
when prompted.

A robust configuration for Apache is possible but tricky; please Google around.
(Or help update this Guide if you have a good configuration example for Apache.)

### Local Precompilation

There are several reasons why you might want to precompile your assets locally.
Among them are:

* You may not have write access to your production file system.
* You may be deploying to more than one server, and want to avoid
duplication of work.
* You may be doing frequent deploys that do not include asset changes.

Local compilation allows you to commit the compiled files into source control,
and deploy as normal.

There are two caveats:

* You must not run the Capistrano deployment task that precompiles assets.
* You must change the following two application configuration settings.

In `config/environments/development.rb`, place the following line:

```ruby
config.assets.prefix = "/dev-assets"
```

The `prefix` change makes Sprockets use a different URL for serving assets in
development mode, and pass all requests to Sprockets. The prefix is still set to
`/assets` in the production environment. Without this change, the application
would serve the precompiled assets from `/assets` in development, and you would
not see any local changes until you compile assets again.

You will also need to ensure any necessary compressors or minifiers are
available on your development system.

In practice, this will allow you to precompile locally, have those files in your
working tree, and commit those files to source control when needed.  Development
mode will work as expected.

### 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:

```ruby
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 more poorly than the default and is not
recommended.

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

```ruby
group :production do
  gem 'therubyracer'
end
```

### CDNs

If your assets are being served by a CDN, ensure they don't stick around in your
cache forever. This can cause problems. If you use
`config.action_controller.perform_caching = true`, Rack::Cache will use
`Rails.cache` to store assets. This can cause your cache to fill up quickly.

Every cache is different, so evaluate how your CDN handles caching and make sure
that it plays nicely with the pipeline. You may find quirks related to your
specific set up, you may not. The defaults nginx uses, for example, should give
you no problems when used as an HTTP cache.

Customizing the Pipeline
------------------------

### CSS Compression

There is currently one option for compressing CSS, YUI. The [YUI CSS
compressor](http://yui.github.io/yuicompressor/css.html) provides
minification.

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

```ruby
config.assets.css_compressor = :yui
```

### 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](https://github.com/lautis/uglifier).
This gem wraps [UglifyJS](https://github.com/mishoo/UglifyJS) (written for
NodeJS) in Ruby. It compresses your code by removing white space and comments,
shortening local variable names, and performing other micro-optimizations such
as changing `if` and `else` statements to ternary operators where possible.

The following line invokes `uglifier` for JavaScript compression.

```ruby
config.assets.js_compressor = :uglifier
```

NOTE: You will need an [ExecJS](https://github.com/sstephenson/execjs#readme)
supported runtime in order to use `uglifier`. If you are using Mac OS X or
Windows you have a JavaScript runtime installed in your operating system.

NOTE: The `config.assets.compress` initialization option is no longer used in
Rails 4 to enable either CSS or JavaScript compression. Setting it will have no
effect on the application. Instead, setting `config.assets.css_compressor` and
`config.assets.js_compressor` will control compression of CSS and JavaScript
assets.

### 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.

```ruby
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`:

```ruby
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:

```ruby
config.assets.prefix = "/some_other_path"
```

This is a handy option if you are updating an older project that didn't use the
asset pipeline and 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 web server to ignore the response
from the application, and instead serve a specified file from disk. 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 can be enabled in
`config/environments/production.rb`:

```ruby
# config.action_dispatch.x_sendfile_header = "X-Sendfile" # for apache
# 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 any other environments you define with production behavior (not
`application.rb`).

Assets Cache Store
------------------

The default Rails cache store will be used by Sprockets to cache assets in
development and production. This can be changed by setting
`config.assets.cache_store`:

```ruby
config.assets.cache_store = :memory_store
```

The options accepted by the assets cache store are the same as the application's
cache store.

```ruby
config.assets.cache_store = :memory_store, { size: 32.megabytes }
```

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
------------------------------------------

As Sprockets uses [Tilt](https://github.com/rtomayko/tilt) as a generic
interface to different templating engines, your gem should just implement the
Tilt template protocol. Normally, you would subclass `Tilt::Template` and
reimplement `evaluate` method to return final output. Template source is stored
at `@code`. Have a look at
[`Tilt::Template`](https://github.com/rtomayko/tilt/blob/master/lib/tilt/template.rb)
sources to learn more.

```ruby
module BangBang
  class Template < ::Tilt::Template
    # Adds a "!" to original template.
    def evaluate(scope, locals, &block)
      "#{@code}!"
    end
  end
end
```

Now that you have a `Template` class, it's time to associate it with an
extension for template files:

```ruby
Sprockets.register_engine '.bang', BangBang::Template
```

Upgrading from Old Versions of Rails
------------------------------------

There are a few issues when upgrading from Rails 3.0 or Rails 2.x. The first is
moving the files from `public/` to the new locations. See [Asset
Organization](#asset-organization) above for guidance on the correct locations
for different file types.

Next will be avoiding duplicate JavaScript files. Since jQuery is the default
JavaScript library from Rails 3.1 onwards, you don't need to copy `jquery.js`
into `app/assets` and it will be included automatically.

The third is updating the various environment files with the correct default
options.

In `application.rb`:

```ruby
# Version of your assets, change this if you want to expire all your assets
config.assets.version = '1.0'

# Change the path that assets are served from config.assets.prefix = "/assets"
```

In `development.rb`:

```ruby
# Expands the lines which load the assets
config.assets.debug = true
```

And in `production.rb`:

```ruby
# Choose the compressors to use (if any) config.assets.js_compressor  =
# :uglifier config.assets.css_compressor = :yui

# Don't fallback to assets pipeline if a precompiled asset is missed
config.assets.compile = false

# Generate digests for assets URLs. This is planned for deprecation.
config.assets.digest = true

# Precompile additional assets (application.js, application.css, and all
# non-JS/CSS are already added) config.assets.precompile += %w( search.js )
```

Rails 4 no longer sets default config values for Sprockets in `test.rb`, so
`test.rb` now requies Sprockets configuration. The old defaults in the test
environment are: `config.assets.compile = true`, `config.assets.compress =
false`, `config.assets.debug = false` and `config.assets.digest = false`.

The following should also be added to `Gemfile`:

```ruby
gem 'sass-rails',   "~> 3.2.3"
gem 'coffee-rails', "~> 3.2.1"
gem 'uglifier'
```