This guide covers the asset pipeline.
After reading this guide, you will know:
How to understand what the asset pipeline is and what it does. How to properly organize your application assets. How to understand the benefits of the asset pipeline. How to add a pre-processor to the pipeline. How to package assets with a gem.
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.
Making the asset pipeline a core feature of Rails means that 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.
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 the asset pipeline while creating a new application by
passing the --skip-sprockets
option.
rails new appname --skip-sprockets
You should use the defaults for all new applications unless you have a specific reason to avoid the asset pipeline.
The first feature of the pipeline is to concatenate assets. This is important in a production environment, because it 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.
Rails 2.x introduced the ability to concatenate JavaScript and CSS assets
by placing cache: true
at the end of the
javascript_include_tag
and stylesheet_link_tag
methods. But this technique has some limitations. For example, it cannot
generate the caches in advance, and it is not able to transparently include
assets provided by third-party libraries.
Starting with version 3.1, Rails defaults to concatenating 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 that 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.
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 that Rails 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
could be renamed with an MD5 digest of its
contents:
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:
Not all caches will reliably cache content where the filename only differs by query parameters<br /> 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.
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.
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 Revving Filenames: don’t use querystring
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.
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.
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.
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 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] %>
. Ensure
that you are not using the require_tree
directive though, as
this will result in your assets being included more than once.
WARNING: When using asset precompilation (the production default), 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 will be compiled on the fly. When running in production however, you will see 500 errors since live compilation is turned off by default. See 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 documentation to know all supported JavaScript runtimes.
You can also disable the generation of asset files when generating a
controller by adding the following to your
config/application.rb
configuration:
config.generators do |g| g.assets false end
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.
WARNING: If you are upgrading from Rails 3, please take into account that
assets under lib/assets
or vendor/assets
are
available for inclusion via the application manifests but no longer part of
the precompile array. See Precompiling
Assets for guidance.
When a file is referenced from a manifest or a helper, Sprockets searches the three default asset locations for it.
The default locations are: app/assets/images
and the
subdirectories javascripts
and stylesheets
in all
three asset locations, 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:
//= require home //= require moovinator //= require slider //= require phonebox
Assets inside subdirectories can also be accessed.
app/assets/javascripts/sub/something.js
is referenced as:
//= 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:
config.assets.paths << Rails.root.join("lib", "videoplayer", "flash")
Paths are traversed in the order that 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.
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 site's application manifest like so:
//= require library_name
This simplifies maintenance and keeps things clean by allowing related code to be grouped before inclusion elsewhere.
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 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 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" %>
WARNING: If you're precompiling your assets (see 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.
The asset pipeline automatically evaluates ERB. This means that 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:
.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 -%>
.
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"
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 (e.g.,
application.js.coffee.erb
):
$('#logo').attr src: "<%= asset_path('logo.png') %>"
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 the file
size enabling the browser to download it faster.
For example, a new Rails application includes a default
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.
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:
/* ... = require_self = require_tree .
The directives that work in the 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 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:
/* ...
= 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/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
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, then 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 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:
<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.
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:
<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 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 the production environment Rails uses the fingerprinting scheme outlined above. By default Rails assumes 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:
<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 setting of
config.assets.digest
setting in Rails (which defaults to
true
for production and 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.
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
. By default, this is the
public/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:
$ RAILS_ENV=production bundle exec rake assets:precompile
For faster asset precompiles, you can partially load your application by
setting config.assets.initialize_on_precompile
to false in
config/application.rb
, though in that case templates cannot
see application objects or methods. Heroku requires this to be
false.
WARNING: If you set config.assets.initialize_on_precompile
to
false, be sure to test rake assets:precompile
locally before
deploying. It may expose bugs where your assets reference application
objects or methods, since those are still in scope in development mode
regardless of the value of this flag. Changing this flag also affects
engines. Engines can define assets for precompilation as well. Since the
complete environment is not loaded, engines (or other gems) will not be
loaded, which can cause missing assets.
Capistrano (v2.8.0 and above) includes 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.
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:
[ 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 that 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
:
config.assets.precompile += ['admin.js', 'admin.css', 'swfObject.js']
Or you can opt to precompile all assets with something like this:
# 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.yml
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:
--- 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).
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).
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 them.
For 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:
location ~ ^/assets/ { expires 1y; add_header Cache-Control public; add_header ETag ""; break; }
When files are precompiled, Sprockets also creates a gzipped (.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
:
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 packages, even
nginx-light
have the module compiled. Otherwise, you may need
to perform a manual compilation:
./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 example configuration for Apache.)
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 the 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:
config.assets.prefix = "/dev-assets"
You will also need this in application.rb:
config.assets.initialize_on_precompile = false
The prefix
change makes Rails 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
public/assets
in development, and you would not see any local
changes until you compile assets again.
The initialize_on_precompile
change tells the precompile task
to run without invoking Rails. This is because the precompile task runs in
production mode by default, and will attempt to connect to your specified
production database. Please note that you cannot have code in pipeline
files that relies on Rails resources (such as the database) when compiling
locally with this option.
You will also need to ensure that any 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.
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 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:
group :production do gem 'therubyracer' end
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.
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.
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. It also includes other optimizations such as changing your
if
and else
statements to ternary operators where
possible.
The following line invokes uglifier
for JavaScript
compression.
config.assets.js_compressor = :uglifier
Note that config.assets.compress
must be set to
true
to enable JavaScript compression
NOTE: You will need an ExecJS 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.
Check the ExecJS
documentation for information on all of the supported JavaScript runtimes.
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
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 are updating an older project that didn't use the asset pipeline and that already uses this path or you wish to use this path for a new resource.
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
.
# 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
).
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
.
config.assets.cache_store = :memory_store
The options accepted by the assets cache store are the same as the application's cache store.
config.assets.cache_store = :memory_store, { size: 32.megabytes }
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.
As Sprockets uses 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
sources to learn more.
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:
Sprockets.register_engine '.bang', BangBang::Template
There are a few issues when upgrading. The first is moving the files from
public/
to the new locations. See 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. The following changes reflect the defaults in version 3.1.0.
In application.rb
:
# Enable the asset pipeline config.assets.enabled = true # 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
:
# Do not compress assets config.assets.compress = false # Expands the lines which load the assets config.assets.debug = true
And in production.rb
:
# Compress JavaScripts and CSS config.assets.compress = true # Choose the compressors to use # 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. 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 )
You should not need to change 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
:
# Gems used only for assets and not required # in production environments by default. group :assets do gem 'sass-rails', "~> 3.2.3" gem 'coffee-rails', "~> 3.2.1" 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 you precompile assets before deploying to production, use this line Bundler.require *Rails.groups(:assets => %w(development test)) # If you want your assets lazily compiled in production, use this line # Bundler.require(:default, :assets, Rails.env)
Instead of the generated version:
# Require the gems listed in Gemfile, including any gems # you've limited to :test, :development, or :production. Bundler.require(:default, Rails.env)