The Asset Pipeline

1 What is the Asset Pipeline?

The asset pipeline provides a framework to handle the delivery of JavaScript and CSS assets. This is done by leveraging technologies like HTTP/2 and techniques like concatenation and minification. Finally, it allows your application to be automatically combined with assets from other gems.

The asset pipeline is implemented by the importmap-rails, sprockets and sprockets-rails gems, and is enabled by default. You can disable it while creating a new application by passing the --skip-asset-pipeline option.

$ rails new appname --skip-asset-pipeline

This guide focuses on the default asset pipeline using only sprockets for CSS and importmap-rails for JavaScript processing. The main limitation of those two is that there is no support for transpiling so you can't use things like Babel, TypeScript, Sass, React JSX format, or Tailwind CSS. We encourage you to read the Alternative Libraries section if you need transpiling for your JavaScript/CSS.

2 Main Features

The asset pipeline's first feature is inserting a SHA256 fingerprinting into each filename so that the file is cached by the web browser and CDN. This fingerprint is automatically updated when you change the file contents, which invalidates the cache.

The second feature of the asset pipeline is to use import maps when serving JavaScript files. This lets you build modern applications using JavaScript libraries made for ES modules (ESM) without the need for transpiling and bundling. In turn, this eliminates the need for Webpack, yarn, node or any other part of the JavaScript toolchain.

The third feature of the asset pipeline is to concatenate all CSS files into one main .css file, which is then minified or compressed. As you'll learn later in this guide, you can customize this strategy to group files any way you like. In production, Rails inserts a SHA256 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 fourth feature of the asset pipeline is it allows coding assets via a higher-level language for CSS.

2.1 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 static or infrequently changed content, 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.

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

2.2 What are Import Maps and Why Should I Care?

Import maps let you import JavaScript modules using logical names that map to versioned/digested files – directly from the browser. So you can build modern JavaScript applications using JavaScript libraries made for ES modules (ESM) without the need for transpiling or bundling.

With this approach, you'll ship many small JavaScript files instead of one big JavaScript file. Thanks to HTTP/2 that no longer carries a material performance penalty during the initial transport, and in fact offers substantial benefits over the long run due to better caching dynamics.

3 How to use Import Maps as JavaScript Asset Pipeline

Import Maps are the default JavaScript processor, the logic of generating import maps is handled by the importmap-rails gem.

Import maps are used only for JavaScript files and can not be used for CSS delivery. Check the Sprockets section to learn about CSS.

You can find detailed usage instructions on the Gem homepage, but it's important to understand the basics of importmap-rails.

3.1 How it works

Import maps are essentially a string substitution for what is referred to as "bare module specifiers". They allow you to standardize the names of JavaScript module imports.

Take for example such an import definition, it won't work without an import map:

import React from "react"

You would have to define it like this to make it work:

import React from "https://ga.jspm.io/npm:react@17.0.2/index.js"

Here comes the import map, we define the react name to be pinned to the https://ga.jspm.io/npm:react@17.0.2/index.js address. With such information our browser accepts the simplified import React from "react" definition. Think of import map as about an alias for the library source address.

3.2 Usage

With importmap-rails you create the importmap configuration file pinning the library path to a name:

# config/importmap.rb
pin "application"
pin "react", to: "https://ga.jspm.io/npm:react@17.0.2/index.js"

All of the configured import maps should be attached in <head> element of your application by adding <%= javascript_importmap_tags %> . The javascript_importmap_tags renders a bunch of scripts in the head element:

JSON with all configured import maps:

<script type="importmap">
{
  "imports": {
    "application": "/assets/application-39f16dc3f3....js"
    "react": "https://ga.jspm.io/npm:react@17.0.2/index.js"
  }
}
</script>

Es-module-shims acting as polyfill ensuring support for import maps on older browsers:

<script src="/assets/es-module-shims.min" async="async" data-turbo-track="reload"></script>

Entrypoint for loading JavaScript from app/javascript/application.js:

<script type="module">import "application"</script>

3.3 Using npm packages via JavaScript CDNs

You can use the bin/importmap command that's added as part of the importmap-rails install to pin, unpin, or update npm packages in your import map. The binstub uses JSPM.org.

It works like so:

$ bin/importmap pin react react-dom
Pinning "react" to https://ga.jspm.io/npm:react@17.0.2/index.js
Pinning "react-dom" to https://ga.jspm.io/npm:react-dom@17.0.2/index.js
Pinning "object-assign" to https://ga.jspm.io/npm:object-assign@4.1.1/index.js
Pinning "scheduler" to https://ga.jspm.io/npm:scheduler@0.20.2/index.js

bin/importmap json

{
  "imports": {
    "application": "/assets/application-37f365cbecf1fa2810a8303f4b6571676fa1f9c56c248528bc14ddb857531b95.js",
    "react": "https://ga.jspm.io/npm:react@17.0.2/index.js",
    "react-dom": "https://ga.jspm.io/npm:react-dom@17.0.2/index.js",
    "object-assign": "https://ga.jspm.io/npm:object-assign@4.1.1/index.js",
    "scheduler": "https://ga.jspm.io/npm:scheduler@0.20.2/index.js"
  }
}

As you can see, the two packages react and react-dom resolve to a total of four dependencies, when resolved via the jspm default.

Now you can use these in your application.js entrypoint like you would any other module:

import React from "react"
import ReactDOM from "react-dom"

You can also designate a specific version to pin:

$ bin/importmap pin react@17.0.1
Pinning "react" to https://ga.jspm.io/npm:react@17.0.1/index.js
Pinning "object-assign" to https://ga.jspm.io/npm:object-assign@4.1.1/index.js

Or even remove pins:

$ bin/importmap unpin react
Unpinning "react"
Unpinning "object-assign"

You can control the environment of the package for packages with separate "production" (the default) and "development" builds:

$ bin/importmap pin react --env development
Pinning "react" to https://ga.jspm.io/npm:react@17.0.2/dev.index.js
Pinning "object-assign" to https://ga.jspm.io/npm:object-assign@4.1.1/index.js

You can also pick an alternative, supported CDN provider when pinning, like unpkg or jsdelivr (jspm is the default):

$ bin/importmap pin react --from jsdelivr
Pinning "react" to https://cdn.jsdelivr.net/npm/react@17.0.2/index.js

Remember, though, that if you switch a pin from one provider to another, you may have to clean up dependencies added by the first provider that isn't used by the second provider.

Run bin/importmap to see all options.

Note that this command is merely a convenience wrapper to resolving logical package names to CDN URLs. You can also just lookup the CDN URLs yourself, and then pin those. For example, if you wanted to use Skypack for React, you could just add the following to config/importmap.rb:

pin "react", to: "https://cdn.skypack.dev/react"

3.4 Preloading pinned modules

To avoid the waterfall effect where the browser has to load one file after another before it can get to the deepest nested import, importmap-rails supports modulepreload links. Pinned modules can be preloaded by appending preload: true to the pin.

It's a good idea to preload libraries or frameworks that are used throughout your app, as this will tell the browser to download them sooner.

Example:

# config/importmap.rb
pin "@github/hotkey", to: "https://ga.jspm.io/npm:@github/hotkey@1.4.4/dist/index.js", preload: true
pin "md5", to: "https://cdn.jsdelivr.net/npm/md5@2.3.0/md5.js"

# app/views/layouts/application.html.erb
<%= javascript_importmap_tags %>

# will include the following link before the importmap is setup:
<link rel="modulepreload" href="https://ga.jspm.io/npm:@github/hotkey@1.4.4/dist/index.js">
...

Refer to importmap-rails repository for the most up-to-date documentation.

4 How to Use Sprockets

The naive approach to expose your application assets to the web would be to store them in subdirectories of public folder such as images and stylesheets. Doing so manually would be difficult as most of the modern web applications require the assets to be processed in specific way for eg. compressing and adding fingerprints to the assets.

Sprockets is designed to automatically preprocess your assets stored in the configured directories and after processing expose them in the public/assets folder with fingerprinting, compression, source maps generation and other configurable features.

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 when config.public_file_server.enabled is set to true. You must define manifest.js directives 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 the app/assets are never served directly in production.

4.1 Manifest Files and Directives

When compiling assets with Sprockets, Sprockets needs to decide which top-level targets to compile, usually application.css and images. The top-level targets are defined in the Sprockets manifest.js file, by default it looks like this:

//= link_tree ../images
//= link_directory ../stylesheets .css
//= link_tree ../../javascript .js
//= link_tree ../../../vendor/javascript .js

It contains directives - instructions that tell Sprockets which files to require in order to build a single CSS or JavaScript file.

This is meant to include the contents of all files found in the ./app/assets/images directory or any subdirectories as well as any file recognized as JS directly at ./app/javascript or ./vendor/javascript.

It will load any CSS from the ./app/assets/stylesheets directory (not including subdirectories). Assuming that you have application.css and marketing.css files in the ./app/assets/stylesheets folder it will allow you to load those stylesheets with <%= stylesheet_link_tag "application" %> or <%= stylesheet_link_tag "marketing" %> from your views.

You might notice that our JavaScript files aren't loaded from the assets directory by default, it's because ./app/javascript is the default entry point for importmap-rails gem and the vendor folder is the place where downloaded JS packages would be stored.

In the manifest.js you could also specify the link directive to load a specific file instead of the whole directory. link directive requires providing explicit file extension.

Sprockets loads the files specified, processes them if necessary, concatenates them into one single file, and then compresses them (based on the value of config.assets.css_compressor or config.assets.js_compressor). Compression reduces file size, enabling the browser to download the files faster.

4.2 Controller Specific Assets

When you generate a scaffold or a controller, Rails also generates a Cascading Style Sheet file for that controller. Additionally, when generating a scaffold, Rails generates the file scaffolds.css.

For example, if you generate a ProjectsController, Rails will also add a new file at app/assets/stylesheets/projects.css. By default, these files will be ready to use by your application immediately using the link_directory directive in the manifest.js file.

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

<%= stylesheet_link_tag params[:controller] %>

When doing this, ensure you are not using the require_tree directive in your application.css, as that could result in your controller-specific assets being included more than once.

4.3 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 or stylesheets.
app/javascript is for your JavaScript code
vendor/[assets|javascript] is for assets that are owned by outside entities, such as CSS frameworks or JavaScript libraries. Keep in mind that third-party code with references to other files also processed by the asset Pipeline (images, stylesheets, etc.), will need to be rewritten to use helpers like asset_path.

Other locations could be configured in the manifest.js file, refer to the Manifest Files and Directives.

4.3.1 Search Paths

When a file is referenced from a manifest or a helper, Sprockets searches all of the locations specified in manifest.js for it. You can view the search path by inspecting Rails.application.config.assets.paths in the Rails console.

4.3.2 Using Index Files as proxies for folders

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

For example, if you have a CSS library with many modules, which is stored in lib/assets/stylesheets/library_name, the file lib/assets/stylesheets/library_name/index.css 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.

It is also somewhat similar to the way that a file in public/library_name/index.html can be reached by a request to /library_name. This means that you cannot directly use an index file.

The library as a whole can be accessed in the .css files like so:

/* ...
*= require library_name
*/

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

4.4 Coding Links to Assets

Sprockets does not add any new methods to access your assets - you still use the familiar stylesheet_link_tag:

<%= stylesheet_link_tag "application", media: "all" %>

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

<%= stylesheet_link_tag "application", "data-turbo-track": "reload" %>

In regular views you can access images in the app/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 SHA256 hash such as public/assets/rails-f90d8a84c707a8dc923fca1ca1895ae8ed0a09237f6992015fef1e11be77c023.png is treated the same way. How these hashes are generated is covered in the In Production section later on in this guide.

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

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

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.

4.4.1 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:

.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 -%>.

4.5 Raise an Error When an Asset is Not Found

If you are using sprockets-rails >= 3.2.0 you can configure what happens when an asset lookup is performed and nothing is found. If you turn off "asset fallback" then an error will be raised when an asset cannot be found.

config.assets.unknown_asset_fallback = false

If "asset fallback" is enabled then when an asset cannot be found the path will be output instead and no error raised. The asset fallback behavior is disabled by default.

4.6 Turning Digests Off

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

config.assets.digest = false

When this option is true, digests will be generated for asset URLs.

4.7 Turning Source Maps On

You can turn on source maps by updating config/environments/development.rb to include:

config.assets.debug = true

When debug mode is on, Sprockets will generate a Source Map for each asset. This allows you to debug each file individually in your browser's developer tools.

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 change between requests, the server responds with a new compiled file.

5 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 a SHA256 is generated from the contents of the compiled files, and inserted into the filenames as they are written to disk. These fingerprinted names are used by the Rails helpers in place of the manifest name.

For example this:

<%= stylesheet_link_tag "application" %>

generates something like this:

<link href="/assets/application-4dd5b109ee3439da54f5bdfd78a80473.css" rel="stylesheet" />

The fingerprinting behavior is controlled by the config.assets.digest initialization option (which defaults to true).

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.

5.1 Precompiling Assets

Rails comes bundled with a command 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 command 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 command is:

$ RAILS_ENV=production rails assets:precompile

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 command.

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.

Always specify an expected compiled filename that ends with .js or .css.

The command also generates a .sprockets-manifest-randomhex.json (where randomhex is a 16-byte random hex string) 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 this:

{"files":{"application-<fingerprint>.js":{"logical_path":"application.js","mtime":"2016-12-23T20:12:03-05:00","size":412383,
"digest":"<fingerprint>","integrity":"sha256-<random-string>"}},
"assets":{"application.js":"application-<fingerprint>.js"}}

In your application, there will be more files and assets listed in the manifest, <fingerprint> and <random-string> will also be generated.

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

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

5.1.1 Far-future Expires Header

Precompiled assets exist on the file system 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:

# 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 "";
}

5.2 Local Precompilation

Sometimes, you may not want or be able to compile assets on the production server. For instance, you may have limited write access to your production filesystem, or you may plan to deploy frequently without making any changes to your assets.

In such cases, you can precompile assets locally — that is, add a finalized set of compiled, production-ready assets to your source code repository before pushing to production. This way, they do not need to be precompiled separately on the production server upon each deployment.

As above, you can perform this step using

$ RAILS_ENV=production rails assets:precompile

Note the following caveats:

If precompiled assets are available, they will be served — even if they no longer match the original (uncompiled) assets, even on the development server.
To ensure that the development server always compiles assets on-the-fly (and thus always reflects the most recent state of the code), the development environment must be configured to keep precompiled assets in a different location than production does. Otherwise, any assets precompiled for use in production will clobber requests for them in development (i.e., subsequent changes you make to assets will not be reflected in the browser).
You can do this by adding the following line to config/environments/development.rb:
```
config.assets.prefix = "/dev-assets"
```
The asset precompile task in your deployment tool (e.g., Capistrano) should be disabled.
Any necessary compressors or minifiers must be available on your development system.

You can also set ENV["SECRET_KEY_BASE_DUMMY"] to trigger the use of a randomly generated secret_key_base that's stored in a temporary file. This is useful when precompiling assets for production as part of a build step that otherwise does not need access to the production secrets.

$ SECRET_KEY_BASE_DUMMY=1 bundle exec rails assets:precompile

5.3 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 Assets Cache Store, and the manifest names used in the helpers are altered to include the SHA256 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.

5.4 CDNs

CDN stands for Content Delivery Network, they are primarily designed to cache assets all over the world so that when a browser requests the asset, a cached copy will be geographically close to that browser. If you are serving assets directly from your Rails server in production, the best practice is to use a CDN in front of your application.

A common pattern for using a CDN is to set your production application as the "origin" server. This means when a browser requests an asset from the CDN and there is a cache miss, it will grab the file from your server on the fly and then cache it. For example if you are running a Rails application on example.com and have a CDN configured at mycdnsubdomain.fictional-cdn.com, then when a request is made to mycdnsubdomain.fictional-cdn.com/assets/smile.png, the CDN will query your server once at example.com/assets/smile.png and cache the request. The next request to the CDN that comes in to the same URL will hit the cached copy. When the CDN can serve an asset directly the request never touches your Rails server. Since the assets from a CDN are geographically closer to the browser, the request is faster, and since your server doesn't need to spend time serving assets, it can focus on serving application code as fast as possible.

5.4.1 Set up a CDN to Serve Static Assets

To set up your CDN you have to have your application running in production on the internet at a publicly available URL, for example example.com. Next you'll need to sign up for a CDN service from a cloud hosting provider. When you do this you need to configure the "origin" of the CDN to point back at your website example.com. Check your provider for documentation on configuring the origin server.

The CDN you provisioned should give you a custom subdomain for your application such as mycdnsubdomain.fictional-cdn.com (note fictional-cdn.com is not a valid CDN provider at the time of this writing). Now that you have configured your CDN server, you need to tell browsers to use your CDN to grab assets instead of your Rails server directly. You can do this by configuring Rails to set your CDN as the asset host instead of using a relative path. To set your asset host in Rails, you need to set config.asset_host in config/environments/production.rb:

config.asset_host = 'mycdnsubdomain.fictional-cdn.com'

You only need to provide the "host", this is the subdomain and root domain, you do not need to specify a protocol or "scheme" such as http:// or https://. When a web page is requested, the protocol in the link to your asset that is generated will match how the webpage is accessed by default.

You can also set this value through an environment variable to make running a staging copy of your site easier:

config.asset_host = ENV['CDN_HOST']

You would need to set CDN_HOST on your server to mycdnsubdomain .fictional-cdn.com for this to work.

Once you have configured your server and your CDN, asset paths from helpers such as:

<%= asset_path('smile.png') %>

Will be rendered as full CDN URLs like http://mycdnsubdomain.fictional-cdn.com/assets/smile.png (digest omitted for readability).

If the CDN has a copy of smile.png, it will serve it to the browser, and your server doesn't even know it was requested. If the CDN does not have a copy, it will try to find it at the "origin" example.com/assets/smile.png, and then store it for future use.

If you want to serve only some assets from your CDN, you can use custom :host option your asset helper, which overwrites value set in config.action_controller.asset_host.

<%= asset_path 'image.png', host: 'mycdnsubdomain.fictional-cdn.com' %>

5.4.2 Customize CDN Caching Behavior

A CDN works by caching content. If the CDN has stale or bad content, then it is hurting rather than helping your application. The purpose of this section is to describe general caching behavior of most CDNs. Your specific provider may behave slightly differently.

5.4.2.1 CDN Request Caching

While a CDN is described as being good for caching assets, it actually caches the entire request. This includes the body of the asset as well as any headers. The most important one being Cache-Control, which tells the CDN (and web browsers) how to cache contents. This means that if someone requests an asset that does not exist, such as /assets/i-dont-exist.png, and your Rails application returns a 404, then your CDN will likely cache the 404 page if a valid Cache-Control header is present.

5.4.2.2 CDN Header Debugging

One way to check the headers are cached properly in your CDN is by using curl. You can request the headers from both your server and your CDN to verify they are the same:

$ curl -I http://www.example/assets/application-
d0e099e021c95eb0de3615fd1d8c4d83.css
HTTP/1.1 200 OK
Server: Cowboy
Date: Sun, 24 Aug 2014 20:27:50 GMT
Connection: keep-alive
Last-Modified: Thu, 08 May 2014 01:24:14 GMT
Content-Type: text/css
Cache-Control: public, max-age=2592000
Content-Length: 126560
Via: 1.1 vegur

Versus the CDN copy:

$ curl -I http://mycdnsubdomain.fictional-cdn.com/application-
d0e099e021c95eb0de3615fd1d8c4d83.css
HTTP/1.1 200 OK Server: Cowboy Last-
Modified: Thu, 08 May 2014 01:24:14 GMT Content-Type: text/css
Cache-Control:
public, max-age=2592000
Via: 1.1 vegur
Content-Length: 126560
Accept-Ranges:
bytes
Date: Sun, 24 Aug 2014 20:28:45 GMT
Via: 1.1 varnish
Age: 885814
Connection: keep-alive
X-Served-By: cache-dfw1828-DFW
X-Cache: HIT
X-Cache-Hits:
68
X-Timer: S1408912125.211638212,VS0,VE0

Check your CDN documentation for any additional information they may provide such as X-Cache or for any additional headers they may add.

5.4.2.3 CDNs and the Cache-Control Header

The Cache-Control header describes how a request can be cached. When no CDN is used, a browser will use this information to cache contents. This is very helpful for assets that are not modified so that a browser does not need to re-download a website's CSS or JavaScript on every request. Generally we want our Rails server to tell our CDN (and browser) that the asset is "public". That means any cache can store the request. Also we commonly want to set max-age which is how long the cache will store the object before invalidating the cache. The max-age value is set to seconds with a maximum possible value of 31536000, which is one year. You can do this in your Rails application by setting

config.public_file_server.headers = {
  'Cache-Control' => 'public, max-age=31536000'
}

Now when your application serves an asset in production, the CDN will store the asset for up to a year. Since most CDNs also cache headers of the request, this Cache-Control will be passed along to all future browsers seeking this asset. The browser then knows that it can store this asset for a very long time before needing to re-request it.

5.4.2.4 CDNs and URL-based Cache Invalidation

Most CDNs will cache contents of an asset based on the complete URL. This means that a request to

http://mycdnsubdomain.fictional-cdn.com/assets/smile-123.png

Will be a completely different cache from

http://mycdnsubdomain.fictional-cdn.com/assets/smile.png

If you want to set far future max-age in your Cache-Control (and you do), then make sure when you change your assets that your cache is invalidated. For example when changing the smiley face in an image from yellow to blue, you want all visitors of your site to get the new blue face. When using a CDN with the Rails asset pipeline config.assets.digest is set to true by default so that each asset will have a different file name when it is changed. This way you don't have to ever manually invalidate any items in your cache. By using a different unique asset name instead, your users get the latest asset.

6 Customizing the Pipeline

6.1 CSS Compression

One of the options for compressing CSS is YUI. The YUI CSS compressor provides minification.

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

config.assets.css_compressor = :yui

6.2 JavaScript Compression

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

Take the terser gem, for example. This gem wraps Terser (written for Node.js) 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 terser for JavaScript compression.

config.assets.js_compressor = :terser

You will need an ExecJS supported runtime in order to use terser. If you are using macOS or Windows you have a JavaScript runtime installed in your operating system.

The JavaScript compression will also work for your JavaScript files when you are loading your assets through importmap-rails or jsbundling-rails gems.

6.3 GZipping your assets

By default, gzipped version of compiled assets will be generated, along with the non-gzipped version of assets. Gzipped assets help reduce the transmission of data over the wire. You can configure this by setting the gzip flag.

config.assets.gzip = false # disable gzipped assets generation

Refer to your web server's documentation for instructions on how to serve gzipped assets.

6.4 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

6.5 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 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.

6.6 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. Have a look at send_file on how to use this feature.

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

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).

For further details have a look at the docs of your production web server:

Apache
NGINX

7 Assets Cache Store

By default, Sprockets caches assets in tmp/cache/assets in development and production environments. This can be changed as follows:

config.assets.configure do |env|
  env.cache = ActiveSupport::Cache.lookup_store(:memory_store,
                                                { size: 32.megabytes })
end

To disable the assets cache store:

config.assets.configure do |env|
  env.cache = ActiveSupport::Cache.lookup_store(:null_store)
end

8 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. 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.

9 Making Your Library or Gem a Pre-Processor

Sprockets uses Processors, Transformers, Compressors, and Exporters to extend Sprockets functionality. Have a look at Extending Sprockets to learn more. Here we registered a preprocessor to add a comment to the end of text/css (.css) files.

module AddComment
  def self.call(input)
    { data: input[:data] + "/* Hello From my sprockets extension */" }
  end
end

Now that you have a module that modifies the input data, it's time to register it as a preprocessor for your MIME type.

Sprockets.register_preprocessor 'text/css', AddComment

10 Alternative Libraries

Over the years there have been multiple default approaches for handling the assets. The web evolved and we started to see more and more JavaScript-heavy applications. In The Rails Doctrine we believe that The Menu Is Omakase so we focused on the default setup: Sprockets with Import Maps.

We are aware that there are no one-size-fits-it-all solutions for the various JavaScript and CSS frameworks/extensions available. There are other bundling libraries in the Rails ecosystem that should empower you in the cases where the default setup isn't enough.

10.1 jsbundling-rails

jsbundling-rails is a JavaScript run-time dependent alternative to the importmap-rails way of bundling JS with Bun, esbuild, rollup.js, or Webpack.

The gem provides a build task in package.json to watch for changes and automatically generate output in development. For production, it automatically hooks javascript:build task into assets:precompile task to ensure that all your package dependencies have been installed and JavaScript has been built for all entry points.

When to use instead of importmap-rails? If your JavaScript code depends on transpiling so if you are using Babel, TypeScript or React JSX format then jsbundling-rails is the correct way to go.

10.2 Webpacker/Shakapacker

Webpacker was the default JavaScript pre-processor and bundler for Rails 5 and 6. It has now been retired. A successor called shakapacker exists, but is not maintained by the Rails team or project.

Unlike other libraries in this list webpacker/shakapacker is completely independent of Sprockets and could process both JavaScript and CSS files.

Read the Comparison with Webpacker document to understand the differences between jsbundling-rails and webpacker/shakapacker.

10.3 cssbundling-rails

cssbundling-rails allows bundling and processing of your CSS using Tailwind CSS, Bootstrap, Bulma, PostCSS, or Dart Sass, then delivers the CSS via the asset pipeline.

It works in a similar way to jsbundling-rails so adds the Node.js dependency to your application with yarn build:css --watch process to regenerate your stylesheets in development and hooks into assets:precompile task in production.

What's the difference between Sprockets? Sprockets on its own is not able to transpile the Sass into CSS, Node.js is required to generate the .css files from your .sass files. Once the .css files are generated then Sprockets is able to deliver them to your clients.

cssbundling-rails relies on Node to process the CSS. The dartsass-rails and tailwindcss-rails gems use standalone versions of Tailwind CSS and Dart Sass, meaning no Node dependency. If you are using importmap-rails to handle your JavaScripts and dartsass-rails or tailwindcss-rails for CSS you could completely avoid the Node dependency resulting in a less complex solution.

10.4 dartsass-rails

If you want to use Sass in your application, dartsass-rails comes as a replacement for the legacy sassc-rails gem. dartsass-rails uses the Dart Sass implementation in favour of deprecated in 2020 LibSass used by sassc-rails.

Unlike sassc-rails the new gem is not directly integrated with Sprockets. Please refer to the gem homepage for installation/migration instructions.

The popular sassc-rails gem is unmaintained since 2019.

10.5 tailwindcss-rails

tailwindcss-rails is a wrapper gem for the standalone executable version of Tailwind CSS v3 framework. Used for new applications when --css tailwind is provided to the rails new command. Provides a watch process to automatically generate Tailwind output in development. For production it hooks into assets:precompile task.

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