activeadmin-quill_editor/docs/propshaft-readme.md

Propshaft: A Modern Asset Pipeline for Rails

Propshaft is an asset pipeline library for Rails. It's built for an era where bundling assets to save on HTTP connections is no longer urgent, where JavaScript and CSS are either compiled by dedicated Node.js bundlers or served directly to the browsers, and where increases in bandwidth have made the need for minification less pressing. These factors allow for a dramatically simpler and faster asset pipeline compared to previous options, like Sprockets.

Table of Contents

1. Overview
2. Architecture
3. Asset Serving in Development/Test Mode
4. Configuration and Asset Paths
5. Test Environment Specifics
6. JavaScript and CSS Bundling Integration
7. Common Patterns and Best Practices

Overview

So that's what Propshaft doesn't do. Here's what it does provide:

1. Configurable load path: You can register directories from multiple places in your app and gems, and reference assets from all of these paths as though they were one.
2. Digest stamping: All assets in the load path will be copied (or compiled) in a precompilation step for production that also stamps all of them with a digest hash, so you can use long-expiry cache headers for better performance. The digested assets can be referred to through their logical path because the processing leaves a manifest file that provides a way to translate.
3. Development server: There's no need to precompile the assets in development. You can refer to them via the same asset_path helpers and they'll be served by a development server.
4. Basic compilers: Propshaft was explicitly not designed to provide full transpiler capabilities. You can get that better elsewhere. But it does offer a simple input->output compiler setup that by default is used to translate url(asset) function calls in CSS to url(digested-asset) instead and source mapping comments likewise.

Architecture

Core Components

Propshaft::Assembly
├── LoadPath          # Asset discovery and caching
├── Resolver           # Path resolution (Dynamic/Static)
│   ├── Dynamic        # Development/test mode
│   └── Static         # Production with manifest
├── Server            # Rack middleware for asset serving
├── Processor         # Precompilation and digesting
├── Compilers         # Asset transformation
└── Manifest          # Asset mapping and integrity hashes

Key Classes

Propshaft::Assembly

Central coordinator that manages all components. Created during Rails initialization and accessible via Rails.application.assets.

# Core assembly configuration
Rails.application.configure do
  app.assets = Propshaft::Assembly.new(app.config.assets)
end

Propshaft::LoadPath

Manages asset discovery across multiple directories. Automatically includes:

• app/assets/**/* (application assets)
• lib/assets/**/* (library assets)
• vendor/assets/**/* (third-party assets)
• Engine assets from all loaded gems

Propshaft::Resolver::Dynamic

Used in development and test environments. Resolves assets on-demand without requiring precompilation.

Propshaft::Resolver::Static

Used in production. Relies on .manifest.json for fast asset resolution.

Asset Serving in Development/Test Mode

Dynamic Resolution Process

1. Request Interception: Propshaft::Server middleware catches requests to /assets/*
2. Asset Discovery: LoadPath#find searches configured paths for matching assets
3. Compilation: Assets pass through registered compilers
4. Cache Headers: Aggressive caching with ETags and immutable cache-control
5. Response: Compiled content served with appropriate MIME type

# Server middleware in action (simplified)
def call(env)
  if path.start_with?(@assembly.prefix) && (asset = @assembly.load_path.find(path))
    [200, {
      'Content-Type' => asset.content_type,
      'ETag' => "\"#{asset.digest}\"",
      'Cache-Control' => 'public, max-age=31536000, immutable'
    }, [asset.compiled_content]]
  end
end

Cache Sweeping

In development/test, Propshaft monitors file changes:

config.assets.sweep_cache = Rails.env.development?

When enabled, before each request:

1. File watcher checks for modifications
2. Asset cache cleared if changes detected
3. New assets discovered and cached

Configuration and Asset Paths

Default Configuration (from Railtie)

config.assets.paths          = []           # Auto-populated
config.assets.excluded_paths = []           # Paths to exclude
config.assets.version        = "1"          # Cache invalidation
config.assets.prefix         = "/assets"    # URL prefix
config.assets.server         = Rails.env.development? || Rails.env.test?
config.assets.sweep_cache    = Rails.env.development?

Path Resolution Order

1. Application paths: app/assets/**/*
2. Library paths: lib/assets/**/*
3. Vendor paths: vendor/assets/**/*
4. Engine paths: From all loaded Rails engines/gems

Paths are automatically prioritized:

• Application assets take precedence over engine assets
• Later additions to load path have lower priority

Installation

With Rails 8, Propshaft is the default asset pipeline for new applications. With Rails 7, you can start a new application with propshaft using rails new myapp -a propshaft. For existing applications, check the upgrade guide which contains step-by-step instructions.

Usage

Propshaft makes all the assets from all the paths it's been configured with through config.assets.paths available for serving and will copy all of them into public/assets when precompiling. This is unlike Sprockets, which did not copy over assets that hadn't been explicitly included in one of the bundled assets.

You can however exempt directories that have been added through the config.assets.excluded_paths. This is useful if you're for example using app/assets/stylesheets exclusively as a set of inputs to a compiler like Dart Sass for Rails, and you don't want these input files to be part of the load path. (Remember you need to add full paths, like Rails.root.join("app/assets/stylesheets")).

These assets can be referenced through their logical path using the normal helpers like asset_path, image_tag, javascript_include_tag, and all the other asset helper tags. These logical references are automatically converted into digest-aware paths in production when assets:precompile has been run (through a JSON mapping file found in public/assets/.manifest.json).

Referencing digested assets in CSS and JavaScript

Propshaft will automatically convert asset references in CSS to use the digested file names. So background: url("/bg/pattern.svg") is converted to background: url("/assets/bg/pattern-2169cbef.svg") before the stylesheet is served.

For JavaScript, you'll have to manually trigger this transformation by using the RAILS_ASSET_URL pseudo-method. It's used like this:

export default class extends Controller {
  init() {
    this.img = RAILS_ASSET_URL("/icons/trash.svg")
  }
}

That'll turn into:

export default class extends Controller {
  init() {
    this.img = "/assets/icons/trash-54g9cbef.svg"
  }
}

Bypassing the digest step

If you need to put multiple files that refer to each other through Propshaft, like a JavaScript file and its source map, you have to digest these files in advance to retain stable file names. Propshaft looks for the specific pattern of -[digest].digested.js as the postfix to any asset file as an indication that the file has already been digested.

Subresource Integrity (SRI)

Propshaft supports Subresource Integrity (SRI) to help protect against malicious modifications of assets. SRI allows browsers to verify that resources fetched from CDNs or other sources haven't been tampered with by checking cryptographic hashes.

Enabling SRI

To enable SRI support, configure the hash algorithm in your Rails application:

config.assets.integrity_hash_algorithm = "sha384"

Valid hash algorithms include:

• "sha256" - SHA-256 (most common)
• "sha384" - SHA-384 (recommended for enhanced security)
• "sha512" - SHA-512 (strongest)

Using SRI in your views

Once configured, you can enable SRI by passing the integrity: true option to asset helpers:

<%= stylesheet_link_tag "application", integrity: true %>
<%= javascript_include_tag "application", integrity: true %>

This generates HTML with integrity hashes:

<link rel="stylesheet" href="/assets/application-abc123.css"
      integrity="sha384-xyz789...">
<script src="/assets/application-def456.js"
        integrity="sha384-uvw012..."></script>

Important: SRI only works in secure contexts (HTTPS) or during local development. The integrity hashes are automatically omitted when serving over HTTP in production for security reasons.

Bulk stylesheet inclusion with SRI

Propshaft extends stylesheet_link_tag with special symbols for bulk inclusion:

<%= stylesheet_link_tag :all, integrity: true %>  <!-- All stylesheets -->
<%= stylesheet_link_tag :app, integrity: true %>  <!-- Only app/assets stylesheets -->

Improving performance in development

Before every request Propshaft checks if any asset was updated to decide if a cache sweep is needed. This verification is done using the application's configured file watcher which, by default, is ActiveSupport::FileUpdateChecker.

If you have a lot of assets in your project, you can improve performance by adding the listen gem to the development group in your Gemfile, and this line to the development.rb environment file:

config.file_watcher = ActiveSupport::EventedFileUpdateChecker

Migrating from Sprockets

Propshaft does a lot less than Sprockets, by design, so it might well be a fair bit of work to migrate if it's even desirable. This is particularly true if you rely on Sprockets to provide any form of transpiling, like CoffeeScript or Sass, or if you rely on any gems that do. You'll need to either stop transpiling or use a Node-based transpiler, like those in jsbundling-rails and cssbundling-rails.

On the other hand, if you're already bundling JavaScript and CSS through a Node-based setup, then Propshaft is going to slot in easily. Since you don't need another tool to bundle or transpile. Just to digest and serve.

But for greenfield apps using the default import-map approach, Propshaft can also work well, if you're able to deal with vanilla CSS.

License

Propshaft is released under the MIT License.

Compilation and Digesting

Asset Processing Pipeline

1. Discovery: LoadPath finds all assets matching patterns
2. Compilation: Each asset processed through registered compilers
3. Digesting: Content hash generated using SHA1 + version string
4. Output: Files written to config.assets.output_path (default: public/assets/)

Built-in Compilers

CSS Asset URL Compiler

Transforms relative URLs to digested versions:

/* Input */
background: url('./hero.jpg');

/* Output */
background: url('/assets/hero-abc123.jpg');

JavaScript Asset URL Compiler

Processes RAILS_ASSET_URL() pseudo-functions:

// Input
const icon = RAILS_ASSET_URL('./icon.svg');

// Output  
const icon = '/assets/icon-def456.svg';

Source Map Compiler

Updates source map references to match digested filenames.

Manifest Generation

The manifest file (.manifest.json) maps logical paths to digested paths:

{
  "application.js": {
    "digested_path": "application-abc123.js",
    "integrity": "sha384-xyz789..."
  }
}

Asset Organization

app/assets/
├── builds/              # jsbundling-rails/cssbundling-rails output
│   ├── application.js
│   └── application.css
├── images/             # Static assets
│   └── logo.svg
├── stylesheets/        # SCSS source (often excluded)
│   └── application.scss
└── javascripts/        # JS source (often excluded)  
    └── application.js

Error Handling

Missing Asset Handling

# Propshaft raises MissingAssetError for missing assets
begin
  asset_path('nonexistent.js')
rescue Propshaft::MissingAssetError => e
  Rails.logger.error "Missing asset: #{e.message}"
  # Fallback logic
end

Development Debugging

# Show all available assets
rake assets:reveal

# Show asset full paths
rake assets:reveal:full