Bundle Caching


Building webpack bundles is often time-consuming, and the same bundles are built many times. For example, you might build the production bundles during CI, then for a Review app, then for Staging, and maybe even for Production. Or you might want to deploy a small Ruby only change to production, but you will have to wait minutes for your bundles to be built again.


React on Rails 2.1.0 introduces bundle caching based on a digest of all the source files, defined in the config/webpacker.yml file, plus other files defined with config.dependency_globs and excluding any files from config.excluded_dependency_globs. Creating this hash key takes at most a few seconds for even large projects. Additionally, the cache key includes

  2. Version of React on Rails Pro
  3. Configurable additional env values by supplying an array in method cache_keys on the remote_bundle_cache_adapter. See examples below.

This cache key is used for saving files to some remote storage, typically S3.

Bonus for local development with multiple directories building production builds

Bundle caching can help save time if you have multiple directories for the same repository.

The bundles are cached in Rails.root.join('tmp', 'bundle_cache')

So, if you have sibling directories for the same project, you can make a sym link so both directories use the same bundle cache directory.

cd my_project2/tmp
ln -s ../../my_project/tmp/bundle_cache


1. React on Rails Configuration

First, we need to tell React on Rails to use a custom build module. In config/initializers/react_on_rails, set this value:

config.build_production_command = ReactOnRailsPro::AssetsPrecompile

Alternatively, if you need to run something after the files are built or extracted from the cache, you can do something like this:

ReactOnRails.configure do |config|
  # This configures the script to run to build the production assets by webpack. Set this to nil
  # if you don't want react_on_rails building this file for you.
  config.build_production_command = CustomBuildCommand

And define it like this:

module CustomBuildCommand
  def self.call

2. React on Rails Pro Configuration

Next, we need to configure the config/initializers/react_on_rails_pro.rb with some module, say called S3BundleCacheAdapter.

config.remote_bundle_cache_adapter = S3BundleCacheAdapter

This module needs four class methods: cache_keys (optional), build, fetch, upload. See two examples of this below.

Also, add whatever file the remote_bundle_cache_adapter module is defined in to config.dependency_globs.

If there are any other files for which changes should bust the fragment cache for cached_react_component and cached_react_component_hash, add those as well to config.dependency_globs. This should include any files used to generate the JSON props, webpack and/or Shakapacker configuration files, and package lockfiles.

To simplify your configuration, entire directories can be added to config.dependency_globs & then any irrelevant files or subdirectories can be added to config.excluded_dependency_globs

For example:

  config.dependency_globs = [ File.join(Rails.root, "app", "views", "**", "*.jbuilder") ]
  config.excluded_dependency_globs = [ File.join(Rails.root, "app", "views", "**", "dont_hash_this.jbuilder") ]

will hash all files in app/views that have the jbuilder extension except for any file named dont_hash_this.jbuilder.

The goal is that Ruby only changes that don't affect your webpack bundles don't change the cache keys, and anything that could affect the bundles MUST change the cache keys!

3. Remove any call to rake task react_on_rails_pro:pre_stage_bundle_for_node_renderer

This task is called automaticaly if you're using bundle caching.


Custom ENV cache keys

Check your webpack config for the webpack.DefinePlugin. That allows JS code to use process.env.MY_ENV_VAR resulting in bundles that differ depending on the ENV value set.

Thus, if you access these process.env.MY_ENV_VAR in your JS code, then you need to include such ENV vars in return value of the cache keys method.

A much better approach than accessing process.env is to use the config/initializers/react_on_rails.rb setting for theconfig.rendering_extension to always pass some values into the rendering props.


Also, if your webpack build process depends on any ENV values, then you will also need to add those to return value of the cache_keys method.

Note, the NODE_ENV value is always included in the cache_keys.

Another use of the ENV values would be a cache version, so incrementing this ENV value would force a new cache value.

Disabling via an ENV value

Once configured for bundle caching, ReactOnRailsPro::AssetsPrecompile's caching functionality can be disabled by setting ENV["DISABLE_PRECOMPILE_CACHE"] equal to "true"

Examples of remote_bundle_cache_adapter:


Example of a module for custom methods for the remote_bundle_cache_adapter.

Note, S3UploadService is your own code that fetches and uploads.

class S3BundleCacheAdapter
  # Optional
  # return an Array of Strings that should get added to the cache key.
  # These are values to put in the cache key based on either using the webpack.DefinePlugin
  # or webpack compilation varying by the ENV values.
  # See the use of the webpack.DefinePlugin. That allows JS code to use
  # process.env.MY_ENV_VAR resulting in bundles that differ depending on the ENV value set
  # when building the bundles.
  # Note, NODE_ENV is automatically included in the default cache key.
  # Also, we can have an ENV value be a cache version, so incrementing this ENV value
  # would force a new cache value.
  def self.cache_keys
    [Rails.env, ENV['SOME_ENV_VALUE']]
  # return value is unused
  # This command should build the bundles
  def self.build
    Rake.sh(ReactOnRails::Utils.prepend_cd_node_modules_directory('yarn start build.prod').to_s)

  # parameter zipped_bundles_filename will be a string
  # should return the zipped file as a string if successful & nil if not
  def self.fetch(zipped_bundles_filename:)
    result = S3UploadService.new.fetch_object(zipped_bundles_filename)
    result.get.body.read if result
  # Optional: method to return an array of extra files paths, that require caching. 
  # These files get placed at the `extra_files` directory at the top of the zipfile
  # and are moved to the original places after unzipping the bundles.
  def self.extra_files_to_cache
      [ Rails.root.join("app", "javascript", "utils", "operationStore.json") ]

  # parameter zipped_bundles_filepath will be a Pathname
  # return value is unused
  def self.upload(zipped_bundles_filepath:)
    return unless ENV['UPLOAD_BUNDLES_TO_S3'] == 'true'

    zipped_bundles_filename = zipped_bundles_filepath.basename.to_s
    puts "Bundles are being uploaded to s3 as #{zipped_bundles_filename}"
    starting = Process.clock_gettime(Process::CLOCK_MONOTONIC)
                                      File.read(zipped_bundles_filepath, mode: 'rb'),
                                      'application/zip', expiration_months: 12)
    ending = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    elapsed = (ending - starting).round(2)
    puts "Bundles uploaded to s3 as #{zipped_bundles_filename} in #{elapsed} seconds"


Example of a module for custom methods for the remote_bundle_cache_adapter that does not save files remotely. Only local files are used.

class LocalBundleCacheAdapter
  def self.cache_keys
    # if no additional cache keys, return an empty array
  def self.build
    Rake.sh(ReactOnRails::Utils.prepend_cd_node_modules_directory('yarn start build.prod').to_s)

  def self.fetch(zipped_bundles_filename:)
    # no-op

  def self.upload(zipped_bundles_filepath:)
    # no-op