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Zeitwerk is an efficient and thread-safe code loader for Ruby.

Given a conventional file structure, Zeitwerk is able to load your project's classes and modules on demand (autoloading), or upfront (eager loading). You don't need to write require calls for your own files, rather, you can streamline your programming knowing that your classes and modules are available everywhere. This feature is efficient, thread-safe, and matches Ruby's semantics for constants.

Zeitwerk is also able to reload code, which may be handy while developing web applications. Coordination is needed to reload in a thread-safe manner. The documentation below explains how to do this.

The gem is designed so that any project, gem dependency, application, etc. can have their own independent loader, coexisting in the same process, managing their own project trees, and independent of each other. Each loader has its own configuration, inflector, and optional logger.

Internally, Zeitwerk issues require calls exclusively using absolute file names, so there are no costly file system lookups in $LOAD_PATH. Technically, the directories managed by Zeitwerk do not even need to be in $LOAD_PATH.

Furthermore, Zeitwerk does at most one single scan of the project tree, and it descends into subdirectories lazily, only if their namespaces are used.


Main interface for gems:

# lib/my_gem.rb (main file)

require "zeitwerk"
loader = Zeitwerk::Loader.for_gem
loader.setup # ready!

module MyGem
  # ...

loader.eager_load # optionally

Main generic interface:

loader = Zeitwerk::Loader.new
loader.setup # ready!

The loader variable can go out of scope. Zeitwerk keeps a registry with all of them, and so the object won't be garbage collected.

You can reload if you want to:

loader = Zeitwerk::Loader.new
loader.enable_reloading # you need to opt-in before setup

and you can eager load all the code:


It is also possible to broadcast eager_load to all instances:


File structure

To have a file structure Zeitwerk can work with, just name files and directories after the name of the classes and modules they define:

lib/my_gem.rb         -> MyGem
lib/my_gem/foo.rb     -> MyGem::Foo
lib/my_gem/bar_baz.rb -> MyGem::BarBaz
lib/my_gem/woo/zoo.rb -> MyGem::Woo::Zoo

Every directory configured with push_dir acts as root namespace. There can be several of them. For example, given


Zeitwerk understands that their respective files and subdirectories belong to the root namespace:

app/models/user.rb                        -> User
app/controllers/admin/users_controller.rb -> Admin::UsersController

Alternatively, you can associate a custom namespace to a root directory by passing a class or module object in the optional namespace keyword argument.

For example, Active Job queue adapters have to define a constant after their name in ActiveJob::QueueAdapters.

So, if you declare

require "active_job"
require "active_job/queue_adapters"
loader.push_dir("#{__dir__}/adapters", namespace: ActiveJob::QueueAdapters)

your adapter can be stored directly in that directory instead of the canonical #{__dir__}/active_job/queue_adapters.

Please, note that the given namespace must be non-reloadable, though autoloaded constants in that namespace can be. That is, if you associate app/api with an existing Api module, that module should not be reloadable. However, if the project defines and autoloads the class Api::V2::Deliveries, that one can be reloaded.

Implicit namespaces

Directories without a matching Ruby file get modules autovivified automatically by Zeitwerk. For example, in

app/controllers/admin/users_controller.rb -> Admin::UsersController

Admin is autovivified as a module on demand, you do not need to define an Admin class or module in an admin.rb file explicitly.

Explicit namespaces

Classes and modules that act as namespaces can also be explicitly defined, though. For instance, consider

app/models/hotel.rb         -> Hotel
app/models/hotel/pricing.rb -> Hotel::Pricing

There, app/models/hotel.rb defines Hotel, and thus Zeitwerk does not autovivify a module.

The classes and modules from the namespace are already available in the body of the class or module defining it:

class Hotel < ApplicationRecord
  include Pricing # works

An explicit namespace must be managed by one single loader. Loaders that reopen namespaces owned by other projects are responsible for loading their constants before setup.

Collapsing directories

Say some directories in a project exist for organizational purposes only, and you prefer not to have them as namespaces. For example, the actions subdirectory in the next example is not meant to represent a namespace, it is there only to group all actions related to bookings:

booking.rb                -> Booking
booking/actions/create.rb -> Booking::Create

To make it work that way, configure Zeitwerk to collapse said directory:


This method accepts an arbitrary number of strings or Pathname objects, and also an array of them.

You can pass directories and glob patterns. Glob patterns are expanded when they are added, and again on each reload.

To illustrate usage of glob patterns, if actions in the example above is part of a standardized structure, you could use a wildcard:


Nested root directories

Root directories should not be ideally nested, but Zeitwerk supports them because in Rails, for example, both app/models and app/models/concerns belong to the autoload paths.

Zeitwerk detects nested root directories, and treats them as roots only. In the example above, concerns is not considered to be a namespace below app/models. For example, the file:


should define Geolocatable, not Concerns::Geolocatable.




Loaders are ready to load code right after calling setup on them:


This method is synchronized and idempotent.

Customization should generally be done before that call. In particular, in the generic interface you may set the root directories from which you want to load files:



Zeitwerk::Loader.for_gem is a convenience shortcut for the common case in which a gem has its entry point directly under the lib directory:

lib/my_gem.rb         # MyGem
lib/my_gem/version.rb # MyGem::VERSION
lib/my_gem/foo.rb     # MyGem::Foo

Neither a gemspec nor a version file are technically required, this helper works as long as the code is organized using that standard structure.

If the entry point of your gem lives in a subdirectory of lib because it is reopening a namespace defined somewhere else, please use the generic API to setup the loader, and make sure you check the section Reopening third-party namespaces down below.

Conceptually, for_gem translates to:

# lib/my_gem.rb

require "zeitwerk"
loader = Zeitwerk::Loader.new
loader.tag = File.basename(__FILE__, ".rb")
loader.inflector = Zeitwerk::GemInflector.new(__FILE__)

except that this method returns the same object in subsequent calls from the same file, in the unlikely case the gem wants to be able to reload.

If the main module references project constants at the top-level, Zeitwerk has to be ready to load them. Their definitions, in turn, may reference other project constants. And this is recursive. Therefore, it is important that the setup call happens above the main module definition:

# lib/my_gem.rb (main file)

require "zeitwerk"
loader = Zeitwerk::Loader.for_gem

module MyGem
  # Since the setup has been performed, at this point we are already able
  # to reference project constants, in this case MyGem::MyLogger.
  include MyLogger


After setup, you are able to reference classes and modules from the project without issuing require calls for them. They are all available everywhere, autoloading loads them on demand. This works even if the reference to the class or module is first hit in client code, outside your project.

Let's revisit the example above:

# lib/my_gem.rb (main file)

require "zeitwerk"
loader = Zeitwerk::Loader.for_gem

module MyGem
  include MyLogger # (*)

That works, and there is no require "my_gem/my_logger". When (*) is reached, Zeitwerk seamlessly autoloads MyGem::MyLogger.

If autoloading a file does not define the expected class or module, Zeitwerk raises Zeitwerk::NameError, which is a subclass of NameError.

Eager loading

Zeitwerk instances are able to eager load their managed files:


That skips ignored files and directories, and you can also tell Zeitwerk that certain files or directories are autoloadable, but should not be eager loaded:

db_adapters = "#{__dir__}/my_gem/db_adapters"
loader.eager_load # won't eager load the database adapters

In gems, the method needs to be invoked after the main namespace has been defined, as shown in Synopsis.

Eager loading is synchronized and idempotent.

If eager loading a file does not define the expected class or module, Zeitwerk raises Zeitwerk::NameError, which is a subclass of NameError.

If you want to eager load yourself and all dependencies using Zeitwerk, you can broadcast the eager_load call to all instances:


This may be handy in top-level services, like web applications.

Note that thanks to idempotence Zeitwerk::Loader.eager_load_all won't eager load twice if any of the instances already eager loaded.


Zeitwerk is able to reload code, but you need to enable this feature:

loader = Zeitwerk::Loader.new
loader.enable_reloading # you need to opt-in before setup

There is no way to undo this, either you want to reload or you don't.

Enabling reloading after setup raises Zeitwerk::Error. Attempting to reload without having it enabled raises Zeitwerk::ReloadingDisabledError.

Generally speaking, reloading is useful while developing running services like web applications. Gems that implement regular libraries, so to speak, or services running in testing or production environments, won't normally have a use case for reloading. If reloading is not enabled, Zeitwerk is able to use less memory.

Reloading removes the currently loaded classes and modules and resets the loader so that it will pick whatever is in the file system now.

It is important to highlight that this is an instance method. Don't worry about project dependencies managed by Zeitwerk, their loaders are independent.

In order for reloading to be thread-safe, you need to implement some coordination. For example, a web framework that serves each request with its own thread may have a globally accessible RW lock. When a request comes in, the framework acquires the lock for reading at the beginning, and the code in the framework that calls loader.reload needs to acquire the lock for writing.

On reloading, client code has to update anything that would otherwise be storing a stale object. For example, if the routing layer of a web framework stores controller class objects or instances in internal structures, on reload it has to refresh them somehow, possibly reevaluating routes.


Each individual loader needs an inflector to figure out which constant path would a given file or directory map to. Zeitwerk ships with two basic inflectors.


This is a very basic inflector that converts snake case to camel case:

user             -> User
users_controller -> UsersController
html_parser      -> HtmlParser

The camelize logic can be overridden easily for individual basenames:

  "html_parser"   => "HTMLParser",
  "mysql_adapter" => "MySQLAdapter"

The inflect method can be invoked several times if you prefer this other style:

loader.inflector.inflect "html_parser" => "HTMLParser"
loader.inflector.inflect "mysql_adapter" => "MySQLAdapter"

Overrides need to be configured before calling setup.

There are no inflection rules or global configuration that can affect this inflector. It is deterministic.

Loaders instantiated with Zeitwerk::Loader.new have an inflector of this type, independent of each other.


This inflector is like the basic one, except it expects lib/my_gem/version.rb to define MyGem::VERSION.

Loaders instantiated with Zeitwerk::Loader.for_gem have an inflector of this type, independent of each other.

Custom inflector

The inflectors that ship with Zeitwerk are deterministic and simple. But you can configure your own:

# frozen_string_literal: true

class MyInflector < Zeitwerk::Inflector
  def camelize(basename, abspath)
    if basename =~ /\Ahtml_(.*)/
      "HTML" + super($1, abspath)

The first argument, basename, is a string with the basename of the file or directory to be inflected. In the case of a file, without extension. In the case of a directory, without trailing slash. The inflector needs to return this basename inflected. Therefore, a simple constant name without colons.

The second argument, abspath, is a string with the absolute path to the file or directory in case you need it to decide how to inflect the basename. Paths to directories don't have trailing slashes.

Then, assign the inflector:

loader.inflector = MyInflector.new

This needs to be done before calling setup.

If a custom inflector definition in a gem takes too much space in the main file, you can extract it. For example, this is a simple pattern:

# lib/my_gem/inflector.rb
module MyGem
  class Inflector < Zeitwerk::GemInflector

# lib/my_gem.rb
require "zeitwerk"
require_relative "my_gem/inflector"

loader = Zeitwerk::Loader.for_gem
loader.inflector = MyGem::Inflector.new(__FILE__)

module MyGem
  # ...

Since MyGem is referenced before the namespace is defined in the main file, it is important to use this style:

# Correct, effectively defines MyGem.
module MyGem
  class Inflector < Zeitwerk::GemInflector
    # ...

instead of:

# Raises uninitialized constant MyGem (NameError).
class MyGem::Inflector < Zeitwerk::GemInflector
  # ...

The on_load callback

The usual place to run something when a file is loaded is the file itself. However, sometimes you'd like to be called, and this is possible with the on_load callback.

For example, let's imagine this class belongs to a Rails application:

class SomeApiClient
  class << self
    attr_accessor :endpoint

With on_load, it is easy to schedule code at boot time that initializes endpoint according to the configuration:

# config/environments/development.rb
loader.on_load("SomeApiClient") do
  SomeApiClient.endpoint = "https://api.dev"

# config/environments/production.rb
loader.on_load("SomeApiClient") do
  SomeApiClient.endpoint = "https://api.prod"

Uses cases:

  • Doing something with an autoloadable class or module in a Rails application during initialization, in a way that plays well with reloading. As in the previous example.
  • Delaying the execution of the block until the class is loaded for performance.
  • Delaying the execution of the block until the class is loaded because it follows the adapter pattern and better not to load the class if the user does not need it.
  • Etc.

However, let me stress that the easiest way to accomplish that is to write whatever you have to do in the actual target file. on_load use cases are edgy, use it only if appropriate.

on_load receives the name of the target class or module as a string. The given block is executed every time its corresponding file is loaded. That includes reloads.

Multiple callbacks on the same target are supported, and they run in order of definition.

The block is executed once the loader has loaded the target. In particular, if the target was already loaded when the callback is defined, the block won't run. But if you reload and load the target again, then it will. Normally, you'll want to define on_load callbacks before setup.

Defining a callback for a target not managed by the receiver is not an error, the block simply won't ever be executed.


Zeitwerk is silent by default, but you can ask loaders to trace their activity. Logging is meant just for troubleshooting, shouldn't normally be enabled.

The log! method is a quick shortcut to let the loader log to $stdout:


If you want more control, a logger can be configured as a callable

loader.logger = method(:puts)
loader.logger = ->(msg) { ... }

as well as anything that responds to debug:

loader.logger = Logger.new($stderr)
loader.logger = Rails.logger

In both cases, the corresponding methods are going to be passed exactly one argument with the message to be logged.

It is also possible to set a global default this way:

Zeitwerk::Loader.default_logger = method(:puts)

If there is a logger configured, you'll see traces when autoloads are set, files loaded, and modules autovivified. While reloading, removed autoloads and unloaded objects are also traced.

As a curiosity, if your project has namespaces you'll notice in the traces Zeitwerk sets autoloads for directories. That's a technique used to be able to descend into subdirectories on demand, avoiding that way unnecessary tree walks.

Loader tag

Loaders have a tag that is printed in traces in order to be able to distinguish them in globally logged activity:

[email protected]: autoload set for User, to be loaded from ...

By default, a random tag like the one above is assigned, but you can change it:

loader.tag = "grep_me"

The tag of a loader returned by for_gem is the basename of the root file without extension:

[email protected]_gem: constant MyGem::Foo loaded from ...

Ignoring parts of the project

Zeitwerk ignores automatically any file or directory whose name starts with a dot, and any files that do not have extension ".rb".

However, sometimes it might still be convenient to tell Zeitwerk to completely ignore some particular Ruby file or directory. That is possible with ignore, which accepts an arbitrary number of strings or Pathname objects, and also an array of them.

You can ignore file names, directory names, and glob patterns. Glob patterns are expanded when they are added and again on each reload.

Let's see some use cases.

Use case: Files that do not follow the conventions

Let's suppose that your gem decorates something in Kernel:

# lib/my_gem/core_ext/kernel.rb

Kernel.module_eval do
  # ...

That file does not define a constant path after the path name and you need to tell Zeitwerk:

kernel_ext = "#{__dir__}/my_gem/core_ext/kernel.rb"

You can also ignore the whole directory:

core_ext = "#{__dir__}/my_gem/core_ext"

Use case: The adapter pattern

Another use case for ignoring files is the adapter pattern.

Let's imagine your project talks to databases, supports several, and has adapters for each one of them. Those adapters may have top-level require calls that load their respective drivers:

# my_gem/db_adapters/postgresql.rb
require "pg"

but you don't want your users to install them all, only the one they are going to use.

On the other hand, if your code is eager loaded by you or a parent project (with Zeitwerk::Loader.eager_load_all), those require calls are going to be executed. Ignoring the adapters prevents that:

db_adapters = "#{__dir__}/my_gem/db_adapters"

The chosen adapter, then, has to be loaded by hand somehow:

require "my_gem/db_adapters/#{config[:db_adapter]}"

Note that since the directory is ignored, the required adapter can instantiate another loader to manage its subtree, if desired. Such loader would coexist with the main one just fine.

Use case: Test files mixed with implementation files

There are project layouts that put implementation files and test files together. To ignore the test files, you can use a glob pattern like this:

tests = "#{__dir__}/**/*_test.rb"

Edge cases

A class or module that acts as a namespace:

# trip.rb
class Trip
  include Geolocation

# trip/geolocation.rb
module Trip::Geolocation

has to be defined with the class or module keywords, as in the example above.

For technical reasons, raw constant assignment is not supported:

# trip.rb
Trip = Class.new { ... }  # NOT SUPPORTED
Trip = Struct.new { ... } # NOT SUPPORTED

This only affects explicit namespaces, those idioms work well for any other ordinary class or module.

Reopening third-party namespaces

Projects managed by Zeitwerk can work with namespaces defined by third-party libraries. However, they have to be loaded in memory before calling setup.

For example, let's imagine you're writing a gem that implements an adapter for Active Job that uses AwesomeQueue as backend. By convention, your gem has to define a class called ActiveJob::QueueAdapters::AwesomeQueue, and it has to do so in a file with a matching path:

# lib/active_job/queue_adapters/awesome_queue.rb
module ActiveJob
  module QueueAdapters
    class AwesomeQueue
      # ...

It is very important that your gem reopens the modules ActiveJob and ActiveJob::QueueAdapters instead of defining them. Because their proper definition lives in Active Job. Furthermore, if the project reloads, you do not want any of ActiveJob or ActiveJob::QueueAdapters to be reloaded.

Bottom line, Zeitwerk should not be managing those namespaces. Active Job owns them and defines them. Your gem needs to reopen them.

In order to do so, you need to make sure those modules are loaded before calling setup. For instance, in the entry file for the gem:

# Ensure these namespaces are reopened, not defined.
require "active_job"
require "active_job/queue_adapters"

require "zeitwerk"
loader = Zeitwerk::Loader.for_gem

With that, when Zeitwerk scans the file system and reaches the gem directories lib/active_job and lib/active_job/queue_adapters, it detects the corresponding modules already exist and therefore understands it does not have to manage them. The loader just descends into those directories. Eventually will reach lib/active_job/queue_adapters/awesome_queue.rb, and since ActiveJob::QueueAdapters::AwesomeQueue is unknown, Zeitwerk will manage it. Which is what happens regularly with the files in your gem. On reload, the namespaces are safe, won't be reloaded. The loader only reloads what it manages, which in this case is the adapter itself.

Rules of thumb

  1. Different loaders should manage different directory trees. It is an error condition to configure overlapping root directories in different loaders.

  2. Think the mere existence of a file is effectively like writing a require call for them, which is executed on demand (autoload) or upfront (eager load).

  3. In that line, if two loaders manage files that translate to the same constant in the same namespace, the first one wins, the rest are ignored. Similar to what happens with require and $LOAD_PATH, only the first occurrence matters.

  4. Projects that reopen a namespace defined by some dependency have to ensure said namespace is loaded before setup. That is, the project has to make sure it reopens, rather than define. This is often accomplished just loading the dependency.

  5. Objects stored in reloadable constants should not be cached in places that are not reloaded. For example, non-reloadable classes should not subclass a reloadable class, or mixin a reloadable module. Otherwise, after reloading, those classes or module objects would become stale. Referring to constants in dynamic places like method calls or lambdas is fine.

  6. In a given process, ideally, there should be at most one loader with reloading enabled. Technically, you can have more, but it may get tricky if one refers to constants managed by the other one. Do that only if you know what you are doing.



Zeitwerk works fine with @gsamokovarov's Break debugger.


Zeitwerk and Byebug are incompatible, classes or modules that belong to explicit namespaces are not autoloaded inside a Byebug session. See this issue for further details.


"Zeitwerk" is pronounced this way.

Supported Ruby versions

Zeitwerk works with MRI 2.4.4 and above.


In order to run the test suite of Zeitwerk, cd into the project root and execute


To run one particular suite, pass its file name as an argument:

bin/test test/lib/zeitwerk/test_eager_load.rb

Furthermore, the project has a development dependency on minitest-focus. To run an individual test mark it with focus:

test "capitalizes the first letter" do
  assert_equal "User", camelize("user")

and run bin/test.


Since require has global side-effects, and there is no static way to verify that you have issued the require calls for code that your file depends on, in practice it is very easy to forget some. That introduces bugs that depend on the load order. Zeitwerk provides a way to forget about require in your own code, just name things following conventions and done.

On the other hand, autoloading in Rails is based on const_missing, which lacks fundamental information like the nesting and the resolution algorithm that was being used. Because of that, Rails autoloading is not able to match Ruby's semantics and that introduces a series of gotchas. The original goal of this project was to bring a better autoloading mechanism for Rails 6.


I'd like to thank @matthewd for the discussions we've had about this topic in the past years, I learned a couple of tricks used in Zeitwerk from him.

Also, would like to thank @Shopify, @rafaelfranca, and @dylanahsmith, for sharing this PoC. The technique Zeitwerk uses to support explicit namespaces was copied from that project.

Jean Boussier (@casperisfine, @byroot) deserves special mention. Jean migrated autoloading in Shopify when Zeitwerk integration in Rails was yet unreleased. His work and positive attitude have been outstanding, and thanks to his feedback the interface and performance of Zeitwerk are way, way better. Kudos man ❤️.

Finally, many thanks to @schurig for recording an audio file with the pronunciation of "Zeitwerk" in perfect German. 💯


Released under the MIT License, Copyright (c) 2019–ω Xavier Noria.