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Zeitwerk implements constant autoloading with Ruby semantics. Each gem and application may have their own independent autoloader, with its own configuration, inflector, and logger. Supports autoloading, reloading, and eager loading.
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Gem Version Build Status

  • Introduction
  • Synopsis
  • File structure
    • The idea: File paths match constant paths
    • Inner simple constants
    • Root directories and root namespaces
      • The default root namespace is Object
      • Custom root namespaces
      • Nested root directories
    • Implicit namespaces
    • Explicit namespaces
    • Collapsing directories
    • Testing compliance
  • Usage
    • Setup
      • Generic
      • for_gem
    • Autoloading
    • Eager loading
      • Eager load exclusions
      • Global eager load
    • Reloading
    • Inflection
      • Zeitwerk::Inflector
      • Zeitwerk::GemInflector
      • Custom inflector
    • Callbacks
      • The on_setup callback
      • The on_load callback
      • The on_unload callback
      • Technical details
    • Logging
      • Loader tag
    • Ignoring parts of the project
      • Use case: Files that do not follow the conventions
      • Use case: The adapter pattern
      • Use case: Test files mixed with implementation files
    • Edge cases
    • Beware of circular dependencies
    • Reopening third-party namespaces
    • Encodings
    • Rules of thumb
    • Debuggers
      • debug.rb
      • Byebug
      • Break
  • Pronunciation
  • Supported Ruby versions
  • Testing
  • Motivation
    • Kernel#require is brittle
    • Rails autoloading was brittle
  • Awards
  • Thanks
  • License


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 =
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 =
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

The idea: File paths match constant paths

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

You can tune that a bit by collapsing directories, or by ignoring parts of the project, but that is the main idea.

Inner simple constants

While a simple constant like HttpCrawler::MAX_RETRIES can be defined in its own file:

# http_crawler/max_retries.rb
HttpCrawler::MAX_RETRIES = 10

that is not required, you can also define it the regular way:

# http_crawler.rb
class HttpCrawler

The first example needs a custom inflection rule:

loader.inflector.inflect("max_retries" => "MAX_RETRIES")

Otherwise, Zeitwerk would expect the file to define MaxRetries.

In the second example, no custom rule is needed.

Root directories and root namespaces

Every directory configured with push_dir is called a root directory, and they represent root namespaces.

The default root namespace is Object

By default, the namespace associated to a root directory is the top-level one: Object.

For example, given


these are the expected classes and modules being defined by these files:

models/user.rb                 -> User
serializers/user_serializer.rb -> UserSerializer

Custom root namespaces

While Object is by far the most common root namespace, you can associate a different one to a particular root directory. The method push_dir accepts a class or module object in the optional namespace keyword argument.

For example, given:

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

a file defining ActiveJob::QueueAdapters::MyQueueAdapter does not need the conventional parent directories, you can (and have to) store the file directly below adapters:

adapters/my_queue_adapter.rb -> ActiveJob::QueueAdapters::MyQueueAdapter

Please, note that the given root 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::Deliveries, that one can be reloaded.

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.

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.

For a directory to implicitly define a namespace, it has to contain at least one non-ignored Ruby file, either directly or recursively. For example, tasks/newsletter.rake does not define a Tasks namespace. If you'd like the loader to not even inspect the contents of some of these, remember you can manually ignore them.

If you need an empty module, please define it in a file.

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:


Testing compliance

When a managed file is loaded, Zeitwerk verifies the expected constant is defined. If it is not, Zeitwerk::NameError is raised.

So, an easy way to ensure compliance in the test suite is to eager load the project:

  loader.eager_load(force: true)
rescue Zeitwerk::NameError => e
  flunk e.message
  assert true




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.

Conceptually, for_gem translates to:

# lib/my_gem.rb

require "zeitwerk"
loader =
loader.tag = File.basename(__FILE__, ".rb")
loader.inflector =

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

Loaders returned by Zeitwerk::Loader.for_gem issue warnings if lib has extra Ruby files or directories.

For example, if the gem has Rails generators under lib/generators, by convention that directory defines a Generators Ruby module. If generators is just a container for non-autoloadable code and templates, not acting as a project namespace, you need to setup things accordingly.

If the warning is legit, just tell the loader to ignore the offending file or directory:


Otherwise, there's a flag to say the extra stuff is OK:

Zeitwerk::Loader.for_gem(warn_on_extra_files: false)

This method is idempotent when invoked from the same file, to support gems that want to reload (unlikely).

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.


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.

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.

Eager load exclusions

You can 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

However, that can be overridden with force:

loader.eager_load(force: true) # database adapters are eager loaded

Which may be handy if the project eager loads in the test suite to ensure project layout compliance.

The force flag does not affect ignored files and directories, those are still ignored.

Global eager load

If you want to eager load yourself and all dependencies that use 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.

This method does not accept the force flag, since in general it wouldn't be a good idea to force eager loading in 3rd party code.


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

loader =
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.

Reloading is not thread-safe:

  • You should not reload while another thread is reloading.
  • You should not autoload while another thread is reloading.

Zeitwerk raises Zeitwerk::UnsynchronizedReloadError if any of these situations are detected. This is a fatal exception that signals a fundamental bug, you cannot rescue it and expect things to work.

In order for reloading to be thread-safe, frameworks need to implement some coordination. For example, a web framework that serves each request with its own thread may have a globally accessible read/write lock: When a request comes in, the framework acquires the lock for reading at the beginning, and releases it at the end. On the other hand, the code in the framework responsible for the call to Zeitwerk::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 reloadable 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, and you can define your own.


Each loader instantiated with has an inflector of this type by default.

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.

The inflectors of different loaders are independent of each other. There are no global inflection rules or global configuration that can affect this inflector. It is deterministic.


Each loader instantiated with Zeitwerk::Loader.for_gem has an inflector of this type by default.

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

The inflectors of different loaders are independent of each other. There are no global inflection rules or global configuration that can affect this inflector. It is deterministic.

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 =

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 =

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_setup callback

The on_setup callback is fired on setup and on each reload:

loader.on_setup do
  # Ready to autoload here.

Multiple on_setup callbacks are supported, and they run in order of definition.

If setup was already executed, the callback is fired immediately.

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 |klass, _abspath|
  klass.endpoint = ""

# config/environments/production.rb
loader.on_load("SomeApiClient") do |klass, _abspath|
  klass.endpoint = ""

Some uses cases:

  • Doing something with a reloadable 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.

on_load gets a target constant path as a string (e.g., "User", or "Service::NotificationsGateway"). When fired, its block receives the stored value, and the absolute path to the corresponding file or directory as a string. The callback is executed every time the target 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.

It is also possible to be called when any constant managed by the loader is loaded:

loader.on_load do |cpath, value, abspath|
  # ...

The block gets the constant path as a string (e.g., "User", or "Foo::VERSION"), the value it stores (e.g., the class object stored in User, or "2.5.0"), and the absolute path to the corresponding file or directory as a string.

Multiple callbacks like these are supported, and they run in order of definition.

There are use cases for this last catch-all callback, but they are rare. If you just need to understand how things are being loaded for debugging purposes, please remember that Zeitwerk::Loader#log! logs plenty of information.

If both types of callbacks are defined, the specific ones run first.

Since on_load callbacks are executed right after files are loaded, even if the loading context seems to be far away, in practice the block is subject to circular dependencies. As a rule of thumb, as far as loading order and its interdependencies is concerned, you have to program as if the block was executed at the bottom of the file just loaded.

The on_unload callback

When reloading is enabled, you may occasionally need to execute something before a certain autoloaded class or module is unloaded. The on_unload callback allows you to do that.

For example, let's imagine that a Country class fetches a list of countries and caches them when it is loaded. You might want to clear that cache if unloaded:

loader.on_unload("Country") do |klass, _abspath|

on_unload gets a target constant path as a string (e.g., "User", or "Service::NotificationsGateway"). When fired, its block receives the stored value, and the absolute path to the corresponding file or directory as a string. The callback is executed every time the target is unloaded.

on_unload blocks are executed before the class is unloaded, but in the middle of unloading, which happens in an unspecified order. Therefore, that callback should not refer to any reloadable constant because there is no guarantee the constant works there. Those blocks should rely on objects only, as in the example above, or regular constants not managed by the loader. This remark is transitive, applies to any methods invoked within the block.

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

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

It is also possible to be called when any constant managed by the loader is unloaded:

loader.on_unload do |cpath, value, abspath|
  # ...

The block gets the constant path as a string (e.g., "User", or "Foo::VERSION"), the value it stores (e.g., the class object stored in User, or "2.5.0"), and the absolute path to the corresponding file or directory as a string.

Multiple callbacks like these are supported, and they run in order of definition.

If both types of callbacks are defined, the specific ones run first.

Technical details

Zeitwerk uses the word "unload" to ease communication and for symmetry with on_load. However, in Ruby you cannot unload things for real. So, when does on_unload technically happen?

When unloading, Zeitwerk issues Module#remove_const calls. Classes and modules are no longer reachable through their constants, and on_unload callbacks are executed right before those calls.

Technically, though, the objects themselves are still alive, but if everything is used as expected and they are not stored in any non-reloadable place (don't do that), they are ready for garbage collection, which is when the real unloading happens.


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 =$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:

Zeitwerk@9fa54b: 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:

Zeitwerk@my_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
  # ...

Kernel is already defined by Ruby so the module cannot be autoloaded. Also, that file does not define a constant path after the path name. Therefore, Zeitwerk should not process it at all.

The extension can still coexist with the rest of the project, you only need to tell Zeitwerk to ignore it:

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

You can also ignore the whole directory:

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

Now, that file has to be loaded manually with require or require_relative:

require_relative "my_gem/core_ext/kernel"

and you can do that anytime, before configuring the loader, or after configuring the loader, does not matter.

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 = { ... }  # NOT SUPPORTED
Trip = { ... } # NOT SUPPORTED

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

Beware of circular dependencies

In Ruby, you can't have certain top-level circular dependencies. Take for example:

# c.rb
class C < D

# d.rb
class D

In order to define C, you need to load D. However, the body of D refers to C.

Circular dependencies like those do not work in plain Ruby, and therefore do not work in projects managed by Zeitwerk either.

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.


Zeitwerk supports projects whose files and file system are in UTF-8. The encoding of the file system can be checked this way:

% ruby -e "puts Encoding.find('filesystem')"

The test suite passes on Windows with codepage Windows-1252 if all the involved absolute paths are ASCII. Other supersets of ASCII may work too, but you have to try.

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.



The new debug.rb gem and Zeitwerk are mostly compatible. This is the new debugger that is going to ship with Ruby 3.1.

There's one exception, though: Due to a technical limitation of tracepoints, explicit namespaces are not autoloaded while expressions are evaluated in the REPL. See ruby/debug#408.


Zeitwerk and Byebug have a similar edge incompatibility.


Zeitwerk works fine with @gsamokovarov's Break debugger.


"Zeitwerk" is pronounced this way.

Supported Ruby versions

Zeitwerk works with CRuby 2.5 and above.

On TruffleRuby all is good except for thread-safety. Right now, in TruffleRuby Module#autoload does not block threads accessing a constant that is being autoloaded. CRuby prevents such access to avoid concurrent threads from seeing partial evaluations of the corresponding file. Zeitwerk inherits autoloading thread-safety from this property. This is not an issue if your project gets eager loaded, or if you lazy load in single-threaded environments. (See oracle/truffleruby#2431.)

JRuby is almost there. As of this writing, the test suite of Zeitwerk passes on JRuby except for three tests. (See jruby/jruby#6781.)


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.


Kernel#require is brittle

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.

Also, if the project has namespaces, setting things up and getting client code to load things in a consistent way needs discipline. For example, require "foo/bar" may define Foo, instead of reopen it. That may be a broken window, giving place to superclass mismatches or partially-defined namespaces.

With Zeitwerk, you just name things following conventions and done. Things are available everywhere, and descend is always orderly. Without effort and without broken windows.

Rails autoloading was brittle

Autoloading in Rails was based on const_missing up to Rails 5. That callback lacks fundamental information like the nesting or the resolution algorithm being used. Because of that, Rails autoloading was not able to match Ruby's semantics, and that introduced a series of issues. Zeitwerk is based on a different technique and fixed Rails autoloading starting with Rails 6.


Zeitwerk has been awarded an "Outstanding Performance Award" Fukuoka Ruby Award 2022.


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.