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Ettin manages loading and accessing settings from your configuration files, in an environment-aware fashion. It has only a single dependency on deep_merge, and provides only the functionality you actually need. It does not monkey-patch ruby nor does it pollute the global namespace.

Why should I use this over other options?

  • Ettin has far fewer dependencies than the top configuration gems.
  • Ettin does not pollute the global namespace.
  • Ettin provides only the features you need; or, put another way, Ettin does not offer you paths that should not be followed.
  • Ettin is just plain ruby. No magic. No DSL.
  • Ettin works everywhere.


  • ruby 2.6.x
  • ruby 2.5.x
  • ruby 2.4.x
  • ruby 2.3.x

As Ettin does not rely on any specific runtime environment other than the ruby core and standard library, it is compatible with every ruby library and framework.


  1. Add it to your bundle and install like any other gem.
  2. Ettin provides an executable that will create the recommended configuration files for you. These files will be empty. You can also create them yourself, or simply specify your own files when you load Ettin.

bundle exec ettin -v -p some/path

Loading Settings

Ettin is just plain ruby. There's nothing special about the objects it creates or how it creates them. As such, it's up to the application to decide how it provides access to the configuration object. That may seem scary or confusing, but it's not--it's just plain ruby. A few examples are below:

Assign to a global constant using the default files:

Settings = Ettin.for(Ettin.settings_files("config", "development"))

Assign with custom files:

Settings = Ettin.for("config/path/1.yml", "config/path/2.yml")

Declare and assign to a top-level module:

module MyApp
  class << self
    def config
      @config ||= Ettin.for(Ettin.settings_files("config"), ENV["MYAPP_ENV"])

Use one of the above variants in a Rails app's application.rb, making settings available to your environment.rb, development.rb, and initializers:

module MyApp
  class << self
    def config
      @config ||= Ettin.for(Ettin.settings_files("config", Rails.env))

  class Application < Rails::Application
    # ...

Add a section to the Rails configuration in an initializer (noting that load order is alphanumeric):

Rails.application.configure do |config|
  config.settings = Ettin.for(...)

The provided ettin executable will create the following files, including a file for each environment of production, development, and test.

The name of the environment is not special, so you can easily create more.



Environment-specific settings take precedence over common, and the .local files take precedence over those. The local files are intendended to be gitignored.

Ettin will also read from the following files commonly placed by other config gems. However, inclusion of these files is redundant, and can be confusing. Their inclusion is not recommended.


Using the Settings


Entries are available via dot-notation:

config.some_setting               #=> 5
config.some.nested.setting        #=> "my nested string"

...or [] notation:

config[:some_setting]             #=> 5
config["some_setting"]            #=> 5
config[:some][:nested][:setting]  #=> "my nested string"

When a setting is not present, the returned value will be nil. We find that this is what most people expect. If you'd like an exception to be thrown, you can use dot-notation with a bang added:

config.some_missing_setting!      #=> raises a KeyError


You can also change settings at runtime via a merge:

config.some_setting               #=> 5
config.merge!({some_setting: 22})
config.some_setting               #=> 22

...or direction assignment:

config.some_setting               #=> 5
config.some_setting = 22
config.some_setting               #=> 22

Both of these methods work for any level of nesting.


In Ettin, YAML files support ERB by default.

# in settings.yml

  hostname: <%= ENV["REDIS_HOST"] %>

Environment-specific Configuration Files

Environment-specific configuration files are supported. These files take precedence over the common configuration, as you'd expect. The


How can I reload the entire setting object?

Ettin's settings object is just a plain ruby object, so you should simply assign your settings reference to something else.

Why these specific files?

Ettin is designed to be an easy transition from users of config. We also think that these locations are quite sensible.

How do I validate my settings?

Validation is a concern that is driven by the application itself. Placing the responsibility for that validation in Ettin would violate the single-responsibility principle. You should validate the settings where they're used, such as in an initialization step.

How do I load environment variables into my settings?

Just use ERB. See the ERB docs for more information.

How can I pull in settings from another source?

Ettin supports hashes and paths to yaml files out of the box. You can extend this support by creating a subclass of Ettin::Source. Your subclass will need to define ::handles?(target), make a call of register(self), and define a #load method that returns a hash.



Copyright (c) 2018 The Regents of the University of Michigan.
All Rights Reserved.
Licensed according to the terms of the Revised BSD License.
See for details.