KingKonf gives you a way of declaratively specifying configuration variables for your application or library. It is focused on simplicity and being able to work well with environment variables, meaning that there is no nesting or fancy structures: all configuration can be passed as strings.


Add this line to your application's Gemfile:

gem 'king_konf'

And then execute:

$ bundle

Or install it yourself as:

$ gem install king_konf


In order to specify a set configuration variables, simply subclass KingKonf::Config and use the DSL:

require "king_konf"

class MyApplication::Config < KingKonf::Config
  # The prefix is used to identify environment variables. Here, we require
  # that all environment variables used for config start with `MY_APP_`,
  # followed by the all caps name of the variable.
  env_prefix :my_app

  # Strings are the simplest. This variable is required and *must* be set. By default,
  # a variable is optional.
  string :title, required: true

  # Integer variables require the value to be a valid integer:
  integer :score

  # Booleans by default use "true", "false", "1", and "0" as valid values:
  boolean :promoted

  # These can be configured:
  boolean :allow_comments, true_values: ["yes"], false_values: ["no"]

  # Lists are by default comma-separated arrays of strings:
  list :tags

  # You can separate with other characters, and decode each value as another type:
  list :codes, sep: ";", items: :integer

  # You can also provide a default value to any variable:
  string :body, default: "N/A"

  # You can restrict the set of allowed values:
  string :category, allowed_values: ["news", "stuff", "accouncements"]

  # You can provide a custom validation function:
  integer :even_number, validate_with: ->(int) { int % 2 == 0 }

Now that we've defined a configuration class, we can initialize it. KingKonf will read the ENV and detect any variables that match the prefix:

# These would normally be passed by the system running your app:
ENV["MY_APP_TITLE"] = "Hello, World!"
ENV["MY_APP_SCORE"] = "85"
ENV["MY_APP_TAGS"] = "greetings,introductions,articles"
ENV["MY_APP_CODES"] = "435;2342;8678"

config =

# This validates that all required variables have been set, raising
# KingKonf::ConfigError if one is missing.

config.title #=> "Hello, World!"
config.score #=> 85
config.promoted #=> true
config.allow_comments #=> false
config.tags #=> ["greetings", "introductions", "articles"] #=> [435, 2342, 8678]

# Boolean variables also get a nice query method alias:
config.promoted? #=> true
config.allow_comments? #=> false

If you prefer to use a config file, that's also possible. Simply load a YAML file with #load_file:


A common pattern is to store config for all runtime environments in a single file and select the config based on the current environment, e.g.:

config.load_file("config/my_app.yml", Rails.environment)

In that case, structure the config file like so:

  title: hello
  score: 25

  title: yolo
  score: 13

  title: yeah
  score: 99


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.


Bug reports and pull requests are welcome on GitHub at


Copyright 2017 Daniel Schierbeck

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License.

You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.