A Simple File-Based Feature Toggler

feature_toggle_service.rb

# A service to properly route feature toggles, similar to Promote's Gate mechanism,
# but greatly simplified.
# @see https://martinfowler.com/articles/feature-toggles.html
class FeatureToggleService

  attr_reader :config
  DEFAULT_CONFIG_PATH = Rails.root.join('config', 'feature_toggles.yml')

  def initialize(config: nil)
    @config =
      if config.blank? || !config.is_a?(Hash)
        YAML.load_file(DEFAULT_CONFIG_PATH)
      else
        config
      end
  end

  # Sets enabled/disabled state of a feature in this instance of service only, great for test.
  # To permanently enable or disable a feature, update the `config/feature_toggles.yml` file.
  # @param feature, required, a String with dot notation,
  #        e.g. <code>'scheduled_send.use_local_time'</code>
  # @param enabled, Boolean value, default to <code>true</code>
  def set(feature:, enabled: true)
    if feature.present?
      keys = feature.to_s.split('.').map(&:to_sym)
      last_config_node = @config
      keys.each_with_index do |key, index|
        # last key gets assigned the toggle
        if index == keys.size - 1
          last_config_node[key] = enabled
        else
          last_config_node[key] ||= {}
          last_config_node = last_config_node[key]
        end
      end
      return true
    end
    false
  end

  # Gets enabled/disabled state of a feature.
  # @return <code>true</code> if a feature key is found with a <code>Boolean</code> true value.
  def enabled?(feature:)
    if feature.present?
      keys = feature.to_s.split('.').map(&:to_sym)
      last_config_node = config
      keys.each_with_index do |key, index|
        if index == keys.size - 1
          return last_config_node.try(:[], key) == true
        else
          return false unless last_config_node.try(:[], key)
          last_config_node = last_config_node[key]
        end
      end
    end
    false
  end

end

feature_toggle_service_helper.rb

require 'rails_helper'

describe FeatureToggleService do
  let(:service) { described_class.new(config: config) }
  let(:config) do
    {
      scheduled_send: {
        use_local_time: false,
        finders: {
          utc_offset_finder: true,
        }
      }
    }
  end

  describe 'Constructor' do
    subject { service }

    it { is_expected.to be_a described_class }

    it 'stores the provided config' do
      expect(subject.config).to eq config
    end
  end

  describe '#set' do
    subject { service.set(feature: feature, enabled: enabled) }
    let(:enabled) { true }

    context 'with empty feature string' do
      let(:feature) { nil }
      it { is_expected.to be false }
    end

    context 'with existing feature string' do
      let(:feature) { 'scheduled_send.use_local_time' }

      it { is_expected.to be true }

      it 'sets the feature state to `true`' do
        subject
        expect(service.config[:scheduled_send][:use_local_time]).to be true
      end
    end

    context 'with new feature string' do
      let(:feature) { 'triggered_email.new_feature' }
      let(:enabled) { false }

      it { is_expected.to be true }

      it 'sets the new feature state to `false`' do
        subject
        expect(service.config[:triggered_email][:new_feature]).to be false
      end
    end
  end

  describe '#enabled?' do
    subject { service.enabled?(feature: feature) }

    context 'feature of using local time' do
      let(:feature) { 'scheduled_send.use_local_time ' }
      it { is_expected.to be false }
    end

    context 'feature of utc_offset_finder' do
      let(:feature) { 'scheduled_send.finders.utc_offset_finder' }
      it { is_expected.to be true }
    end
  end
end

feature_toggles.yml

---
scheduled_send:
  use_local_time: false