An example of an app-specific configuration wrapper around ENV and Rails.env. This consolidates access to config vars, while providing a seam for adding conditional logic, data munging, type coercion, etc...

config.rb

module Example
  class Config
    def initialize(host_env: ENV, rails_env: Rails.env)
      @host_env = host_env.to_hash
      @rails_env = rails_env
    end

    def admin_qa_tooling_enabled?
      coerce_boolean(host_env.fetch("ADMIN_QA_TOOLING", "false"))
    end
    
    def app_auth_url(token)
      "#{app_url_scheme}://example.com/in-app-auth?authcode=#{token}"
    end

    def aws_assets_bucket
      host_env.fetch("AWS_ASSETS_BUCKET")
    end

    def aws_access_key_id
      host_env.fetch("AWS_ACCESS_KEY_ID")
    end

    def aws_secret_access_key
      host_env.fetch("AWS_SECRET_ACCESS_KEY")
    end

    def aws_region
      host_env.fetch("AWS_REGION")
    end

    def aws_sns_service_provider_application_arn
      host_env.fetch("AWS_SNS_SERVICE_PROVIDER_APPLICATION_ARN")
    end

    def google_maps_geocoding_api_key
      if rails_env.production?
        host_env.fetch("GOOGLE_MAPS_GEOCODING_API_KEY")
      else
        # A nil key results in using the free tier with lower quotas
        host_env.fetch("GOOGLE_MAPS_GEOCODING_API_KEY", nil)
      end
    end

    def intercom_access_token
      host_env.fetch("INTERCOM_ACCESS_TOKEN")
    end

    def intercom_app_id
      host_env.fetch("INTERCOM_APP_ID")
    end

    def intercom_enabled?
      coerce_boolean(host_env.fetch("INTERCOM_ENABLED", "false"))
    end

    def intercom_secure_mode_secret_key
      host_env.fetch("INTERCOM_SECURE_MODE_SECRET_KEY")
    end

    def oldest_supported_version
      Integer(host_env.fetch("OLDEST_SUPPORTED_VERSION"))
    end

    def qualified_technician_max_distance_in_miles
      Float(host_env.fetch("QUALIFIED_TECHNICIAN_MAX_DISTANCE_IN_MILES", 100))
    end

    def service_provider_onboarding_enabled?
      coerce_boolean(host_env.fetch("SERVICE_PROVIDER_ONBOARDING_ENABLED", "false"))
    end

    def service_provider_stripe_enabled?
      coerce_boolean(host_env.fetch("SERVICE_PROVIDER_STRIPE_ENABLED", "false"))
    end

    def subscription_stripe_enabled?
      coerce_boolean(host_env.fetch("SUBSCRIPTION_STRIPE_ENABLED", "false"))
    end

    def stripe_connect_client_id
      host_env.fetch("STRIPE_CONNECT_CLIENT_ID")
    end

    def stripe_connect_oauth_redirect_uri
      "#{current_uri_scheme}://#{web_domain}#{STRIPE_CONNECT_CALLBACK_PATH}"
    end

    def stripe_publishable_key
      host_env.fetch("STRIPE_PUBLISHABLE_KEY")
    end

    def stripe_secret_key
      host_env.fetch("STRIPE_SECRET_KEY")
    end

    def web_domain
      host = if rails_env.production?
        host_env.fetch("WEB_DOMAIN")
      else
        host_env.fetch("WEB_DOMAIN", "localhost:5000")
      end

      override_for_heroku_review_app(host)
    end

    def web_domain_url
      "#{current_uri_scheme}://#{web_domain}"
    end

    def web_push_vapid_contact
      host_env.fetch("VAPID_CONTACT", "mailto:developers+webpush@example.com")
    end

    def web_push_vapid_private_key
      host_env.fetch("VAPID_PRIVATE_KEY")
    end

    def web_push_vapid_public_key
      host_env.fetch("VAPID_PUBLIC_KEY")
    end

    def zip_tax_api_key
      host_env.fetch("ZIP_TAX_API_KEY")
    end

    private

    HEROKU_REVIEW_APP_INDICATOR = "this-is-a-heroku-review-app"
    STRIPE_CONNECT_CALLBACK_PATH = "/oauth/stripe/connect"
    TRUE_VALUES = %w[true t yes y 1].freeze
    URI_SCHEME = {true => "https", false => "http"}.freeze
    private_constant :HEROKU_REVIEW_APP_INDICATOR, :STRIPE_CONNECT_CALLBACK_PATH, :TRUE_VALUES, :URI_SCHEME

    attr_reader :host_env, :rails_env

    def app_url_scheme
      host_env.fetch("URL_SCHEME")
    end

    def coerce_boolean(value)
      TRUE_VALUES.include?(String(value).downcase)
    end

    def current_uri_scheme
      URI_SCHEME.fetch(rails_env.production?)
    end

    def heroku_review_app?(host)
      host == HEROKU_REVIEW_APP_INDICATOR
    end

    # A workaround for Heroku Review Apps
    # (https://devcenter.heroku.com/articles/github-integration-review-apps).
    # Review Apps don't currently have a way to populate one host_env var
    # (e.g., WEB_DOMAIN) from another (e.g., HEROKU_APP_NAME).
    def override_for_heroku_review_app(host)
      if heroku_review_app?(host)
        return "#{host_env.fetch("HEROKU_APP_NAME")}.herokuapp.com"
      end

      host
    end
  end
end