Confex simplifies reading configuration at run-time with adapter-based system for fetch values from any source. It's inspired by Phoenix {:system, value} definition for HTTP port.

It's available on hex.pm and can be installed as project dependency:

1. Add confex to your list of dependencies in mix.exs:

def deps do
  [{:confex, "~> 3.2.2"}]
end

2. Ensure confex is started before your application:

def application do
  [applications: [:confex]]
end

1. Replace values with configuration tuples

Define configuration in your config.exs:

config :my_app, MyApp.MyQueue,
  queue: [
    name:        {:system, "OUT_QUEUE_NAME", "MyQueueOut"},
    error_name:  {:system, "OUT_ERROR_QUEUE_NAME", "MyQueueOut.Errors"},
    routing_key: {:system, "OUT_ROUTING_KEY", ""},
    durable:     {:system, "OUT_DURABLE", false},
    port:        {:system, :integer, "OUT_PORT", 1234},
  ]

Configuration tuples examples:

• var - any bare values will be left as-is.
• {:system, "ENV_NAME", "default"} - read string from system environment or fallback to "default" if it is not set.
• {:system, "ENV_NAME"} - same as above, but raise error if ENV_NAME is not set.

Additionally you can cast string values to common types:

• {:system, :string, "ENV_NAME", "default"} (string is a default type).
• {:system, :string, "ENV_NAME"}.
• {:system, :integer, "ENV_NAME", 123}.
• {:system, :integer, "ENV_NAME"}.
• {:system, :float, "ENV_NAME", 123.5}.
• {:system, :float, "ENV_NAME"}.
• {:system, :boolean, "ENV_NAME", true}.
• {:system, :boolean, "ENV_NAME"}.
• {:system, :atom, "ENV_NAME"}.
• {:system, :atom, "ENV_NAME", :default}.
• {:system, :module, "ENV_NAME"}.
• {:system, :module, "ENV_NAME", MyDefault}.
• {:system, :list, "ENV_NAME"}.
• {:system, :list, "ENV_NAME", [1, 2, 3]}.

:system can be replaced with a {:via, adapter} tuple, where adapter is a module that implements Confex.Adapter behaviour.

2. Read configuration by replacing Application.fetch_env/2, Application.fetch_env!/2 and Application.get_env/3 calls with Confex functions

Fetch string values:

iex> Confex.fetch_env(:myapp, MyKey)
{:ok, "abc"}

Fetch integer values:

iex> Confex.fetch_env(:myapp, MyIntKey)
{:ok, 123}

Fetch configuration from maps or keywords:

iex> Confex.fetch_env(:myapp, MyIntKey)
{:ok, [a: 123, b: "abc"]}

Ecto has a init/2 callback, you can use it with Confex to read environment variables. We used to have all our repos to look like this:

defmodule MyApp do
  use Ecto.Repo, otp_app: :my_app

  @doc """
  Dynamically loads the repository configuration from the environment variables.
  """
  def init(_, config) do
    url = System.get_env("DATABASE_URL")
    config =
      if url do
        Keyword.merge(config, Ecto.Repo.Supervisor.parse_url(url))
      else
        {:ok, config} = Confex.Resolver.resolve(config)
        config
      end

    unless config[:database] do
      raise "Set DB_NAME environment variable!"
    end

    unless config[:username] do
      raise "Set DB_USER environment variable!"
    end

    unless config[:password] do
      raise "Set DB_PASSWORD environment variable!"
    end

    unless config[:hostname] do
      raise "Set DB_HOST environment variable!"
    end

    unless config[:port] do
      raise "Set DB_PORT environment variable!"
    end

    {:ok, config}
  end
end

1. Set on_init callback in your prod.exs:

   config :my_app, MyApp.Web.Endpoint,
     on_init: {MyApp.Web.Endpoint, :load_from_system_env, []}

2. Add load_from_system_env function to your endpoint:

   defmodule Mithril.Web.Endpoint do
   
     # Some code here
   
     @doc """
     Dynamically loads configuration from the system environment
     on startup.
   
     It receives the endpoint configuration from the config files
     and must return the updated configuration.
     """
     def load_from_system_env(config) do
       {:ok, config} = Confex.Resolver.resolve(config)
   
       unless config[:secret_key_base] do
         raise "Set SECRET_KEY environment variable!"
       end
   
       {:ok, config}
     end
   end

Confex is supplied with helper macros that allow to attach configuration to specific modules of your application.

defmodule Connection do
  use Confex, otp_app: :myapp
end

It will add config/0 function to Connection module that reads configuration at run-time for :myapp OTP application with key Connection.

You can add defaults by extending macro options:

defmodule Connection do
  use Confex,
    otp_app: :myapp,
    some_value: {:system, "ENV_NAME", "this_will_be_default value"}
end

If application environment contains values in Keyword or Map structs, default values will be recursively merged with application configuration.

We recommend to avoid using tuples without default values in this case, since config/0 calls will raise exceptions if they are not resolved.

You can validate configuration by overriding validate_config!/1 function, which will receive configuration and must return it back to caller function. It will be evaluated each time config/1 is called.

defmodule Connection do
  use Confex, otp_app: :myapp

  def validate_config!(config) do
    unless config[:password] do
      raise "Password is not set!"
    end

    config
  end
end

Currently Confex supports two embedded adapters:

• :system - read configuration from system environment;
• :system_file - read file path from system environment and read configuration from this file. Useful when you want to resolve Docker, Swarm or Kubernetes secrets that are stored in files.

You can create adapter by implementing Confex.Adapter behaviour with your own logic.

• Docs
• REPLACE_OS_VARS in Distillery
• How to config environment variables with Elixir and Exrm by Platformatech.