Confex

This is helper module that provides a nice way to read environment configuration at runtime. It's inspired by Phoenix {:system, value} definition for HTTP port.

Installation

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, "~> 2.0.0"}]
end

2. Ensure confex is started before your application:

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

Usage

1. Defining configurations

Define your configuration in config.ex of your application.

config :ap_cfpredictor, AssetProcessor.AMQP.Producer,
  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},
  ],

List of supported formats:

• var - any bare values will be left as-is.
• {:system, "ENV_NAME", default} - read string from system ENV, return default if it's nil.
• {:system, "ENV_NAME"} - read string from system ENV, returns nil if environment variables doesn't exist.
• {:system, :string, "ENV_NAME", default} - same as {:system, "ENV_NAME", default}.
• {:system, :string, "ENV_NAME"} - same as {:system, "ENV_NAME"}.
• {:system, :integer, "ENV_NAME", default} - same as {:system, "ENV_NAME", default}, but will convert value to integer if it's not nil. Default value type will not be changed.
• {:system, :integer, "ENV_NAME"} - same as {:system, :integer, "ENV_NAME", nil}.
• {:system, :boolean, "ENV_NAME", default} - same as {:system, "ENV_NAME", default}, but will convert value to boolean if it's not nil. Default value type will not be changed.
• {:system, :boolean, "ENV_NAME"} - same as {:system, :boolean, "ENV_NAME", nil}.
• {:system, :atom, "ENV_NAME"}
• {:system, :atom, "ENV_NAME", :default}
• {:system, :module, "ENV_NAME"}
• {:system, :module, "ENV_NAME", Default}
• {:system, :list, "ENV_NAME"} - same as {:system, :list, "ENV_NAME", nil}.
• {:system, :list, "ENV_NAME", Default} - same as {:system, "ENV_NAME", default}, but will convert value to list if it's not nil, splitting at commas. Default value type will not be changed.

2. Reading configuration

Read string values:

iex> Confex.get(:myapp, MyKey)
"abc"

Read integer values:

Confex.get(:myapp, MyIntKey)
123

Read map values:

Confex.get_map(:myapp, MyIntKey)
[a: 123, b: "abc"]

3. Using macros

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

Connection in this case will read configuration from app :myapp with key Connection. Also it will provide helper function config/0 that will return values at run-time.

4. Configuration validation

Sometimes you want to validate configuration, for this you can define def validate_config(config) method, that will be called on each config/0 usage.

Confex doesn't give opinions on a validator to be used in overrided methods.

Integrating with Ecto

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: Confex.process_env(config)

    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

Integrating with Phoenix

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
       config = Confex.process_env(config)
   
       unless config[:secret_key_base] do
         raise "Set SECRET_KEY environment variable!"
       end
   
       {:ok, config}
     end
   end

Configuration priorities

By using Confex macro in your module, you allow to provide compile-time defaults for it.

1. Declare module

defmodule MyModule do
  use Confex
end

2. Provide defaults when using it

use MyModule,
  otp_all: :myapp,
  host: {:system, "HOST"}

This configs will overwritten by any configuration that you have in your application env.

Helpful links

• Docs