start confex (and my application) before logger ?

Confex

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 and have no external dependencies.

Installation

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

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

def deps do
  [{:confex, "~> 3.5.0"}]
end

Ensure confex is started before your application either by adding it to applications list as shown below or by making sure you use extra_applications option instead of applications (this feature is available since Elixir 1.4 and enabled by default for new projects):
```
def application do
  [applications: [:confex]]
end
```

Usage

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", ["a", "b", "c"]}.
- {:system, :charlist, "ENV_NAME"}.
- {:system, :charlist, "ENV_NAME", 'default'}.
:system can be replaced with a {:via, adapter} tuple, where adapter is a module that implements Confex.Adapter behaviour.

Type can be replaced with {module, function, arguments} tuple, in this case Confex will use external function to resolve the type. Function must returns either {:ok, value} or {:error, reason :: String.t} tuple.
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"]}
```

Integrating with Ecto

Ecto.Repo 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: [url: url] ++ config, else: Confex.Resolver.resolve!(config)

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

    {:ok, config}
  end
end

Integrating with Phoenix

Same for Phoenix, use init/2 callback of Phoenix.Endpoint:

defmodule MyApp.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 init(_type, 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

Populating configuration at start-time

In case you want to keep using Application.get_env/2 and other methods to keep accessing configuration, you can resolve it one-time when application is started:

defmodule MyApp do
  use Application

  def start(_type, _args) do
    # Replace Application environment with resolved values
    Confex.resolve_env!(:my_app)

    # ...
  end
end

However, don't drink too much Kool-Aid. Direct calls to the Confex are more explicit and should be default way to go, you don't want your colleagues to waste their time finding out how that resolved value got into the configuration, right?

Using Confex 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

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

Adapters

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.

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