Specifies a provider API that loads configuration during boot.

Config providers are typically used during releases to load external configuration while the system boots. This is done by starting the VM with the minimum amount of applications running, then invoking all of the providers, and then restarting the system. This requires a mutable configuration file on disk, as the results of the providers are written to the file system. For more information on runtime configuration, see mix release.

Sample config provider

For example, imagine you need to load some configuration from a JSON file and load that into the system. Said configuration provider would look like:

defmodule JSONConfigProvider do
  @behaviour Config.Provider

  # Let's pass the path to the JSON file as config
  def init(path) when is_binary(path), do: path

  def load(config, path) do
    # We need to start any app we may depend on.
    {:ok, _} = Application.ensure_all_started(:jason)

    json = path |> File.read!() |> Jason.decode!()

    Config.Reader.merge(
      config,
      my_app: [
        some_value: json["my_app_some_value"],
        another_value: json["my_app_another_value"],
      ]
    )
  end
end

Then when specifying your release, you can specify the provider in the release configuration:

releases: [
  demo: [
    # ...,
    config_providers: [{JSONConfigProvider, "/etc/config.json"}]
  ]
]

Now once the system boots, it will invoke the provider early in the boot process, save the merged configuration to the disk, and reboot the system with the new values in place.