#
View Source Config.Provider behaviour (Elixir v1.15.7)

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`.

## Multiple config files

One common use of config providers is to specify multiple configuration files in a release. Elixir ships with one provider, called `Config.Reader`, which is capable of handling Elixir's built-in config files.

For example, imagine you want to list some basic configuration on Mix's built-in `config/runtime.exs` file, but you also want to support additional configuration files. To do so, you can add this inside the `def project` portion of your `mix.exs`:

    releases: [
      demo: [
        config_providers: [
          {Config.Reader, {:system, "RELEASE_ROOT", "/extra_config.exs"}}
        ]
      ]
    ]

You can place this `extra_config.exs` file in your release in multiple ways:

1. If it is available on the host when assembling the release, you can place it on "rel/overlays/extra_config.exs" and it will be automatically copied to the release root

2. If it is available on the target during deployment, you can simply copy it to the release root as a step in your deployment

Now once the system boots, it will load both `config/runtime.exs` and `extra_config.exs` early in the boot process. You can learn more options on `Config.Reader`.

## Custom config provider

You can also implement custom config providers, similar to how `Config.Reader` works. 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
      @impl true
      def init(path) when is_binary(path), do: path

      @impl true
      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"}
        ]
      ]
    ]

# Summary

## Types

config()
config_path()

A path pointing to a configuration file.

state()

## Callbacks

init(term)

Invoked when initializing a config provider.

load(config, state)

Loads configuration (typically during system boot).

## Functions

resolve_config_path!(path)

Resolves a `config_path/0` to an actual path.

validate_config_path!(path)

Validates a `config_path/0`.

# Types

Link to this type

# config()

View Source

    @type config() :: keyword()

Link to this type

# config_path()

View Source

    @type config_path() :: {:system, binary(), binary()} | binary()

A path pointing to a configuration file.

Since configuration files are often accessed on target machines, it can be expressed either as:

- a binary representing an absolute path

- a `{:system, system_var, path}` tuple where the config is the concatenation of the environment variable `system_var` with the given `path`

Link to this type

# state()

View Source

    @type state() :: term()

# Callbacks

Link to this callback

# init(term)

View Source

    @callback init(term()) :: state()

Invoked when initializing a config provider.

A config provider is typically initialized on the machine where the system is assembled and not on the target machine. The `init/1` callback is useful to verify the arguments given to the provider and prepare the state that will be given to `load/2`.

Furthermore, because the state returned by `init/1` can be written to text-based config files, it should be restricted only to simple data types, such as integers, strings, atoms, tuples, maps, and lists. Entries such as PIDs, references, and functions cannot be serialized.

Link to this callback

# load(config, state)

View Source

    @callback load(config(), state()) :: config()

Loads configuration (typically during system boot).

It receives the current `config` and the `state` returned by `init/1`. Then, you typically read the extra configuration from an external source and merge it into the received `config`. Merging should be done with `Config.Reader.merge/2`, as it performs deep merge. It should return the updated config.

Note that `load/2` is typically invoked very early in the boot process, therefore if you need to use an application in the provider, it is your responsibility to start it.

# Functions

Link to this function

# resolve_config_path!(path)

View Source (since 1.9.0)

    @spec resolve_config_path!(config_path()) :: binary()

Resolves a `config_path/0` to an actual path.

Link to this function

# validate_config_path!(path)

View Source (since 1.9.0)

    @spec validate_config_path!(config_path()) :: :ok

Validates a `config_path/0`.