Config.Provider behaviour (Elixir v1.13.0-dev) View Source

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 some additional configuration files. To do so, you can do this in 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.

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
  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"}
    ]
  ]
]

Link to this section Summary

Types

A path pointing to a configuration file.

Callbacks

Invoked when initializing a config provider.

Loads configuration (typically during system boot).

Link to this section Types

Specs

config() :: keyword()

Specs

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

Specs

state() :: term()

Link to this section Functions

Link to this function

resolve_config_path!(path)

View Source (since 1.9.0)

Specs

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)

Specs

validate_config_path!(config_path()) :: :ok

Validates a config_path/0.

Link to this section Callbacks

Specs

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.

Specs

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.