A simple keyword-based configuration API.

Example

This module is most commonly used to define application configuration, typically in config/config.exs:

import Config

config :some_app,
  key1: "value1",
  key2: "value2"

import_config "#{config_env()}.exs"

import Config will import the functions config/2, config/3 config_env/0, config_target/0, and import_config/1 to help you manage your configuration.

config/2 and config/3 are used to define key-value configuration for a given application. Once Mix starts, it will automatically evaluate the configuration file and persist the configuration above into :some_app's application environment, which can be accessed in as follows:

"value1" = Application.fetch_env!(:some_app, :key1)

Finally, the line import_config "#{config_env()}.exs" will import other config files based on the current configuration environment, such as config/dev.exs and config/test.exs.

Config also provides a low-level API for evaluating and reading configuration, under the Config.Reader module.

Avoid application environment in libraries

If you are writing a library to be used by other developers, it is generally recommended to avoid the application environment, as the application environment is effectively a global storage. Also note that the config/config.exs of a library is not evaluated when the library is used as a dependency, as configuration is always meant to configure the current project. For more information, read our library guidelines.

Migrating from use Mix.Config

The Config module in Elixir was introduced in v1.9 as a replacement to use Mix.Config, which was specific to Mix and has been deprecated.

You can leverage Config instead of use Mix.Config in three steps. The first step is to replace use Mix.Config at the top of your config files by import Config.

The second is to make sure your import_config/1 calls do not have a wildcard character. If so, you need to perform the wildcard lookup manually. For example, if you did:

import_config "../apps/*/config/config.exs"

It has to be replaced by:

for config <- "../apps/*/config/config.exs" |> Path.expand(__DIR__) |> Path.wildcard() do
  import_config config
end

The last step is to replace all Mix.env() calls in the config files with config_env().

Keep in mind you must also avoid using Mix.env() inside your project files. To check the environment at runtime, you may add a configuration key:

# config.exs
...
config :my_app, env: config_env()

Then, in other scripts and modules, you may get the environment with Application.fetch_env!/2:

# router.exs
...
if Application.fetch_env!(:my_app, :env) == :prod do
  ...
end

The only files where you may access functions from the Mix module are the mix.exs file and inside custom Mix tasks, which always within the Mix.Tasks namespace.

config/runtime.exs

For runtime configuration, you can use the config/runtime.exs file. It is executed right before applications start in both Mix and releases (assembled with mix release).