The environment is set by mix when starting up, both in compile time and when running your application. If you’re using releases, then the config is set at the point where the release is built (except for runtime.exs, which is evaluated at startup in releases as well).

config.exs is still an exs file, so it’s evaluated at runtime.

My guess is that you’re remembering two similar things:

• using System.get_env in some places in modules will result in the “stay until the next compile” behavior you describe:

    defmodule SomeModule do
      @foo System.get_env("FOO")
  
      def foo, do: @foo
   end
  

  SomeModule.foo will “capture” the value from ENV at compile-time

• using System.get_env in config.exs will result in a “stay until the next release” behavior for environment variables, thus the need for runtime.exs

@v0idpwn I am surprised actually, I too was believing that this was compile-time only but indeed the values defined here must be available at runtime and are not stored into a file like in a release.

So the runtime execution erases the value set before compilation.

What is confusing is that if you wanted to have this config:

import Config
config :demo, foo: System.get_env("FOO")

And this code:

defmodule Demo do
  @foo Application.get_env(:demo, :foo)

  def foo do
    @foo
  end
end

You cannot call FOO=bar mix compile and then just iex -S mix and get the expected value, because iex -S mix will always trigger a recompilation. The code above will emit a warning as you should use Application.compile_env. Very different behaviour than calling @foo System.get_env("FOO") directly.

Personnally, I just put everything possible in runtime.exs.