The mechanism that Elixir used for providing different configuration values for dev, test, and prod (via dev.exs, test.exs, and prod.exs files), did not work well with environment variables, since those configuration files were evaluated at build time, which wouldn't work if you built, e.g., a single prod app and wanted to run it with different ENV vars in staging and prod.

Meanwhile, as of Elixir 1.10, there was a file called releases.exs which you could use to configure your release (basically a compiled artifact that included the BEAM VM runtime system that you could drop on your server to run, the preferred way of deploying production apps). However, those values didn't affect running the app not as a release, which is common when developing locally.

In the end, our organization kind of standardized on an approach of reading the ENV vars in the app start up, at the top of the supervision tree, which kind of worked in all scenarios.

But then in Elixir 1.11 there's finally a great (IMO) way of configuring: a new file called runtime.exs which is the best of all worlds. Like the old dev/test/prod files, it's evaluated when you run a mix project, so it's used during local development, and has access to Mix.env, so you can set variables based on environment. But it instead of being evaluated at build time, it's evaluated at run time, so you can refer to environment variables, too. And, it's used by releases!

So now there's a very slick standard way to configure your app, that works in all situations.