Runtime configuration only for production


I have a small team working on one elixir application.

Every developer runs the app locally.

The app is built with mix release inside docker on CI and deployed as a container.

Over time, we’ve settled on this approach to configuration files:

  • config/config.exs provides a common build-time configuration base for all dev/test/prod
  • config/dev.exs provides a build-time configuration for all developers
  • config/dev.local.exs gives a handy way to override development config locally - each developer can put whatever they need in there (per-user config, developer secrets, etc.), this file is put into .gitignore
  • config/test.exs provides all necessary configuration to run tests - this is to ensure that mix test can be run right after git clone without any configuration necessary
  • config/prod.exs provides build-time config for production
  • config/releases.exs provides runtime config for production, which is essentially ENV variables mapping with System.fetch_env!/1 so the app fails to start when some configuration is missing

Config files examples

Click to expand
# config/config.exs - common build-time config for all
use Mix.Config

config :phoenix, :json_library, Jason

import_config "#{Mix.env()}.exs"
# config/dev.exs - all config for development
use Mix.Config

config :myapp, MyAppWeb.BasicAuth, user: "", pass: ""

import_config "dev.local*.exs"
# config/prod.exs - build-time config for production
use Mix.Config

config :myapp, MyAppWeb.Endpoint, server: true
# config/test.exs - all config for test
use Mix.Config

config :myapp, MyAppWeb.Endpoint, server: false
config :myapp, MyAppWeb.BasicAuth, user: "test", pass: "test"
# config/dev.local.exs - local config for development, gitignored
use Mix.Config

config :myapp, MyAppWeb.Endpoint, url: [host: ""]
config :myapp, MyAppWeb.BasicAuth, user: "", pass: ""
# config/releases.exs - runtime config for production
import Config

config :myapp, MyAppWeb.BasicAuth,
  user: System.fetch_env!("ADMIN_USER"),
  pass: System.fetch_env!("ADMIN_PASS")

The problem

The new preferred way of runtime configuration is to use config/runtime.exs.

This file will be executed regardless if run with mix or as a part of the release.

Now, if I simply rename my config/releases.exs to config/runtime.exs it will blow up in dev & test due to System.fetch_env!/1 usage. Nobody have all the ENV values referenced locally (a lot of the are secrets).

I can’t seem to find any reference on how to deal with such case.

Any ideas?

Bonus question

Another, somehow-related issues is using .exs files as runtime configuration.

With elixir 1.10 I could add import_config "foo.exs" at the bottom of config/releases.exs and then mount foo.exs file on a production system at /app/releases/0.1.0/foo.exs (docker on kubernetes via ConfigMap).

With elixir 1.11 it now fails with ** (RuntimeError) import_config/1 is not enabled for this configuration file. Some configuration files do not allow importing other files as they are often copied to external systems, which seems to be a bit ironic, as I do want to “copy to external system” :upside_down:

The question here is: I DO want to use runtime-mounted .exs file as a configuration (alongside ENV vars) simply because it is much better than yaml, but it seems to be discouraged.

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at hexpm we ended up with the following:

# config/runtime.exs
import Config

if config_env() == :prod do
  config :hexpm, ...


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