How to make Ecto Repo configuration work in both local and production using environment variables?

The Solution: Use runtime.exs for Production

The reason your production release fails to read DATABASE_URL is that config/prod.exs is evaluated at compile time. When you package your application as a release, the values from prod.exs are baked into the bytecode. Any call to System.get_env inside that file happens on your build machine, not on your production server.

To load environment variables at startup in a release, you must move production-specific configuration to config/runtime.exs.

Implementation Steps

1. Local Development: Keep your local settings in config/dev.exs. Since you run this via mix, compile-time configuration is acceptable.

   config :my_app, MyApp.Repo, 
     url: "postgres://localhost/my_app_dev"
   

2. Production Runtime: Create or edit config/runtime.exs. This file is executed every time the application starts, regardless of whether it was started via mix or as a release.

   import Config
   
   if config_env() == :prod do
     config :my_app, MyApp.Repo, 
       url: System.get_env("DATABASE_URL")
   end

3. Remove Overrides: Delete any System.get_env or System.put_env calls from config/prod.exs to avoid configuration conflicts.

Technical Explanation

Elixir releases distinguish between compile-time configuration (config.exs, dev.exs, prod.exs) and runtime configuration (runtime.exs).

• Compile-time: Evaluated once during the build process. If you use System.get_env here, the value is fixed at the moment of compilation.
• Runtime: Evaluated every time the BEAM VM starts the application. This is the only place where environment variables provided by Docker, Kubernetes, or systemd will be correctly injected into your Repo configuration.

Verification

To verify the configuration is working without deploying to a full production cluster, you can run your app locally in production mode:

DATABASE_URL=postgres://user:pass@localhost/db_name MIX_ENV=prod mix phx.server

If the application connects successfully, the runtime.exs logic is correct.

Diagnostic Question

Are you using a custom release configuration (e.g., in rel/env.sh.eex) that might be overriding or clearing environment variables before the application boots?