Why can't I do configuration in runtime.exs in my dev environment?

Also, I notice that config/runtime.exs is apparently executed after lib/myapp/application.ex is executed when I start my app locally with mix phx.server. This is not what I was expecting.

I think of lib/myapp/application.ex as the entry point to the actually running app, which should happen after everything is configured. So runtime.exs should be executed after compilation, but before the app actually starts, because I will need to load secret API keys through runtime.exs, which my app will need when it starts. But if I’m already in lib/myapp/application.ex before runtime.exs has executed then isn’t that the wrong order?

What am I not understanding? And how can I set configuration through runtime.exs and have it actually override configuration set in the config files?

Thanks.

What kind of settings are you seeing not being overriden via runtime.exs? Are you by any chance trying to change a compile-time config param via runtime.exs?

in config.exs I have the following lines:

config :myapp,
  secret_api_key: "set-in-config.exs"
IO.inspect('in config.exs...')

in dev.exs I have the following lines:

config :myapp,
  secret_api_key: "set-in-dev.exs"
IO.inspect('in dev.exs...')

in runtime.exs I have the following lines:

config :myapp,
  secret_api_key: "set-in-runtime.exs"
IO.inspect('in runtime.exs...')

When I start the app, I see it print
‘in config.exs…’
‘in dev.exs…’
‘in runtime.exs…’

but the value of secret_api_key is “set-in-dev.exs”, at least as far as I can tell.

Also, if I add IO.inspect to lib/myapp/application.ex then it sometimes prints when I start the app up and it sometimes does not – I would like to be able to reliably IO.inspect my environment in application.ex and I don’t understand why that doesn’t work reliably.

I have the following lines at the bottom of lib/myapp/application.ex:

IO.inspect("in lib/myapp/application.ex")
IO.inspect(Application.get_all_env(:myapp))

When I start the app for the first time after adding those lines it will print “in lib/myapp/application.ex” and then print the environment so I can inspect it – and it will do this BEFORE it prints “in runtime.exs”. But if I then stop and restart the app it will not print the lines from lib/myapp/application.ex.

So yes, I think I am trying to change a compile-time config param via runtime.exs. Is this not possible? If not, then why not?

How are you reading the config value? If you’re reading it during compile-time, then it’s baked into the app and trying to modify at runtime will have no effect.

At the time runtime.exs is read compilation is long done. So you cannot change compile time values through runtime.exs. This is even more apparent when you build a release. Compilation happens when the release is created, but runtime.exs is only evaluated once the release is moved to wherever it’s supposed to run and started there.

If you need things to be available at compile time use config.exs and any additional files imported from there.

I understand that runtime.exs is executed long after the compilation is finished. This is why I am so interested in setting my secret API key through runtime.exs rather than through config.exs or one of dev/test/prod.exs.

I would like for runtime.exs to be evaluated only once the release is moved to wherever it is supposed to run and started there, and it is my understanding that that is what happens.

On my local development environment I see that runtime.exs is in fact evaluated after config.exs and dev.exs are evaluated. Surely it is possible to use runtime.exs to set a secret API key, since that is the whole purpose of the file, as I understand it.

Is it not possible to overwrite values in runtime.exs that have previously been set in config.exs?

How are you reading the config value? If you’re reading it during compile-time, then it’s baked into the app and trying to modify at runtime will have no effect.

I wonder if I don’t understand how to inspect my final runtime configuration properly. How can I inspect a value in my environment while the application is running, long after runtime.exs has finished doing it’s thing?

The issue is here. These lines that are at the bottom of the file will be executed at compile time. Thus runtime.exs will not be evaluated yet. Any values set in runtime.exs are only available at runtime.

I wrote this blog post that attempts to clarify the differences between the configurations (in the article I call “compile time” “build time” but it means the same thing): Elixir: Time for Some Configuration — Random Notes

To simplify, if you want to see your values from runtime.exs, you need to grab them at runtime, i.e. inside a function that is called at runtime.

To clarify this further. In elixir any code, which is not within a function definition (or moved to such via a macro) will be executed at compile time.