[Proposal] Config.config variant that does *not* deep-merge keyword lists

[Hubert Łępicki]

I am stumbling at random intervals, but quite predictably every several months or so, on the issue that someone in some project created a bug, because they didn't realize at the moment that changes made in runtime.exs will be deep-merged with config.exs or prod.exs respectively.

The most recent example for me is when someone wanted to remove global_limit from our Oban engine this way:

# config/config.exs
config :core, :background_jbos, queues: [my_queue: [global_limit: 1, local_limit: 1]]

we had this in runtime.exs:

# config/runtime.exs
config :core, :background_jbos, queues: [my_queue: [global_limit: 100, local_limit: 10]]

The person changed it to:

# config/runtime.exs
config :core, :background_jbos, queues: [my_queue: [local_limit: 10]]

i.e. they removed global_limit configuration option, expecting it to lift the limit completely, instead because configs are deep-merged, it used global_lmit of 1.

Obviously everything locked up as the server is quite busy and we had tens of thousands jobs accumulating in every minute O_o.

I am raising the issue because I have seen this enough times that I am thinking a variant of config/2 function that does *not* deep merge with the existing config key, but replaces it completely could be warranted. 

Obviously naming it is hard but something like Config.put_config/2 could work for me I think.

Let me know if that's useful thing to have for other people, maybe it's a singular issue that only my teams stumble upon xD

[Jon Rowe]

If we were to add something here, I'd suggest `replace_config` or `replace_all_config` as a name, `put_config` is too similar to what we have already, given that if you were to `put` just one key in a map it would ignore other keys.

Cheers

Jon

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[Hubert Łępicki]

I like replace_all_config a lot. I'll wait for some more votes before I start getting my hands dirty implementing it because I'm not entirely convinced it'll be a welcome change. There's always a trade-off between functionality and maintainability that's hard for me to judge.

[José Valim]

This is tricky because at what level do you stop deep merging? What if you had other queues? Do you want to delete them? What if you stored it at:

config :core, queues: [my_queue: [global_limit: 1, local_limit: 1]]

Maybe the best would be "delete_config", so you explicitly delete up to a certain nesting? For example:

delete_config [:core, :queues, :my_queue]

If this sound sensible, a PR is welcome.

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[Hubert Łępicki]

Yes, I would very much like delete_config too. It'd do the same for me.

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Pozdrawiam,
Hubert Łępicki
 -----------------------------------------------
[ http://hubertlepicki.com ]

[Jason Axelson]

+1 for delete_config for me too

I've used a snippet like this before as a workaround:

        # We need to delete all the existing env because `Application.put_all_env` does a deep merge
        for {key, _val} <- Application.get_all_env(app), do: Application.delete_env(app, key)
        Application.put_all_env([{app, original_env}])

Also an earlier version of that code was incorrect because I forgot that config was always deep merged, which makes `Application.put_all_env([{app, []}]` a no-op which perhaps should be a warning instead.

-Jason

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