Validate runtime config on application start

Questions / Help
,
1

In my config/runtime.exs I have the following:

timezone = System.get_env("TZ") || "Etc/UTC"

unless Timex.is_valid_timezone?(timezone) do
  raise """
  The timezone specified in the TZ environment variable is invalid.
  """
end

config :my_app, MyApp, timezone: timezone

My intention is that a valid Timezone will work as expected while an invalid one (such as Europe/Börlin) crashes the application early.In the rest of the App, I can then assume that the user-supplied timezone is always valid.

This works as expected in dev, on prod however, this results in a crash at runtime:

ERROR! Config provider Config.Reader failed with:
** (ArgumentError) errors were found at the given arguments:
  * 1st argument: the table identifier does not refer to an existing ETS table
    (stdlib 6.2) :ets.lookup(:tzdata_current_release, :release_version)
    (tzdata 1.1.3) lib/tzdata/release_reader.ex:74: Tzdata.ReleaseReader.current_release
    (tzdata 1.1.3) lib/tzdata/release_reader.ex:17: Tzdata.ReleaseReader.simple_lookup/1
    (tzdata 1.1.3) lib/tzdata/release_reader.ex:9: Tzdata.ReleaseReader.zone_and_link_li
    (tzdata 1.1.3) lib/tzdata.ex:61: Tzdata.zone_exists?/1
    (timex 3.7.13) lib/timezone/timezone.ex:230: Timex.Timezone.name_of/1
    (timex 3.7.13) lib/timex.ex:857: Timex.is_valid_timezone?/1
    /app/releases/0.1.0/runtime.exs:13: (file)

I’m assuming this happens because the Timex application isn’t started yet when the runtime configuration is evaluated.

So instead, I now have this workaround that achieves the same thing:

defmodule MyApp.Application do
  use Application

  @impl true
  def start(_type, _args) do
    with :ok <- validate_timezone_config() do
      # shortened
      Supervisor.start_link(children, opts)
    end
  end

  defp validate_timezone_config do
    timezone = Briefly.user_timezone()

    case Timex.is_valid_timezone?(timezone) do
      true -> :ok
      false -> {:error, "Configured timezone '#{timezone}' is invalid"}
    end
  end
end

If an invalid timezone is supplied, the app crashes early:

11:39:02.487 [notice] Application briefly exited: Briefly.Application.start(:normal, []) returned an error: "Configured timezone 'Europe/Börlin' is invalid"
** (exit) :terminating
    (kernel 10.2.1) application_controller.erl:511: :application_controller.call/2
    (kernel 10.2.1) application.erl:367: :application."-ensure_all_started/3-lc$^0/1-0-"/1
    (kernel 10.2.1) application.erl:367: :application.ensure_all_started/3
    (mix 1.18.2) lib/mix/tasks/app.start.ex:72: Mix.Tasks.App.Start.start/3
    (mix 1.18.2) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5
    (mix 1.18.2) lib/mix/tasks/run.ex:129: Mix.Tasks.Run.run/5
    (mix 1.18.2) lib/mix/tasks/run.ex:85: Mix.Tasks.Run.run/1
    (mix 1.18.2) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5

Is this considered an anti-pattern? Is there a more appropriate/easier way to validate runtime configuration during startup?

2

You should be able to do Application.ensure_all_started in the runtime config, but that would also mean you cannot configure the started application and its dependencies anymore in runtime.exs. Validating in the application start callback is free of that chicken-egg type problem of what runs first.

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