I’ve been trying to add a custom logger backend to my application using the now-recommended :logger.add_handler/3 function as suggested to me by @LostKobrakai (on Twitter). I’ve ran into an issue with the config of the handler, which changes between adding the handler and logging the first event. This is my logger:

defmodule Demo.FileLogger do
  def adding_handler(config) do
    IO.inspect(config, label: "Initial")
    {:ok, config}
  end

  def log(event, config) do
    IO.inspect(event, label: "Event")
    IO.inspect(config, label: "Config")

    :ok
  end
end

This is how I add the handler to my application:

# lib/demo/application.ex
  @impl true
  def start(_type, _args) do
    :ok = :logger.add_handler(:file_log, Demo.FileLogger, %{})
    # ...
end

Now, the handler logs the following config in the adding_handler/1 callback:

Initial: %{
  id: :file_log,
  module: Demo.FileLogger,
  level: :all,
  filters: [],
  filter_default: :log,
  formatter: {:logger_formatter, %{}}
}

but once the log/2 callback is executed for the first time, config only contains the following values:

Config: %{
  id: :file_log,
  module: Demo.FileLogger,
  formatter: {:logger_formatter, %{}}
}

Note the missing level, filters, and filter_default fields. This is surprising to me, because in the “old” :gen_event behaviour for logger backends, you’d set a config during the init/1 callback and this config would be passed to every callback thereafter, just like in a GenServer. Why is this different in the :logger backend and what’s the suggested solution to pass on a config map between all callbacks?

I also implemented the changing_config/2 callback to see whether the config is changed somehow, but it never executes between the adding_handler/1 and the log/2 callback.