Proposal: Logger Backends and Formatters

[Ebtoulson]

ElixirForum Discussion

The current :console logger backend doesn't support injecting custom formatters.

Issue

Creating a backend seems to be the intended pattern but I think this could be improved. I see a definite separation between outputting (stdio, file, tcp/udp, etc) and formatting, but this seems to be coupled in a lot of existing logger packages (e.g. logger_json_file_backend).

Proposal

Let's create a behavior for formatters, break out the default formatter from :console, and determine formatters from configuration.

We might also want to break out common functionality into the utils module to help with the creation of custom formatters. This would include things like handling metadata.

config :logger,
  backends: [:console]

config :logger, :console,
  level: :debug,
  metadata: [:request_id],
  formatter: Logger.Formatters.Json

Doing something like this while remaining backwards compatible with the existing :console backend and :format configuration might be a little tricky.

It is not possible in the current implementation, we would have to reimplement the whole backend. - Jose

I'm also hoping this creates a pattern for other elixir logger projects to follow.

[fishcakez]

We already support this. Use Logger.Formatter.compile with {module, function} for completely custom format. Then call Logger.Formatter.format. Perhaps the docs are not quite so good in the Logger module itself. I believe this feature has existed since Logger was first introduced.

[josevalim]

Good call @fishcakez. I had totally forgotten about this!

[Ebtoulson]

👍 This is what I was looking for. Thanks