Using push_named_tags for MDC-like logging

[KazWolfe]

I'd like to look at using SemanticLogger#push_named_tags to support MDC-like log contexts:

class ApplicationController < ActionController::Base
  before_action :set_log_context

  def set_log_context
    SemanticLogger.push_named_tags(user: { id: current_user.id }) if user_signed_in?
  end
end

This in and of itself works, but has a few quirks:

1. When using push_named_tags, the log context will leak between requests. As an example, a logged out request following a logged in request will still receive the named tags.
2. The push_named_tags method in general does not perform a deep merge on provided context, meaning if multiple push_named_tags were to set user, only the last one would win.

As is, both of these can be resolved with some tricks that ultimately come down to a special tag named _mdc, a lot of (inefficient) juggling, and a middleware to clear this after every request. This, however, is obviously inelegant and inefficient. A number of monkeypatches can also be applied, but that in turn has its own problems. It would be nicer to be able to use SemanticLogger.tagged, but I'm not sure how (or even if) it's possible to appropriately wrap Rails contexts in that.

Has anyone managed to set up something like this before? Or, better yet, is this something that would be beneficial to SemanticLogger overall? It seems relatively simple to tweak the requisite behavior, but I'm not sure it's desired.

I suspect all that would really be needed would be to change named_tags to properly deep merge, and then inject some middleware accordingly to set/clear things.

[KazWolfe]

And Rails 8.1 solves this problem a bit, yay!

require "semantic_logger"

module ContextualLogger
  class RailsContextBridge
    class << self
      def context
        SemanticLogger.named_tags
      end

      def set_context(context_hash)
        SemanticLogger.push_named_tags(context_hash)
      end

      def clear
        Thread.current[:semantic_logger_named_tags] = []
      end
    end
  end
end

This can then be brought in with:

config.active_support.event_reporter_context_store = ContextualLogger::RailsContextBridge

It's definitely not pretty, but it at least works and integrates perfectly well with existing MDC behaviors. Clearing named tags at the end of each request is still a bit strange and might have side effects for other use cases, but it's a viable proof of concept.

I'd be curious to see how (and if) this library evolves to support the event system.

---

There's also around_action, courtesy of a coworker:

class MyController < ApplicationController
  around_action :add_logs

  def index
    # do things
  end

  private def add_logs
    SemanticLogger.named_tags(key: "value") do
      yield
    end
  end
end