JSON logger for Rails

json_logger.rb

# This logger is having the interface is same to `ActiveSupport::TaggedLogging`.
# @see https://github.com/rails/rails/blob/master/activesupport/lib/active_support/tagged_logging.rb
class JSONLogger < ActiveSupport::Logger
  class Formatter < ActiveSupport::Logger::SimpleFormatter
    def call severity, timestamp, progname, message
      JSON.generate({
                      type: severity,
                      time: timestamp.iso8601,
                      progname: progname,
                      message: message,
                      tags: current_tags
                    }) + "\n"
    end

    def tagged *tags
      new_tags = push_tags(*tags)
      yield self
    ensure
      pop_tags(new_tags.size)
    end

    def push_tags *tags
      tags.flatten.reject(&:blank?).tap do |new_tags|
        current_tags.concat new_tags
      end
    end

    def pop_tags size = 1
      current_tags.pop size
    end

    def clear_tags!
      current_tags.clear
    end

    def current_tags
      # We use our object ID here to avoid conflicting with other instances
      thread_key = @thread_key ||= "activesupport_tagged_logging_tags:#{object_id}"
      Thread.current[thread_key] ||= []
    end
  end

  delegate :push_tags, :pop_tags, :clear_tags!, to: :formatter

  def initialize *_args
    super
    self.formatter = Formatter.new
  end

  def tagged *tags
    formatter.tagged(*tags) { yield self }
  end

  def flush
    clear_tags!
    super if defined?(super)
  end
end