Ruby log formatter with JSON output and Datadog APM trace correlation injection

traced_json_log_formatter.rb

# frozen_string_literal: true

require 'json'

module Utils
  # Ruby Log Formatter that serves two purposes:
  # 1. Formats logs to JSON, so that they can be indexed by our log processor (currently Datadog)
  # 2. Enriches logs with Datadog APM trace information, so logs can be associated back to APM traces
  class TracedJSONLogFormatter
    def initialize
      @tracer = Datadog.tracer if defined?(Datadog) && Datadog.tracer
    end

    def call(severity, datetime, _progname, message)
      message_and_attributes = message.is_a?(Hash) ? message : { message: message.to_s }
      log_line = JSON.dump(
        timestamp: datetime.to_s,
        severity: severity.ljust(5).to_s,
        **datadog_log_to_trace_correlation,
        **message_and_attributes
      )
      "#{log_line}\n"
    end

    def datadog_log_to_trace_correlation
      return {} unless @tracer

      correlation = @tracer.active_correlation
      {
        ddsource: ['ruby'],
        dd: correlation_info(correlation)
      }
    end

    def correlation_info(correlation)
      {
        # To preserve precision during JSON serialization, use strings for large numbers
        trace_id: correlation.trace_id.to_s,
        span_id: correlation.span_id.to_s,
        env: correlation.env.to_s,
        service: correlation.service.to_s,
        version: correlation.version.to_s
      }
    end
  end
end

  # in production.rb
  logger = Logger.new(STDOUT)
  logger.formatter = Utils::TracedJSONLogFormatter.new
  config.logger = logger
  ActiveJob::Base.logger = logger