my_fluent_logger_rails.rb

# -*- coding: utf-8 -*-
require 'fluent-logger'
require 'active_support/core_ext'

module CreativeSurvey
  class FluentLogger < ::Logger
    attr_reader :config_file

    def initialize(config_file = Rails.root.join('config', 'fluent-logger.yml.erb'), device = nil)
      @config_file = config_file
      super(device.nil? ? ProxyLogger.new(config['host'], config['port']) : device)
      @formatter = FluentFormatter.new(config['tag'])
    end

    def config
      @config ||= YAML.load(ERB.new(config_file.read).result)[Rails.env]
    end
  end

  # Fluent::Logger::FluentLogger#write が private methodのため
  class ProxyLogger
    attr_reader :host, :port

    def initialize(host = '0.0.0.0', port = 24224)
      @host, @port = host, port
    end

    def fluent
      @fluent ||= ::Fluent::Logger::FluentLogger.new(
        nil,
        host: host,
        port: port
      )
    end

    def write(msg)
      fluent.send(:write, msg)
    end

    def close
      fluent.close
    end
  end

  class FluentFormatter < ::Logger::Formatter
    attr_reader :tag
    def initialize(tag)
      @tag = tag
      super()
    end

    def call(severity, timestamp, _progname, msg)
      [tag, timestamp.to_i, message(severity, msg)]
    end

    def message(severity, msg)
      msg if msg.is_a? Hash
      { # Rails.config.log_tags が設定されている場合はこのHashにmergeするといい
        messages: msg,
        level: severity || 'ANY'
      }
    end
  end
end