DevNullParserで始めるFluentd パーサプラグイン入門 ref: http://qiita.com/kiyoto/items/9903718147adde4d6c32

file0.txt

<source>
  type tail
  format json
  path /path/to/file
  tag  aoi.yu
</source>

file1.txt

<source>
  type tcp
  format none
  tag akai.yu
</source>

file10.txt

2014-12-01 03:31:44 +0000 something: {"hello":"world"}

file11.txt

vagrant@precise64:~$ echo 'this is not JSON' | nc localhost 13337

file12.txt

2014-12-01 03:33:49 +0000 something: {"message":"this is not JSON"}

file2.txt

module Fluent
  class TextParser
    #パーサープラグインの名前
    class DevNullParser < Parser
      # このプラグインをパーサプラグインとして登録する
      Plugin.register_parser('dev_null', self)
      def initialize
        super
        # いろいろとここで初期化
      end
      def configure(conf={})
        super
        # 設定ファイルによるものはここで
      end
      def parse(text)
        # これが肝となるメソッドで、textをパースする。
        # parser.parse(text) {|time, record| ... } みたいに使う。
        yield Engine.now, {}
      end
    end
  end
end

file3.txt

<source>
  type tcp
  format dev_null
  port 13337
  tag nothing
</source>
<match nothing>
  type stdout
</match>

file4.txt

vagrant@precise64:~$ cat t.conf
<source>
  type tcp
  format dev_null
  port 13337
  tag nothing
</source>
<match nothing>
  type stdout
</match>
vagrant@precise64:~$ ls my_plugins/
parser_dev_null.rb
vagrant@precise64:~$ fluentd -c t.conf -p my_plugins/
2014-12-01 02:46:36 +0000 [info]: starting fluentd-0.10.57
2014-12-01 02:46:36 +0000 [info]: reading config file path="t.conf"
2014-12-01 02:46:37 +0000 [info]: gem 'fluent-plugin-record-reformer' version '0.4.0'
2014-12-01 02:46:37 +0000 [info]: gem 'fluentd' version '0.10.57'
2014-12-01 02:46:37 +0000 [info]: using configuration file: <ROOT>
  <source>
    type tcp
    format dev_null
    port 13337
    tag nothing
  </source>
  <match nothing>
    type stdout
  </match>
</ROOT>
2014-12-01 02:46:37 +0000 [info]: adding source type="tcp"
2014-12-01 02:46:37 +0000 [info]: adding match pattern="nothing" type="stdout"

file5.txt

vagrant@precise64:~$ echo 'this is a loooooooooooooooooooooooooooooooooooong message' | nc localhost 13337

file6.txt

2014-12-01 02:46:43 +0000 nothing: {}

file7.txt

module Fluent
  class TextParser
    #パーサープラグインの名前
    class MaybeParser < Parser
      Plugin.register_parser('maybe', self)

      config_param :parser, :string
      # フォールバックするNoneParserでのキーの名前。
      config_param :fallback_message_field_key, :string, :default => nil

      def initialize
        super
        # いろいろとここで初期化
      end

      def configure(conf={})
        super
        conf['format'] = conf.delete('parser') # parserフィールドがformatの値なので、書き換えてパーサを初期化
        @parser = TextParser.new
        @parser.configure(conf)
        @fallback_parser = NoneParser.new
        @fallback_parser.configure("message_key" => @fallback_message_field_key)
      end

      def parse(text, &block)
        # パースに失敗したらfallback_parserを呼ぶだけ
        @parser.parse(text) do |time, record|
          if time.nil? or record.nil?
            @fallback_parser.call(text, &block)
          else
            yield time, record
          end
        end
      end
    end
  end
end

file8.txt

vagrant@precise64:~$ cat t.conf
<source>
  type tcp
  format maybe
  parser json
  port 13337
  tag something
</source>

<match something>
  type stdout
</match>
vagrant@precise64:~$ ls my_plugins/
parser_devnull.rb  parser_maybe.rb
vagrant@precise64:~$ fluentd -c t.conf -p my_plugins/
2014-12-01 03:30:59 +0000 [info]: starting fluentd-0.10.57
2014-12-01 03:30:59 +0000 [info]: reading config file path="t.conf"
2014-12-01 03:30:59 +0000 [info]: gem 'fluent-plugin-record-reformer' version '0.4.0'
2014-12-01 03:30:59 +0000 [info]: gem 'fluentd' version '0.10.57'
2014-12-01 03:30:59 +0000 [info]: using configuration file: <ROOT>
  <source>
    type tcp
    format maybe
    parser json
    port 13337
    tag something
  </source>
  <match something>
    type stdout
  </match>
</ROOT>
2014-12-01 03:30:59 +0000 [info]: adding source type="tcp"
2014-12-01 03:30:59 +0000 [info]: adding match pattern="something" type="stdout"

file9.txt

vagrant@precise64:~$ echo '{"hello":"world"}' | nc localhost 13337