How to Write Parser Plugin

Fluentd supports pluggable and customizable formats for input plugins. The plugin filenames prefixed parser_ are registered as Parser Plugins.

See Plugin Base Class API for details on the common APIs for all the plugin types.

Here is an example of a custom parser that parses the following newline-delimited log format:


like this:

2014-04-01T00:00:00 name=jake age=100 action=debugging

While it is not hard to write a regular expression to match this format, it is tricky to extract and save key names.

Here is the code to parse this custom format (let's call it time_key_value). It takes one optional parameter called delimiter, which is the delimiter for key/value pairs. It also takes time_format to parse the time string.

require 'fluent/plugin/parser'

module Fluent::Plugin
  class TimeKeyValueParser < Parser
    # Register this parser as 'time_key_value'
    Fluent::Plugin.register_parser('time_key_value', self)

    # `delimiter` is configurable with ' ' as default
    config_param :delimiter, :string, default: ' '

    # `time_format` is configurable
    config_param :time_format, :string, default: nil

    def configure(conf)

      if @delimiter.length != 1
        raise ConfigError, "delimiter must be a single character. #{@delimiter} is not."

      # `TimeParser` class is already available.
      # It takes a single argument as the time format
      # to parse the time string with.
      @time_parser =

    def parse(text)
      time, key_values = text.split(' ', 2)
      time = @time_parser.parse(time)
      record = {}
      key_values.split(@delimiter).each do |kv|
        k, v = kv.split('=', 2)
        record[k] = v
      yield time, record

Save this code as parser_time_key_value.rb in a loadable plugin path.

With in_tail configured as:

# Other lines...
  @type tail
  path /path/to/input/file
    @type time_key_value

this log line:

2014-01-01T00:00:00 k=v a=b

will be parsed as:

2014-01-01 00:00:00 +0000 test: {"k":"v","a":"b"}

For more details on <parse>, see Parse Section Configurations.

How To Use Parsers From Plugins

Parser plugins are designed to be used with other plugins, like Input, Filter and Output. There is a Parser plugin helper solely for this purpose:

# in class definition
helpers :parser

# in #configure
@parser = parser_create(type: 'json')

# in input loop or #filter or ...
@parser.parse do |time, record|
  # ...

See Parser Plugin Helper API for details.


Parser plugins have a method to parse input (text) data to a structured record (Hash) with time.

#parse(text, &block)

It gets input data as text, and call &block to feed the results of the parser. The input text may contain two or more records so that means the parser plugin might call the &block two or more times for one argument.

Parser plugins must implement this method.

Writing Tests

Fluentd parser plugin has one or more points to be tested. Others (parsing configurations, controlling buffers, retries, flushes, etc.) are controlled by the Fluentd core.

Fluentd also provides the test driver for plugins. You can easily write tests for your own plugins:

# test/plugin/test_parser_your_own.rb

require 'test/unit'
require 'fluent/test/driver/parser'

# Your own plugin
require 'fluent/plugin/parser_your_own'

class ParserYourOwnTest < Test::Unit::TestCase
  def setup
    # Common setup

  CONFIG = %[
    pattern apache

  def create_driver(conf = CONF)

  sub_test_case 'configured with invalid configurations' do
    test 'empty' do
      assert_raise(Fluent::ConfigError) do
    # ...

  sub_test_case 'plugin will parse text' do
    test 'record has a field' do
      d = create_driver(CONFIG)
      text = ' - - [28/Feb/2013:12:00:00 +0900] "GET / HTTP/1.1" 200 777'
      expected_time = event_time('28/Feb/2013:12:00:00 +0900', format: '%d/%b/%Y:%H:%M:%S %z')
      expected_record = {
        'method' => 'GET',
        # ...
      d.instance.parse(text) do |time, record|
        assert_equal(expected_time, time)
        assert_equal(expected_record, record)

Overview of Tests

Testing for parser plugins is mainly for:

  • Validation of configuration (i.e. #configure)

  • Validation of the parsed time and record

To make testing easy, the plugin test driver provides a logger and the functionality to override the system and parser configurations, etc.

The lifecycle of plugin and test driver is:

  1. Instantiate plugin driver which then instantiates the plugin

  2. Configure plugin

  3. Run test code

  4. Assert results of tests by data provided by the driver


  • configuration tests, repeat steps # 1-2

  • full feature tests, repeat steps # 1-4

See Testing API for Plugins for details.

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is an open-source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.

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