Versions | v0.14 (td-agent3)

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Writing Parser Plugins

Fluentd supports pluggable, customizable formats for input plugins. The plugin files whose names start with “parser_” are registered as Parser Plugins. See Plugin Base Class API to show details of common API for all plugin types.

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


e.g., something 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"
    Plugin.register_parser("time_key_value", self)

    config_param :delimiter, :string, default: " "   # delimiter is configurable with " " as default
    config_param :time_format, :string, default: nil # time_format is configurable

    def configure(conf)

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

      # TimeParser class is already given. 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 { |kv|
        k, v = kv.split("=", 2)
        record[k] = v
      yield time, record

Then, save this code in parser_time_key_value.rb in a loadable plugin path. Then, if in_tail is configured as

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

Then, the log line like 2014-01-01T00:00:00 k=v a=b is parsed as 2013-01-01 00:00:00 +0000 test: {"k":"v","a":"b"}.

TODO: add description about <parse> section.

Table of Contents

How To Use Parsers From Plugins

Parser plugins are designed to be used from other plugins, like Input, Filter and Output. There is a Parser plugin helper for that purpose (v0.14.1 or later):

# in class definition
helpers :parser

# in #start
@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)

Parser#parse gets input data as an argument, and call callback block to feed results of parser. The input text may contain 2 or more records, so the parser plugin might call block 2 or more times for an argument.

Parser plugins must implement this method.

Writing Tests

TODO: write

Last updated: 2016-06-13 06:11:23 UTC

Versions | v0.14 (td-agent3)

If this article is incorrect or outdated, or omits critical information, please let us know.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

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