Versions | v0.14 (td-agent3)

This page is for v0.14, not the latest stable version which is v0.12. For the latest stable version of this article, click here.

Writing Formatter Plugins

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

Here is an example of a custom formatter that outputs events as CSVs. It takes a required parameter called “csv_fields” and outputs the fields. It assumes that the values of the fields are already valid CSV fields.

require 'fluent/plugin/formatter'

module Fluent::Plugin
  class MyCSVFormatter < Formatter
    # Register MyCSVFormatter as "my_csv".
    Plugin.register_formatter("my_csv", self)

    include HandleTagAndTimeMixin # If you wish to use tag_key, time_key, etc.
    config_param :csv_fields, :string

    # This method does further processing. Configuration parameters can be
    # accessed either via "conf" hash or member variables.
    def configure(conf)
      @csv_fields = conf['csv_fields'].split(",")

    # This is the method that formats the data output.
    def format(tag, time, record)
      values = []

      # Look up each required field and collect them from the record
      @csv_fields.each do |field|
        v = record[field]
        unless v
          log.error "#{field} is missing."
        values << v.to_s

      # Output by joining the fields with a comma

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

# Other lines...
<match test>
  @type file
  path /path/to/output/file
  format my_csv
  csv_fields k1,k2

and if the record {"k1": 100, "k2": 200} is matched, the output file should look like 100,200

TODO: add description about <format> section.

Table of Contents

How To Use Formatters From Plugins

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

# in class definition
helpers :formatter

# in #start
@formatter = formatter_create(type: 'json')

# in #filter, #format or ...
es.each do |time, record|
  row = @formatter.format(@tag, time, record)
  # ...

See Formatter Plugin Helper API for details.


Formatter plugins have a method to format input record (Hash) into a String object.

#format(tag, time, record)

Formatter#format gets an event represented with tag (String), time (Fluent::EventTime or Integer) and record (Hash with String keys), and should return a String object, the result of formatting.

Formatter 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). All components are available under the Apache 2 License.

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