How to Write Formatter Plugin

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".
Fluent::Plugin.register_formatter("my_csv", self)
config_param :csv_fields, :array, value_type: :string
# This method does further processing. Configuration parameters can be
# accessed either via "conf" hash or member variables.
def configure(conf)
# 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
@type my_csv
csv_fields k1,k2

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

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:

# in class definition
helpers :formatter
# in #configure
@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

Fluentd formatter plugin has just one or some points to be tested. Others (parsing configurations, controlling buffers, retries, flushes and many others) are controlled by Fluentd core.

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

# test/plugin/test_formatter_your_own.rb
require 'test/unit'
require 'fluent/test/driver/formatter'
# your own plugin
require 'fluent/plugin/formatter_your_own'
class FormatterYourOwnTest < Test::Unit::TestCase
def setup
# common setup
fields a,b,c
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 format record' do
test 'record has a field' do
d = create_driver(CONFIG)
tag = "test"
time = event_time
record = { "message" => "This is message" }
formatted = d.instance.format(tag, time, record)
expected = "..."
assert_equal(expected, formatted)

Overview of Tests

Testing for formatter plugins are mainly for:

  • Configuration/Validation checks for invalid configurations (about


  • Checks for formatted by formatter plugins

Plugin test driver provides logger and feature to override system configurations, and configuration parser and others to make it easy to test configuration errors or others.

Lifecycle of plugins and test drivers is:

  1. Instantiate plugin driver (and it instantiates plugin)

  2. Configure plugin

  3. Run test code

  4. Assert results of tests by data provided from driver

Test drivers instantiate the plugin. See Testing API for plugins for details.

For configuration tests, repeat 1-2. For full feature tests, repeat 1-4. Test drivers and helper methods will support it.

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.