How to Write Input Plugin

Extend Fluent::Plugin::Input class and implement its methods.

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

In most cases, input plugins start timers, threads, or network servers to listen on ports in #start method and then call router.emit in the callbacks of timers, threads or network servers to emit events.


require 'fluent/plugin/input'

module Fluent::Plugin
  class SomeInput < Input
    # First, register the plugin. 'NAME' is the name of this plugin
    # and identifies the plugin in the configuration file.
    Fluent::Plugin.register_input('NAME', self)

    # `config_param` defines a parameter.
    # You can refer to a parameter like an instance variable e.g. @port.
    # `:default` means that the parameter is optional.
    config_param :port, :integer, default: 8888

    # `configure` is called before `start`.
    # 'conf' is a `Hash` that includes the configuration parameters.
    # If the configuration is invalid, raise `Fluent::ConfigError`.
    def configure(conf)

      # The configured 'port' is referred by `@port` or instance method `#port`.
      if @port < 1024
        raise Fluent::ConfigError, "well-known ports cannot be used."

      # You can also refer to raw parameter via `conf[name]`.
      @port = conf['port']
      # ...

    # `start` is called when starting and after `configure` is successfully completed.
    # Open sockets or files and create threads here.
    def start

      # Startup code goes here!

    # `shutdown` is called while closing down.
    def shutdown
      # Shutdown code goes here!


To submit events, use router.emit(tag, time, record) method, where:

  • tag is a String,

  • time is the Fluent::EventTime or Integer as Unix time; and,

  • record is a Hash object.


tag = "myapp.access"
time =
record = {"message"=>"body"}
router.emit(tag, time, record)

To submit multiple events, use router.emit_stream(tag, es) method, where:

  • tag is a String; and,

  • es is a MultiEventStream object.


es =
records.each do |record|
  es.add(time, record)
router.emit_stream(tag, es)

Record Format

Fluentd plugins assume the record is in JSON format so the key should be the String, not Symbol. If you emit a record with a key as Symbol, it may cause a problem.


# Good
router.emit(tag, time, {'foo' => 'bar'})

# Bad
router.emit(tag, time, {:foo => 'bar'})


There are no specific methods for the Input plugins.

Writing Tests

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

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

# test/plugin/test_in_your_own.rb

require 'test/unit'
require 'fluent/test'
require 'fluent/test/driver/input'

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

class YourOwnInputTest < Test::Unit::TestCase
  def setup
    # This line is required to set up router, and other required components.

  # Default configuration for tests
  CONFIG = %[
    param1 value1
    param2 value2

  def create_driver(conf = CONFIG)

  sub_test_case 'configured with invalid configurations' do
    test 'param1 should reject too short string' do
      assert_raise Fluent::ConfigError do
          param1 a

    test 'param2 is set correctly' do
      d = create_driver
      assert_equal 'value2', d.instance.param2
    # ...

  sub_test_case 'plugin will emit some events' do
    test 'test expects plugin emits events 4 times' do
      d = create_driver

      # This method blocks until the input plugin emits events 4 times
      # or 10 seconds lapse. 4, timeout: 10)

      # An array of `[tag, time, record]`
      events =

      assert_equal 'expected_tag', events[0][0]
      # ...
  # ...

Overview of Tests

Testing for input plugins is mainly for:

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

  • Validation of the emitted events

To make testing easy, the test driver provides a dummy router, a logger and the functionality to override 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. Register conditions to stop/break running tests

  4. Run test code (provided as a block for

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

Test driver calls methods for plugin lifecycle at the beginning of Step # 4 (i.e. #start) and at the end (i.e. #stop, #shutdown, etc.). It can be skipped by optional arguments of #run.

See Testing API for Plugins for details.


  • configuration tests, repeat steps # 1-2

  • full feature tests, repeat steps # 1-5

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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