Plugin Helper: Child Process

child_process helper manages child processes life cycle.

Here is the code example with child_process helper:

require 'fluent/plugin/output'
module Fluent::Plugin
class ExampleOutput < Output
Fluent::Plugin.register_output('example', self)
# 1. load child_process helper
helpers :child_process
# omit configure, shutdown and other plugin API
def start
# 2. execute child process with unique name
child_process_execute(:exec_external_command, "external_command", immediate: true, mode: [:read]) do
# ...

Launched child process is managed by the plugin. No need child process shutdown code in plugin's shutdown. The plugin shutdowns launched child process automatically.


child_process_execute(title, sub_process_name, arguments: nil, subprocess_name: nil, interval: nil, immediate: false, parallel: false, mode: [:read, :write], stderr: :discard, env: {}, unsetenv: false, chdir: nil, internal_encoding: 'utf-8', external_encoding: 'ascii-8bit', scrub: true, replace_string: nil, wait_timeout: nil, on_exit_callback: nil, &block)

This method executes child_process with given parameters and routine

  • title: unique symbol value

  • sub_process_name: sub process name value

  • interval: Second unit integer/float value.

  • immediate: true/false. Default is false.

  • parallel: true/false. Default is false.

  • mode: [:read, :write]. Default is [:read, :write].

  • stderr: Connect stderr or not. Default is :discard.

  • env: Environment valuables. Default is {}.

  • unsetenv: true/false. Default is false

  • chdir: Working directory. Default is nil.

  • internal_encoding: Internal character encoding. Default is


  • external_encoding: External character encoding. Default is


  • scrub: true/false. Default is true.

  • replace_string: Replace invalid code point with specified

    character. Default is nil.

  • wait_timeout: Set timeout seconds. Default is nil.

  • on_exit_callback: Set callback function. Default is nil

Code examples:

# Pass block directly. block is executed in 10 second interval.
child_process_execute(:exec_awesome_command, @command, interval: 10, mode: [:read]) {|io|
# ...
# Pass block with existing method. block is executed one-shot.
child_process_execute(:exec_awesome_command, @command, immediate: true, mode: [:read], &method(:run))
def run(io)
# ...

child_process used plugins

If this article is incorrect or outdated, or omits critical information, please let us know.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.