require 'fluent/plugin/filter'
​
module Fluent::Plugin
  class PassThruFilter < Filter
    # Register this filter as "passthru"
    Fluent::Plugin.register_filter('passthru', self)
​
    # config_param works like other plugins
​
    def configure(conf)
      super
      # do the usual configuration here
    end
​
    # def start
    #   super
    #   # Override this method if anything needed as startup.
    # end
​
    # def shutdown
    #   # Override this method to use it to free up resources, etc.
    #   super
    # end
​
    def filter(tag, time, record)
      # Since our example is a pass-thru filter, it does nothing and just
      # returns the record as-is.
      # If returns nil, that records are ignored.
      record
    end
  end
end

# test/plugin/test_filter_your_own.rb
​
require 'test/unit'
require 'fluent/plugin/test/driver/filter'
​
# your own plugin
require 'fluent/plugin/filter_your_own'
​
class YourOwnFilterTest < Test::Unit::TestCase
  def setup
    Fluent::Test.setup # this is required to setup router and others
  end
​
  # default configuration for tests
  CONFIG = %[
    param1 value1
    param2 value2
  ]
​
  def create_driver(conf = CONFIG)
    Fluent::Test::Driver::Filter.new(Fluent::Plugin::YourOwnFilter).configure(conf)
  end
​
  def filter(config, messages)
    d = create_driver(config)
    d.run(default_tag: "input.access") do
      messages.each do |message|
        d.feed(message)
      end
    end
    d.filtered_records
  end
​
  sub_test_case 'configured with invalid configuration' do
    test 'empty configuration' do
      assert_raise(Fluent::ConfigError) do
         create_driver('')
      end
    end
​
    test 'param1 should reject too short string' do
      conf = %[
        param1 a
      ]
      assert_raise(Fluent::ConfigError) do
         create_driver(conf)
      end
    end
    # ...
  end
​
  sub_test_case 'plugin will add some fields' do
    test 'add hostname to record' do
      conf = CONFIG
      messages = [
        { "message" => "This is test message" }
      ]
      expected = [
        { "message" => "This is test message", "hostname" => "example.com" }
      ]
      filtered_records = filter(conf, messages)
      assert_equal(expected, filtered_records)
    end
    # ...
  end
​
  # ...
end