Selecting a particular field for Fluentd to process further

example.conf

<source>
  type forward
</source>

<match test.**>
  type select_fields
  add_tag_prefix fields_selected
  selected_field foo
</match>

<match fields_selected.**>
  type stdout
</match>

out_select_fields.rb

module Fluent
  class SelectFieldsOutput < BufferedOutput
    Plugin.register_output('select_fields', self)

    def initialize
      super
    end

    config_param :selected_field, :string
    config_param :add_tag_prefix, :string

    def configure(conf)
      super
    end

    def emit(tag, es, chain)
      new_tag = "#{@add_tag_prefix}.#{tag}"
      es.each do |time, record|
        if record[@selected_field].is_a?(Hash)
          Engine.emit(new_tag, time, record[@selected_field])
        end
      end
    end
  end
end