fluent/fluentd · error · NotImplementedError

Implement this method in child class

Error message

Implement this method in child class

What it means

Fluent::Plugin::Storage is the abstract base class for storage plugins; #get is declared but only raises NotImplementedError to force subclasses to override it. Calling storage.get(key) means the storage object's class never provided a concrete getter. Fluentd's built-in LocalStorage and other shipped storages override it, so this error almost always means a custom storage plugin is incomplete or the base class was instantiated directly.

Source

Thrown at lib/fluent/plugin/storage.rb:62

      def synchronized?
        false
      end

      def implementation
        self
      end

      def load
        # load storage data from any data source, or initialize storage internally
      end

      def save
        # save internal data store into data source (to be loaded)
      end

      def get(key)
        raise NotImplementedError, "Implement this method in child class"
      end

      def fetch(key, defval)
        raise NotImplementedError, "Implement this method in child class"
      end

      def put(key, value)
        # return value
        raise NotImplementedError, "Implement this method in child class"
      end

      def delete(key)
        # return deleted value
        raise NotImplementedError, "Implement this method in child class"
      end

      def update(key, &block) # transactional get-and-update
        raise NotImplementedError, "Implement this method in child class"

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Override #get(key) in your Fluent::Plugin::Storage subclass to return the value for the key from your backing store
2. If you did not intend to write a storage plugin, use a built-in implementation such as the default 'local' type (Fluent::Plugin::LocalStorage) instead of the base class
3. Cover the whole abstract API in your subclass: get, fetch, put, delete, and update (lines 61-81), plus load/save when persistence matters
4. In tests, stub the abstract methods or use a test double derived from LocalStorage instead of the raw base class

Example fix

// before (custom storage plugin, incomplete)
class MyStorage < Fluent::Plugin::Storage
  Fluent::Plugin.register_storage('my', self)
  def put(key, value)
    @data[key.to_s] = value
  end
end
storage.get('counter')  # => NotImplementedError

// after
class MyStorage < Fluent::Plugin::Storage
  Fluent::Plugin.register_storage('my', self)
  def put(key, value)
    @data[key.to_s] = value
  end

  def get(key)
    @data[key.to_s]
  end
end

storage = Fluent::Plugin.new_storage('my')
raise 'storage does not implement #get' if storage.method(:get).owner == Fluent::Plugin::Storage

def concrete_storage?(storage)
  %i[get fetch put delete update].all? do |m|
    storage.respond_to?(m) && storage.method(m).owner != Fluent::Plugin::Storage
  end
end

begin
  storage.get('key')
rescue NotImplementedError => e
  logger.error("storage plugin is incomplete: #{e.message} (#{storage.class})")
  raise
end

When it happens

Trigger: Calling storage.get('some_key') on an instance of a class that inherits Fluent::Plugin::Storage without overriding #get (lib/fluent/plugin/storage.rb:61-63). Instantiating Fluent::Plugin::Storage itself via Fluent::Plugin.new_storage('...') with a type that maps to the base class, or writing a custom storage plugin (registered with Fluent::Plugin.register_storage) that omits #get.

Common situations: Writing a custom storage plugin (e.g. Redis- or memcached-backed) and forgetting one of the CRUD methods; test code that exercises a half-finished storage subclass; a plugin author who copied the skeleton but left #get as a stub before shipping.

Related errors

• Implement this method in child class
• key must be a string (or symbol for to_s)
• Optional API #parse_io is not implemented
• Optional API #parse_partial_data is not implemented
• Plugin @id or path for <storage> required when 'persistent'

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/bfad1a1aad0704e2. Report an issue: GitHub.