Deprecated method: this method is going to be deleted. Use F

Error message

Deprecated method: this method is going to be deleted. Use Fluent::Plugin.add_plugin_dir

What it means

Fluent::Engine#add_plugin_dir is a deprecated compatibility shim: it logs this warning and then forwards to Fluent::Plugin.add_plugin_dir. It exists only so older plugins/scripts registering plugin directories via the Engine singleton keep working. Unlike other deprecation shims in the codebase it warns on every call.

Source

Thrown at lib/fluent/engine.rb:121

      end
    end

    def configure(conf)
      @root_agent.configure(conf)

      @fluent_log_event_router = FluentLogEventRouter.build(@root_agent)

      if @fluent_log_event_router.emittable?
        $log.enable_event(true)
      end

      unless @suppress_config_dump
        $log.info :supervisor, "using configuration file: #{conf.to_s.rstrip}"
      end
    end

    def add_plugin_dir(dir)
      $log.warn('Deprecated method: this method is going to be deleted. Use Fluent::Plugin.add_plugin_dir')
      Plugin.add_plugin_dir(dir)
    end

    def emit(tag, time, record)
      raise "BUG: use router.emit instead of Engine.emit"
    end

    def emit_array(tag, array)
      raise "BUG: use router.emit_array instead of Engine.emit_array"
    end

    def emit_stream(tag, es)
      raise "BUG: use router.emit_stream instead of Engine.emit_stream"
    end

    def flush!
      @root_agent_mutex.synchronize do
        @root_agent.flush!

View on GitHub (pinned to dd45c6e18d)

Solutions

Replace the call with Fluent::Plugin.add_plugin_dir(dir).
Alternatively set the FLUENT_PLUGIN environment variable or pass -p/--plugin dir on the command line to register plugin directories without code.
Upgrade third-party gems that call the old API.

Example fix

# before
Fluent::Engine.instance.add_plugin_dir('/opt/fluent-plugins')

# after
Fluent::Plugin.add_plugin_dir('/opt/fluent-plugins')

Defensive patterns

Strategy: validation

Validate before calling

# Grep guard for CI: fail if the deprecated Engine API is used
# grep -rn 'Engine.*add_plugin_dir\|engine\.add_plugin_dir' lib/ && exit 1 || exit 0

Type guard

if defined?(Fluent::Plugin) && Fluent::Plugin.respond_to?(:add_plugin_dir)
  Fluent::Plugin.add_plugin_dir(dir)
end

Prevention

Use Fluent::Plugin.add_plugin_dir or the -p/--plugin CLI flag in all new code.
During upgrades, grep plugins for Fluent::Engine usage — most Engine instance methods are deprecated shims.

When it happens

Trigger: A plugin, initializer, or management script calls Fluent::Engine.instance.add_plugin_dir(dir) (the pre-1.x API) instead of the modern module-level API.

Common situations: Old in-house plugins written for Fluentd v0.x that add custom plugin paths at boot; wrapper tools that call Engine APIs directly during test setup.

Related errors

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/c080fea4f938c855. Report an issue: GitHub.