fluent/fluentd · warning

Ruby DSL configuration format is deprecated. Please use orig

Error message

Ruby DSL configuration format is deprecated. Please use original configuration format. https://docs.fluentd.org/configuration/config-file

What it means

Fluentd supports parsing configuration in a Ruby DSL (files interpreted with eval via Config::DSL::Parser), but that format is deprecated. This warning is printed once by Fluent::Config.parse whenever it is invoked with parser: :ruby (e.g. `fluentd -c fluent.conf.rb`) before the DSL parser runs. The config still loads; only a migration to the standard directive-based format is requested.

Source

Thrown at lib/fluent/config.rb:78

                 when :v1 then :v1
                 when :v0 then :v0
                 else
                   raise ArgumentError, "Unknown Fluentd configuration syntax: '#{syntax}'"
                 end
               elsif v1_config then :v1
               else :v0
               end
      case parser
      when :v1
        require 'fluent/config/v1_parser'
        V1Parser.parse(str, fname, basepath, Kernel.binding, on_file_parsed: on_file_parsed)
      when :v0
        # TODO: show deprecated message in v1
        require 'fluent/config/parser'
        Parser.parse(str, fname, basepath)
      when :ruby
        require 'fluent/config/dsl'
        $log.warn("Ruby DSL configuration format is deprecated. Please use original configuration format. https://docs.fluentd.org/configuration/config-file") if $log
        Config::DSL::Parser.parse(str, File.join(basepath, fname))
      else
        raise "[BUG] unknown configuration parser specification:'#{parser}'"
      end
    end

    def self.new(name = '')
      Element.new(name, '', {}, [])
    end
  end
end

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Rewrite the configuration in the standard @type/directive format documented at https://docs.fluentd.org/configuration/config-file.
2. Rename the file so it is not picked up as Ruby DSL (drop the .rb convention and the ruby parser selection).
3. If you call Config.parse programmatically, stop passing parser: :ruby (use :v1, the default).
4. Audit deployment tooling for hardcoded `-c fluentd.conf.rb` invocations.

Example fix

# before (fluent.conf.rb, Ruby DSL)
source do
  type :tail
  path '/var/log/app.log'
  tag app.log
end

# after (fluent.conf, standard format)
<source>
  @type tail
  path /var/log/app.log
  tag app.log
</source>

# Fail CI if any Fluentd config still uses the Ruby DSL
require 'rbconfig'
configs = Dir.glob(['conf/*.rb', 'etc/*.rb'])
abort 'Ruby DSL configs are deprecated: ' + configs.join(', ') unless configs.empty?

When it happens

Trigger: Running `fluentd -c somefile.rb`, passing a Ruby DSL config to Fluent::Config.parse(str, fname, basepath, parser: :ruby), or using tools (fluentd-ui, supervision wrappers) that select the :ruby parser.

Common situations: Legacy deployments from early Fluentd v0.x era still shipping .rb configs; copied examples from old blog posts; CI pipelines validating configs via the ruby parser.

Related errors

• unknown_key
• buffer path is not configured. specify 'path' in <buffer>
• regexp#{i} does not contain 2 parameters
• regexp#{i} contains a duplicated key, #{key}
• exclude#{i} does not contain 2 parameters

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/f096ccd7a753af7d. Report an issue: GitHub.