fluent/fluentd · error · Fluent::ConfigError

<secondary> section cannot have <buffer> section

Error message

<secondary> section cannot have <buffer> section

What it means

A `<buffer>` block was nested inside the `<secondary>` block. The secondary plugin is driven by the primary's buffer (it receives the primary's failed chunks), so it cannot define its own buffer section; the configuration parser finds `@secondary_config.buffer` set and rejects it.

Source

Thrown at lib/fluent/plugin/output.rb:402

            end
          end

          if (@flush_mode != :interval) && buffer_conf.has_key?('flush_interval')
            if buffer_conf.has_key?('flush_mode')
              raise Fluent::ConfigError, "'flush_interval' can't be specified when 'flush_mode' is not 'interval' explicitly: '#{@flush_mode}'"
            else
              log.warn "'flush_interval' is ignored because default 'flush_mode' is not 'interval': '#{@flush_mode}'"
            end
          end

          if @buffer.queued_chunks_limit_size.nil?
            @buffer.queued_chunks_limit_size = @buffer_config.flush_thread_count
          end
        end

        if @secondary_config
          raise Fluent::ConfigError, "Invalid <secondary> section for non-buffered plugin" unless @buffering
          raise Fluent::ConfigError, "<secondary> section cannot have <buffer> section" if @secondary_config.buffer
          raise Fluent::ConfigError, "<secondary> section cannot have <secondary> section" if @secondary_config.secondary
          if @buffer_config.retry_forever
            log.warn "<secondary> with 'retry_forever', only unrecoverable errors are moved to secondary"
          end

          secondary_type = @secondary_config[:@type]
          unless secondary_type
            secondary_type = conf['@type'] # primary plugin type
          end
          secondary_conf = conf.elements(name: 'secondary').first
          @secondary = Plugin.new_output(secondary_type)
          unless @secondary.respond_to?(:acts_as_secondary)
            raise Fluent::ConfigError, "Failed to setup secondary plugin in '#{conf['@type']}'. '#{secondary_type}' plugin in not allowed due to non buffered output"
          end
          @secondary.acts_as_secondary(self)
          @secondary.configure(secondary_conf)
          if (@secondary.class.to_s != "Fluent::Plugin::SecondaryFileOutput") &&
             (self.class != @secondary.class) &&

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Delete the `<buffer>` section from inside `<secondary>`
2. Leave buffer tuning (flush_interval, retry_*, timekey) on the primary's `<buffer>` only
3. Keep only plugin-specific parameters (e.g. `directory` for secondary_file) inside `<secondary>`

Example fix

# before
<secondary>
  @type secondary_file
  directory /var/log/fluent/backup
  <buffer>
    flush_interval 1s
  </buffer>
</secondary>

# after
<secondary>
  @type secondary_file
  directory /var/log/fluent/backup
</secondary>

fluentd --dry-run -c /etc/fluent/fluent.conf

When it happens

Trigger: `<secondary> @type file <buffer> ... </buffer> </secondary>` — any buffer subsection inside `<secondary>` triggers the `@secondary_config.buffer` check at output.rb:402 during configure.

Common situations: Copy/pasting the primary plugin's full body (including its `<buffer>` tuning) into the `<secondary>` block; assuming the secondary needs matching buffer settings to work.

Understand the failure class

Background: Config validation failed: what "invalid value for {key}" and settings-rejection errors mean across 19 open-source libraries — this error's family across 19 libraries.

Related errors

• secondary plugin '#{self.class}' must support buffering, but
• secondary plugin '#{self.class}' must support buffering, but
• Invalid <secondary> section for non-buffered plugin
• buffer path is not configured. specify 'path' in <buffer>
• This plugin can only be used in the <secondary> section

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/249d936316f751a6. Report an issue: GitHub.