fluent/fluentd · error · Fluent::ConfigError

time_type is :mixed but time_format and time_format_fallback

Error message

time_type is :mixed but time_format and time_format_fallbacks is empty.

What it means

Fluent::ConfigError raised in TimeParameters#configure (lib/fluent/time.rb:177) when time_type is set to :mixed but neither time_format nor time_format_fallbacks provides any format. The :mixed mode works by trying a primary format and then fallback parsers (e.g. unixtime, %iso8601), so with no formats configured there is nothing to parse with and startup fails.

Source

Thrown at lib/fluent/time.rb:177

            conf['localtime'] = Fluent::Config.bool_value(conf['localtime'])
          elsif conf.has_key?('utc')
            conf['localtime'] = !(Fluent::Config.bool_value(conf['utc']))
            # Specifying "localtime false" means using UTC in TimeFormatter
            # And specifying "utc" is different from specifying "timezone +0000"(it's not always UTC).
            # There are difference between "Z" and "+0000" in timezone formatting.
            # TODO: add kwargs to TimeFormatter to specify "using localtime", "using UTC" or "using specified timezone" in more explicit way
          end
        end

        super

        if conf.has_key?('localtime') && conf.has_key?('utc') && !(@localtime ^ @utc)
          raise Fluent::ConfigError, "both of utc and localtime are specified, use only one of them"
        end

        if conf.has_key?('time_type') and @time_type == :mixed
          if @time_format.nil? and @time_format_fallbacks.empty?
            raise Fluent::ConfigError, "time_type is :mixed but time_format and time_format_fallbacks is empty."
          end
        end

        Fluent::Timezone.validate!(@timezone) if @timezone
      end
    end

    module Parser
      def self.included(mod)
        mod.include TimeParameters
      end

      def time_parser_create(type: @time_type, format: @time_format, timezone: @timezone, force_localtime: false)
        return MixedTimeParser.new(type, format, @localtime, timezone, @utc, force_localtime, @time_format_fallbacks) if type == :mixed
        return NumericTimeParser.new(type) if type != :string
        return TimeParser.new(format, true, nil) if force_localtime

        localtime = @localtime && (timezone.nil? && !@utc)

View on GitHub (pinned to dd45c6e18d)

Solutions

1. Add time_format_fallbacks listing the formats to try, e.g. time_type mixed + time_format_fallbacks %iso8601, unixtime
2. Or specify a primary time_format plus fallbacks: time_format %iso8601 and time_type mixed with time_format_fallbacks unixtime
3. If you only ever have one timestamp shape, drop time_type mixed entirely — the default parser with a single time_format is simpler

Example fix

# before
<parse>
  @type regexp
  time_key time
  time_type mixed
</parse>

# after
<parse>
  @type regexp
  time_key time
  time_type mixed
  time_format %iso8601
  time_format_fallbacks unixtime
</parse>

time_type = 'mixed'
time_format = nil
time_format_fallbacks = []
if time_type == :mixed && time_format.nil? && time_format_fallbacks.empty?
  raise 'time_type mixed requires time_format and/or time_format_fallbacks'
end

When it happens

Trigger: A parser/formatter section with time_type mixed and no time_format / time_format_fallbacks lines; also when time_format is present but empty. Valid usage is either 'time_format + time_type mixed (+ optional fallbacks)' or 'time_type mixed + time_format_fallbacks' listing at least one format.

Common situations: Enabling time_type mixed to accept both epoch seconds and ISO8601 but forgetting the fallbacks line; renaming time_format_fallbacks to a wrong key so it is silently ignored and only time_type remains; upgrading a config where time_type mixed was added experimentally.

Related errors

• specifying timezone requires time format
• invalid time format: value = #{value}, even though fallbacks
• out_secondary_file: basename or directory has an incompatibl
• invalid number of workers (must be 1 or unspecified) with --
• <system> is duplicated. <system> should be only one

AI-assisted analysis of fluent/fluentd@dd45c6e18d (2026-08-21). Data as JSON: /api/errors/327b08044762066b. Report an issue: GitHub.