Enhanced Dead Letter Queue

Enhanced Dead Letter Queue feature provides additional functionalities and warranties to the regular Dead Letter Queue feature. It aims to complement it with additional dispatch warranties and additional messages metadata information.

This documentation only covers extra functionalities enhancing the Dead Letter Queue feature.

Please refer to the Dead Letter Queue documentation for more details on its core principles.

Using Enhanced Dead Letter Queue

There are no extra steps needed. If you are using Karafka Pro, Enhanced Dead Letter Queue is configured the same way as the regular one:

class KarafkaApp < Karafka::App
  routes.draw do
    topic :orders_states do
      consumer OrdersStatesConsumer

      dead_letter_queue(
        topic: 'dead_messages',
        max_retries: 2
      )
    end
  end
end

Delaying the DLQ data processing

In some cases, it can be beneficial to delay the processing of messages dispatched to a Dead Letter Queue (DLQ) topic. This can be useful when a message has failed to be processed multiple times, and you want to avoid overwhelming the system with repeated processing attempts. By delaying the processing of these messages, you can avoid consuming valuable resources and prevent potential system failures or downtime.

If you are processing data dispatched to the DLQ topic, all you need to do to make it delayed is to add delay_by to your DLQ topic routing definition as follows:

class KarafkaApp < Karafka::App
  routes.draw do
    topic :orders_states do
      consumer OrdersStatesConsumer

      dead_letter_queue(
        topic: :failed_orders_dlq,
        max_retries: 2
      )
    end

    topic :failed_orders_dlq do
      consumer FailedOrdersRecoveryConsumer
      # Try to process failed orders messages with 5 minutes of a delay
      delay_by(5 * 60_000)
    end
  end
end

Disabling dispatch

For some use cases, you may want to skip messages after retries without dispatching them to an alternative topic.

To do this, you need to set the DLQ topic attribute value to false:

class KarafkaApp < Karafka::App
  routes.draw do
    topic :orders_states do
      consumer OrdersStatesConsumer

      dead_letter_queue(
        topic: false,
        max_retries: 2
      )
    end
  end
end

When that happens, Karafka will retry two times and continue processing despite errors.

Dispatch warranties

Enhanced Dead Letter Queue ensures that messages moved to the DLQ topic will always reach the same partition and in order, even when the DLQ topic has a different number of partitions. This means that you can implement pipelines for processing broken messages and rely on the ordering warranties from the original topic.

*This example illustrates how Enhanced DLQ preserves order of messages from different partitions.

Note: The DLQ topic does not have to have the same number of partitions as the topics from which the broken messages come. Karafka will ensure that all the messages from the same origin partition will end up in the same DLQ topic partition.

Additional headers for increased traceability

Karafka Pro, upon transferring the message to the DLQ topic, aside from preserving the payload, and the headers will add a few additional headers that allow for increased traceability of broken messages:

• original_topic - topic from which the message came
• original_partition - partition from which the message came
• original_offset - offset of the transferred message
• original_consumer_group - id of the consumer group that was consuming this message

Note: Karafka headers values are always strings.

This can be used for debugging or for example when you want to have a single DLQ topic with per topic strategies:

class DlqConsumer
  def consume
    messages.each do |broken_message|
      original_topic = broken_message.headers['original_topic']
      original_partition = broken_message.headers['original_partition'].to_i
      original_offset = broken_message.headers['original_offset'].to_i
      payload = broken_message.raw_payload

      case original_topic
      when 'orders_events'
        BrokenOrders.create!(
          payload: payload,
          source_partition: original_partition,
          source_offset: original_offset
        )
      when 'users_events'
        NotifyDevTeam.call(
          payload: payload,
          source_partition: original_partition,
          source_offset: original_offset
        )
      else
        raise StandardError, "Unsupported original topic: #{original_topic}"
      end

      mark_as_consumed(broken_message)
    end
  end
end

Adding custom details to the DLQ message

If you want to add some extra information or change anything in the message that will be dispatched to the DLQ topic, you can do it by defining a custom method called #enhance_dlq_message in your consumer class.

It accepts two arguments:

• dql_message - a hash with all the details of the DLQ message that will be dispatched
• skippable_message - Karafka message that we skip via the DLQ feature

Let's say you want to add some headers and alter the payload. You can do it in the following way:

class MyConsumer
  def consume
    # some code that can raise an error...
  end

  private

  def enhance_dlq_message(dlq_message, skippable_message)
    # Replace the DLQ message payload with a hash containing the original raw payload as well as
    # process pid
    #
    # Note that payload here needs to be a string
    dlq_message[:payload] = {
      original_raw_payload: skippable_message.raw_payload,
      process_pid: Process.pid
    }.to_json

    # Add one extra header to the message headers
    dlq_message[:headers]['extra-header'] = 'yes'
  end
end

Note: No routing changes are needed to make it work.

DLQ message key enhancements for a compacted DLQ topic

If you use a compact value for Kafka log.cleanup.policy, you may lose messages dispatched to the DLQ topic due to the DLQ compacting limitations.

You can mitigate this by enhancing the DLQ message with a unique key using the #enhance_dlq_message consumer method:

class MyConsumer
  def consume
    # some code that can raise an error...
  end

  private

  def enhance_dlq_message(dlq_message, skippable_message)
    dlq_message[:key] = [
      topic.name,
      skippable_message.partition,
      skippable_message.offset
    ].join('-')
  end
end