Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

...

Describe the problems you are trying to solve.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

KIP-500: Replace ZooKeeper with a Self-Managed Metadata Quorum changes the way cluster metadata is stored and managed in a Kafka cluster. It introduces the concept of a replicated log that is maintained using a custom version of the Raft consensus protocol described in KIP-595: A Raft Protocol for the Metadata Quorum. The controller now utilizes this log to persist and broadcast all metadata related actions in the cluster as described in KIP-631: The Quorum-based Kafka Controller.

With these changes in place, the replicated log containing all metadata changes (henceforth called metadata log) is the source of metadata related information for all nodes in the cluster. Any errors that occur while processing the log could lead to the in-memory state for the node becoming inconsistent. It is important that such errors are made visible. The metrics proposed in the following section aim at doing so. These metrics can be used to set up alerts so that affected nodes can be discovered and needed remedial actions can be performed on them.

Public Interfaces

We propose adding the following new metrics:

Proposed Changes

Controllers

The MetadataErrorCount metric is update for both active and standby controllers. For Active Controllers it is incremented anytime they hit an error in either generating a Metadata log or while applying it to memory. For standby controllers, this metric is incremented when they hit an error in applying the metadata log to memory. This metric will reflect the total count of errors that a controller encountered in metadata log processing since the last restart.

https://github.com/apache/kafka/blob/14d2269471141067dc3c45300187f20a0a051777/metadata/src/main/java/org/apache/kafka/controller/QuorumController.java#L409 

 private Throwable handleEventException(String name,
                                           OptionalLong startProcessingTimeNs,
                                           Throwable exception) {
        if (!startProcessingTimeNs.isPresent()) {
        ...
        ...
        renounce();
		//**** Increment MetadataErrorCount
        return new UnknownServerException(exception);
    }

Brokers

The metadata-apply-error-count metric will be incremented by one every time there is an error in publishing a new MetadataImage. This metric will reflect the count of cumulative errors since the broker started up.

https://github.com/apache/kafka/blob/14d2269471141067dc3c45300187f20a0a051777/core/src/main/scala/kafka/server/metadata/BrokerMetadataPublisher.scala#L125

override def publish(delta: MetadataDelta, newImage: MetadataImage): Unit = {
    val highestOffsetAndEpoch = newImage.highestOffsetAndEpoch()

    try {
      trace(s"Publishing delta $delta with highest offset $highestOffsetAndEpoch")

      // Publish the new metadata image to the metadata cache.
      metadataCache.setImage(newImage)
   	  ...
	  ...
      publishedOffsetAtomic.set(newImage.highestOffsetAndEpoch().offset)
    } catch {
	  //**** Increment metadata-apply-error-count
      case t: Throwable => error(s"Error publishing broker metadata at $highestOffsetAndEpoch", t)	
        throw t
    } finally {
      _firstPublish = false
    }
  }

The metadata-load-error-count metric will be incremented every time there is an error in loading batches and generating MetadataDelta from them. This metric will reflect the count of cumulative errors since the broker started up.

https://github.com/apache/kafka/blob/14d2269471141067dc3c45300187f20a0a051777/core/src/main/scala/kafka/server/metadata/BrokerMetadataListener.scala#L112

class HandleCommitsEvent(reader: BatchReader[ApiMessageAndVersion])
      extends EventQueue.FailureLoggingEvent(log) {
    override def run(): Unit = {
      val results = try {
        val loadResults = loadBatches(_delta, reader, None, None, None)
        ...
        loadResults
      } catch {
		//**** Increment metadata-load-error-count
	  } finally {
        reader.close()
      }

      ...
      _publisher.foreach(publish)
    }
  }

https://github.com/apache/kafka/blob/14d2269471141067dc3c45300187f20a0a051777/core/src/main/scala/kafka/server/metadata/BrokerMetadataListener.scala#L162

  class HandleSnapshotEvent(reader: SnapshotReader[ApiMessageAndVersion])
    extends EventQueue.FailureLoggingEvent(log) {
    override def run(): Unit = {
      try {
        info(s"Loading snapshot ${reader.snapshotId().offset}-${reader.snapshotId().epoch}.")
        _delta = new MetadataDelta(_image) // Discard any previous deltas.
        val loadResults = loadBatches(
       ...
      } catch {
		//**** Increment metadata-load-error-count
	  } finally {
        reader.close()
      }
      _publisher.foreach(publish)
    }
  }

Compatibility, Deprecation, and Migration Plan

These will be newly exposed metrics and there will be no impact on existing kafka versions.

Rejected Alternatives

Instead of adding the specific metrics, we could have added a more generic MetadataProcessingErrorCount Metric which would be incremented when either of these (and any other similar) or any other similar errors are hit on both Brokers and Controllers. The downside to this approach would be the loss in granularity on what exactly failed on a given node. The specific metrics are more meaningful and give better control over any alarming that might be setup on these metricsIf there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.