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Status

Current state:   [One of "Under Discussion", "Accepted", "Rejected"]

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

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

Motivation

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When the leader of a partition changes, the end-to-end latency of produce or fetch client request can increase significantly as a result of the work required to discover the new leader.

The client’s process to recover from NOT_LEADER_OR_FOLLOWER or FENCED_LEADER_EPOCH errors looks like this:

1. Asynchronously refresh the metadata for all topics using the Metadata RPC.

2. Retry the Produce or Fetch RPC after the RETRY_BACKOFF_MS_CONFIG.

3. If the RPC failed with NOT_LEADER_OR_FOLLOWER or FENCED_LEADER_EPOCH , retry again from step 1.

This approach has a few downsides:

• A single Metadata RPC can be slow (order of few 100 milliseconds in extreme cases).

• It might take multiple attempts to refresh the metadata if the latest metadata hasn’t yet propagated throughout the cluster.

• The produce and fetch requests have their own delayed retries on failed attempts (default is 100ms in Java client), to avoid overloading the Kafka brokers with failing RPCs.

Our goal is to minimize the time taken to discover a new leader, and use that information as soon as possible on the client. This can be beneficial in a number of situations such as cluster rolls, reassignments, or preferred leader election.

Note, performance tests are being done to show the improvements with the proposed changes. Results to followDescribe 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.

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• 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

If 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.