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Status

Current state: Under Discussion

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

JIRA: here [Change the link from KAFKA-1 to your own ticket]

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

Motivation

While using "acks=-1", we found the P999 latency is very spiky. Since the produce request can only be acknowledged after all of the "insync.replicas" committed the message, the produce request latency is determined by the slowest replica inside "insync.replicas". In the production environment, we usually set "replica.lag.time.max.ms" to the order of 10 seconds (to protect frequent ISR shrink and expand), and producer P999 can jump to the value of "replica.lag.time.max.ms" even without any failures.

We already have pretty stable and low P99 latency, it will definitely make Kafka more suitable for more use cases if we can have the similar guarantees for P999.

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

I totally understand in the early days, we have the support to specify the "request.required.acks" or "acks=2" on the producer side. We got rid of these supports since it was misleading and can't guarantee there is no data loss in all of the scenarios. The proposed changes here contain two separate parts. 1: introduce a similar config on the producer side as the "request.required.acks"; however, with more strict requirements. 2: improve the new leader election process to achieve the same level of data durability.

1. introduce a new config on the producer side called "quorum.required.acks". This config specifies the number of replicas a message should be committed to before the message got acknowledged. This config should be <= the number of replicas, 
2. select the live replica with the largest LEO as the new leader during leader election. For the most popular configs in today's production environment, we use replicas=3, ack=-1, and min.insync.replicas=2. In this case, we guarantee a committed message will NOT be lost if only 1 broker failed/died. If we use replicas=3 and quorum.required.acks=2, for any message got acknowledged, it has been committed to at least 2 brokers, if the leader died, when we chose the replica with the larger LEO from the two live replicas, it will ensure this replica contains all of the messages from the previous leader.

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?

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.