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Status

Current state: Draft

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

Kafka Streams have a very tight coupling with the input partition counts. There are dimensions that could be scaled:

• Number of consumers
• Number of tasks (thread)
• Number of state stores

Out of all the scaling needs, as we discussed in KIP-X, the scaling out consumer is indeed hard as we have to struggle with server side filtering and a not well scaled offset commit mode. Furthermore, most cases the number of consumers is not the bottleneck - the total parallelism is. The biggest win could in fact come from scaling the 

We don't want constantly synchronizing the states between threads either. So the individual commit feature is still a favored approach to solve the problem. What we need is just a different way of viewing the offset data format. Instead of mapping from offset → committed boolean, we actually need to map from worker-id → committed offsets.

High Level Design

In offset commit protocol, we always have a topic partition → offset mapping to remember our progress. In fact if suppose we build a consumer with multi-threading access, we could actually do the rebalance assignment of key ranges to workers and let those mappings returned and stored on broker side. In this way, say if we have two workers A and B sharing the same consumer, they should be able to commit their progress individually by (worker-id, offset) pairs. Adding the group assignment message which has key range mapping, we could easily do the client side filtering for the first generation if possible. This work also unblocks the potential later if we want consumer level scaling by defining their individual key ranges, so that we could allow concurrent commit.

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?

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.