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Status

Current state: [Under Discussion]

Discussion thread: TBD

JIRA:  TBD

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

Motivation

Currently Kafka Streams uses consumer membership protocol to coordinate the stream task assignment. When we scale up the stream application, KStream group will attempt to revoke active tasks and let the newly spinned up hosts take over them. It takes time for the new host to restore the tasks if assigned ones are stateful, but current strategy is to reassign the tasks upon receiving new member join group requests to achieve application consumption balance. For state heavy application, it is not ideal to give up the tasks immediately once the new player joins the party, instead we should buffer some time to let the new player accept some restoring tasks, and wait until it is “ready” to take over the active tasks. Ideally, we could realize no downtime transition during cluster scaling up if we take this approach. Same situation applies to scale down, when we need to buffer time for migrating the tasks from ready-to-shut-down hosts to retained ones.

Recently the community is promoting cooperative rebalancing to mitigate the pain points in the stop-the-world rebalancing protocol and an initiation for Kafka Connect already started in KIP-415. There is already great discussion around it, but the hard part for KStream is that delayed rebalance is not the most ideal solution. The better approach is to adopt some great design fo KConnect in KIP-415, while let KStream members explicitly announce the state changes and trigger necessary rebalance to migrate the resource ownership, once they are fully ready after task restoring.

Thus we are proposing a dedicated design specifically for KStream rebalancing in order to holistically smooth the scale up/down experience.

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

We will introduce a new type of stream task called `learner task`, which is a task status that one stream instance is learning. 

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.

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