Status
Current state: Draft
Discussion thread: TBD
JIRA: here
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
For stateful applications, one of the biggest performance bottleneck is the state shuffling. In Kafka consumer, there is concept called "rebalance" which means that for given M partitions and N consumers in one consumer group, Kafka will try to balance the load between consumers and ideally have each consumer dealing with M/N partitions. Broker will also adjust the workload dynamically by monitoring consumers' health and handling new consumer join request. The intuition is to avoid processing hot spot and maintain fairness plus liveness of the whole application. However, when the service state is heavy, a rebalance of one partition from instance A to B means huge amount of data transfer. If multiple rebalances are triggered, the whole service could take a very long time to recover.
It may hit multiple state rebalances when the client group is doing multiple rebalances.
Public Interfaces
Right now broker handles consumer state in a two-phase protocol. To be concise, we only list 3 states here: RUNNING, IN_REBALANCE and SYNC.
- When a new member joins the consumer group, if the
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.