...

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

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

Motivation

Describe 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.

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

...

Motivation

KafkaFuture.Function and KafkaFuture.BiCosumer are currently empty public abstract classes with one abstract method.

This means you cannot implement them with a java 8 lambda, as that only works for functional interfaces.

Public Interfaces

From the public interfcate, only org.apache.kafka.common.KafkaFuture has such helper classes.

Proposed Changes

The current proposal implemented in PR#4033 makes from those abstract classes interfaces.

This is a binary incompatible change.  Which might be acceptable as KafkaFuture is annotated as InterfaceStability.Evolving, where compatibility may be broken at minor release.

This change also helps to move KafkaFuture closer to the interface of a java 8 CompletableFuture.  As KafkaFuture's javadoc states:

This will eventually become a thin shim on top of Java 8's CompletableFuture.

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

...

When the change is implemented like proposed, everybody using this api will need to implement instead of extend their implementations of KafkaFuture.Function and KafkaFuture.Biconsumer.

Other Alternatives

• Keep the current interface as is, as a nicer syntax to address this api might not be worth breaking backwards compatibility
• Wait for a kafka release which will not support java 7 anymore.  A backwards incompatible change can then be done to replace KafkaFuture with CompletableFuture
• Spend more effort and update the definition of KafkaFuture in such a way that it gradually switches to CompletableFuture.  By following an approach which looks like this:
  • Create new interface versions of KafkaFuture.Function and KafkaFuture.BiConsumer, create new methods which handle those functional interface and preferably line their names up with those of CompletableFuture.  So for example KafkaFuture.addWaiter(BiConsumer) becomes KafkaFuture.whenComplete(BiConsumerInterface).  Mark the old methods as @Deprecated.
  • When kafka does not support java 7 anymore, let KafkaFuture extend CompletableFuture (or even CompletionStage)
  • Later on the deprecated methods can be removed, and KafkaFuture becomes a shim on Completablefuture or CompletionStage.