Status

Current state: Draft

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

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

Motivation

The Apache Kafka project has retained support for all protocol API versions since Apache Kafka 0.8.0 (released 9+ years ago on December 2013). As time goes on, the cost of maintaining support for all these versions goes up (both in code complexity and the testing matrix) and the value goes down (as users switch to clients that rely on newer protocol API versions). Apache Kafka 4.0 presents a good opportunity to re-evaluate this policy since it includes other modernization initiatives like dropping support for zk mode and dropping support for message formats v0 and v1 (KIP-724). No date has been set for Apache Kafka 4.0, but it's safe to say it won't happen before the end of 2023.

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We believe there is merit in having a general protocol API versioning policy that can be applied mechanically on each new major version - we intend to submit a separate KIP for that. It would only apply from Apache Kafka 5.0 to give the ecosystem plenty of time to adapt.

Public Interfaces

Remove support for the following protocol API versions (the range is inclusive):

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The Java client is part of the Apache Kafka repository, it is released alongside the broker and it generally supports the latest protocol versions exposed by the broker. Given that, we know that the Java client has supported the protocol API versions baseline set by Apache Kafka 2.1 as part of that release in November 2018 and no further analysis is required.

Proposed Changes

TBD

Compatibility, Deprecation, and Migration Plan

TBD (talk about broker metrics users can rely on to know if they're impacted and what happens if an unsupported protocol version is used)

Test Plan

• Client compatibility system tests will updated so that 2.0.x clients fail with an UNSUPPORTED_VERSION error and older versions are removed.
• Protocol API integration tests will be updated so that the highest unsupported version and lowest supported version are tested and older versions are not.
• Protocol API unit tests will continue to cover all versions, but the expectations will be updated for unsupported versions.

Rejected Alternatives

1. Continue with the "support forever" compatibility policy: the benefit cost ratio is low given low usage of the older protocol API versions.