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As stated in the Public Interfaces  sections, a number of protocol API versions would no longer be supported. Kafka brokers would return the UNSUPPORTED_VERSION error when they receive a request with any of the removed API versions. Similarly, the Java clients (producer, consumer and admin) would throw UnsupportedVersionException when interacting with brokers that do not support the newly established minimum protocol API versions.

Compatibility, Deprecation, and Migration Plan

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As stated previously, the UNSUPPORTED_VERSION error will be returned in case incompatible clients are used with Kafka clusters running Apache Kafka 4.0.

Similarly, users should ensure their brokers are running Apache Kafka 2.1 before upgrading their Java clients to Apache Kafka 4.0.

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.
• Client compatibility system tests will updated so that 4.0.x clients fail with an UNSUPPORTED_VERSION error when dealing with Apache Kafka brokers that are too old (2.0 and/or 1.x).
• 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.
• Test simple flows end to end with the clients mentioned in this proposal against a cluster with this KIP implemented.

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