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• The proposed changes does create any breaking changes in public interfaces. There are however changes that are done to the semantics of the listener configuration, i.e. the following configuration examples

"PLAINTEXT://127.0.0.1:9092,PLAINTEXT://[::1]:9092"

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"PLAINTEXT://[::1]:9093,PLAINTEXT://127.0.0.1:9093"

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"PLAINTEXT://127.0.0.1:9094,PLAINTEXT://[::1]:9094,PLAINTEXT://127.0.0.1:9095,PLAINTEXT://[::1]:9095"

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"PLAINTEXT://[::1]:9096,PLAINTEXT://127.0.0.1:9096,PLAINTEXT://[::1]:9097,PLAINTEXT://127.0.0.1:9097"

Are now considered valid which means that Kafka will attempt to start with such configurations rather than short circuiting with an error. Specifically this means if and only if you have 2 listeners on the same port and one is IPv4 and the other is IPv6 then this is considered an accepted listener configuration.

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Apart from adding some extra information that gets thrown when the validation in listenerListToEndPoints  fails there are no changes outside of this specific case.

Examples of listener configurations which will pass the validation proposed in this KIP

"PLAINTEXT://127.0.0.1:9092,PLAINTEXT://[::1]:9092"
"PLAINTEXT://[::1]:9093,PLAINTEXT://127.0.0.1:9093"
"PLAINTEXT://127.0.0.1:9094,PLAINTEXT://[::1]:9094,PLAINTEXT://127.0.0.1:9095,PLAINTEXT://[::1]:9095"
"PLAINTEXT://[::1]:9096,PLAINTEXT://127.0.0.1:9096,PLAINTEXT://[::1]:9097,PLAINTEXT://127.0.0.1:9097"

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And here are examples of listener configurations which will fail the validation

"PLAINTEXT://[::1]:9092,PLAINTEXT://[::1]:9092"
"PLAINTEXT://127.0.0.1:9092,PLAINTEXT://127.0.0.1:9092"
"PLAINTEXT://127.0.0.1:9092,PLAINTEXT://127.0.0.1:9092,PLAINTEXT://127.0.0.1:9092"
"PLAINTEXT://127.0.0.1:9092,SSL://127.0.0.1:9092,SASL_PLAINTEXT://127.0.0.1:9092"

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Compatibility, Deprecation, and Migration Plan

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