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This approach makes selective backporting of protocol versions to previous branch difficult, hard to manage. Requires a lot of manual coordination and so has lots of room for mistakes.

Sub-proposal: improved handling of unsupported requests

This proposal was not intended to solve the main feature detection functionality of KIP-35 but rather to fix the suboptimal handling of unsupported requests in the broker to provide meaningful error messages to the end user.

This is an optional nice-to-have feataure and was removed from KIP-35 to minimize scope.

Original text:

If a request is received for an unknown protocol request type, or for an unsupported version of a known type, the broker should respond with an empty response only including the common Length+CorrId header with Length=0, instead of closing the connection. The effect of this empty message received by the client is that the client will fail to parse the response (since it will expect a proper non-empty reply) and throw an error to the application with the correct context - that of a protocol parsing failure for request type XYZ. All existing clients should be able to handle this gracefully without any alterations to the code, while updated clients supporting the proposals in this KIP can handle the empty response accordingly by returning a "Request not supported by server" error the application. The level of detail in the returned error message to the application naturally varies between client implementations but is still by far better than a closed connection which says nothing of the underlying error.

This proposal is not a good fit for feature discovery since there is no way to reliably and without side-effects check if a certain protocol version is supported. As an example let's say that a future KafkaConsumer needs to commit offsets with OffsetCommit v99 (which features something spiffy), with this proposal the client would need to Join the group, pass through rebalancing, Consume and process message(s) and after that perform an OffsetCommit. If at this point it turns out OffsetCommit v99 wasn't supported the application has processed messages that it now can't commit. This late erroring handling is hard to get right.