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https://issues.apache.org/jira/browse/KAFKA-15869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17788419#comment-17788419
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David Jacot commented on KAFKA-15869:
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[~antonagestam-aiven] No, it is not a mistake. We had a KIP for this:
[https://cwiki.apache.org/confluence/display/KAFKA/KIP-893%3A+The+Kafka+protocol+should+support+nullable+structs.]
However, I agree that we should update the protocol guide. We can use this
Jira for this purpose.
> Document semantics of nullable nested API entities
> --------------------------------------------------
>
> Key: KAFKA-15869
> URL: https://issues.apache.org/jira/browse/KAFKA-15869
> Project: Kafka
> Issue Type: Wish
> Reporter: Anton Agestam
> Priority: Minor
>
> The initial version of ConsumerGroupHeartbeatResponse [introduced the first
> field across the protocol that is a nullable nested
> entity|https://github.com/dajac/kafka/blob/3acd87a3e82e1d2fd4c07218d362e7665b99c547/clients/src/main/resources/common/message/ConsumerGroupHeartbeatResponse.json#L48].
> As the implementor of a third-party schema parser it is not clear how to
> handle this field, where such fields are allowed, and how null is represented
> for such fields.
> As far as I can tell, the [protocol
> guide|https://kafka.apache.org/protocol.html#The_Messages_ConsumerGroupHeartbeat]
> does not mention the nullability at all.
> The reason I ask where such fields are allowed is because if the answer to
> how null is represented here is just omitting writing any bytes, then I
> suspect the only unambiguous place for such field to appear would be as the
> last field of a top-level entity. Even then, how is it discriminated from
> tagged fields?
> Is it possible this field was made nullable by mistake?
>
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