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In this KIP we propose to bump each of these records to a flexible version and backport this to earlier releases, specifically 3.0.3, 3.1.3, 3.2.4, 3.3.3, and 3.4.1 (note: these minor versions may never be releasedwe will need commitment from release managers to perform the minor releases). We will also apply this to 3.5 if the patch is merged to trunk before the code freeze date. Backporting this patch to earlier releases is acceptable because the change is small and is very low risk. One limitation is that we will be unable to downgrade to a version lower than 3.X. We will mention in future release notes where a new record type or a new field is introduced that we can only downgrade to the aforementioned versions. Note that once a new tagged field is introduced in a later version, that version can never be downgraded to below the listed versions.

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This KIP opens the door to backward compatible changes to the record schemas through the use of tagged fields. Any future tagged fields will not require a version bump and older brokers can simply ignore the tagged fields they do not understand. Note that introducing a new non-tagged field in the future will not be backward compatible. We propose to deserialize unknown versions to the highest supported version as a safeguard in the chance that we add a tagged field and also bump the version.

Compatible changes

Compatible changes are straightforward; the added tagged field is truly optional and is not required for the correctness of the coordinator. These fields will be ignored by the downgraded coordinator.

Incompatible changes

There are two cases: 1) we introduce a tagged field that is required for correctness, i.e. a new field that enforces correct access and without it results in incorrect coordinator behavior / data loss or 2) a non-tagged field is added. For both incompatible changes, we propose to have a version bump (already required for non-tagged fields) and add a new isBackwardCompatible field to the MetadataVersion enum so that the operator, if they decide, can downgrade with the --force option knowing that the downgrade is not backward compatible. 

Public Interfaces

__consumer_offsets

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