...

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

The metadata version MetadataVersion (MV) is used to define the on disk and over the wire record format for messages stored in the metadata log. A new metadata version is introduced each time a record format is changed or a new record is introduced to support a new feature. Examples of this are the support of SCRAM in MV IBP_3_5_IV2, support of Delegation Tokens in MV IBP_3_6_IV2 and the support of JBOD in MV IBP_3_7_IV2. All brokers and controllers must support a given MV before the cluster can start using that MV. Once the brokers and controllers agree to use a MV then the record format for that MV and all earlier MV is supported thus also enabling all earlier features, thus if you want to use Delegation Tokens you get the record format changes necessary to support SCRAM also.

The development of a new feature that requires a new MV often requires multiple commits before the feature is complete, sufficiently tested, and ready for production. Sometimes there is a need for multiple features to be developed at the same time and in some cases not all of these features with be complete and ready for production for a given release. In these scenarios we need to reorder MetadataVersions such that production ready features have their MV before those that are not ready and so that customers can specify to use only those features which are stable.

...