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Status

Current state: "Under Discussion"

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here [Change the link from KAFKA-1 to your own ticket]

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

Motivation

The metadata version (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 customers can specify to use only those features which are stable.

Proposed Changes

The idea is to define a production-ready MetadataVersion defined in MetadataVersion.java as LATEST_PRODUCTION such that all MetadataVersions less than or equal to this value are stable and ready for production. By default only those features with a MV less than or equal to the LATEST_PRODUCTION MV will be available to use in a cluster. A cluster can set "unstable.metadata.versions.enable" in the broker and controller properties file to enable the use of a MV greater than LATEST_PRODUCTION in a cluster. Any unstable feature which requires a new MV can have that MV change in a later release of Apache Kafka as other features with a higher MV become stable and ready for production.

Public Interfaces

A new property "unstable.metadata.versions.enable" will be created to allow for developers to create clusters and use features with an unstable MV.

Compatibility, Deprecation, and Migration Plan

• Features with a stable MV will never have that MV change!
• Users will only be able to use features with a stable MV unless they explicitly configure their clusters with the new property.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.