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We shall introduce a FeatureCommand CLI tool backed by a new admin command library. This CLI tool will be maintained in the Kafka repository, alongside the code for the Kafka Broker.
The section on Guidelines on feature versions and workflows, is a recommended read ahead of using the CLI tool .Note the following: The CLI tool internally has knowledge about a map of features to their respective max versions supported by the Broker. The tool's knowledge of features and their version values, is limited to the version of the CLI tool itself i.e. the information is packaged into the CLI tool when it is released. Whenever a Kafka release introduces a new feature version, or modifies an existing feature version, the CLI tool shall also be updated with this information. Newer versions of the CLI tool will be released as part of the Kafka releases.in practice.

The CLI tool will be used by the cluster operator (i.e. a human), and willenable us to do the following:

1. Read cluster-wide finalized feature versions from a broker or a controller via it’s ApiKeys.API_VERSIONS API.
2. Add/update/delete specific or all cluster-wide finalized feature versions by exercising the newly introduced ApiKeys.UPDATE_FEATURES API on a controller.

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The CLI tool is versioned. Whenever a Kafka release introduces a new feature version, or modifies an existing feature version, the CLI tool shall also be updated with this information. Newer versions of the CLI tool will be released as part of the Kafka releases. The tool is handy for both regular usage (upgrade/downgrade all feature max version levels) as well as advanced usage (change specific feature version levels). Later below, we demonstrate both such usages of the CLI tool.

• Regular usage happens typically when after a Kafka release, the cluster operator wants to finalize all latest feature versions. The tool internally has knowledge about a map of features to their respective max versions supported by the Broker. Using this information, the tool provides a facility to upgrade all feature max version levels to latest values known to the tool.
• Downgrade of feature max version levels, after they are finalized using the tool, are a rare occurrence. To facilitate emergency downgrade of all feature versions (ex: during emergency roll back to a previous Kafka release), the tool provides a downgrade-all facility. To achieve this, the user needs to run the version of the tool packaged with the Kafka release that he/she needs to downgrade to. This is because the tool's knowledge of features and their version values, is limited to the version of the CLI tool itself (i.e. the information is packaged into the CLI tool when it is released).

We shall introduce 3 new APIs in the Admin interface, which enables us to read the feature versions and finalize feature version upgrades/downgrades. Below is Java-ish pseudocode for the same.

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