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• Upgrades: Most common workflow for the feature versioning system would involve finalizing a cluster upgrade to max feature version levels for all features, after completing the release of a new Kafka Broker binary on a cluster. This is explained  under Regular Basic CLI tool usage section.
• Downgrades: Emergency downgrades of max version levels of features are rare, especially after they are finalized  (because these are breaking changes, and finalizing these changes can have consequences).  If the user has set the 'allowDowngrade' boolean field in an UpdateFeaturesRequest, then they must have had a good reason to attempt downgrading. Also, the CLI tool only allows a downgrade in conjunction with specific flags and sub-commands, as explained below:
  • In the CLI tool, if the user passes '--allow-downgrade' flag when updating a specific feature, then the tool will translate this ask to setting 'allowDowngrade' field in the request to the server.
  • Emergency downgrade of all features can be attempted using the CLI tool belonging to a specific Broker release. This is by using the 'downgrade-all' sub-command. For examples of these, please refer to the Tooling support section.
  • Note: Please note that some of the downgrades may still be blocked by Controller if it is deemed impossible to downgrade. The controller (on it's end) can maintain rules in the code, that, for safety reasons would outright reject certain downgrades from a specific max_version_level for a specific feature. Such rejections may happen depending on the feature being downgraded, and from what version level.

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As such, we don’t support downgrades in this versioning scheme/system (see Non-goals and Future work). So, there is a danger if the cluster operator tries to downgrade the group_coordinator feature flag from v2 to v1 using the --allow-downgrade flag (see these examples). This is because the stream client side shall not downgrade at all, and this would crash the EOS applications. A safe practice is to first downgrade the stream clients, before downgrading  the group_coordinator feature flag.

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