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Use case: group_coordinator feature flag (for KIP-447)

As described in the motivation for this KIP, the Kafka Streams use case is going to be immediately benefitted by programmatically learning the cluster-wide version of EOS feature on the brokers (KIP-447). We shall use the group_coordinator feature flag to achieve the same, as explained below. This feature flag will cover both schema and protocol changes related to group coordinator (KIP-447 falls under a change to the group coordinator).

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As such, we don’t support downgrades in this versioning scheme/system (see Non-goals). So, there is a danger if the cluster operator tries to downgrade the group_coordinator feature flag from v1 to v0. This is because the stream client side shall not downgrade at all, and this would crash the EOS applications. As part of future work, we may consider adding constraints to the controller to restrict the downgrade of specific flags (unless there is a sign that it is done consciously). For example, group_coordinator feature is tied to data schemas and EOS processing semantics that’s not forward compatible.

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