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Compatibility, Deprecation, and Migration Plan

This would be a passive (opt-in) change. No existing functionality will be deprecated.

Rejected Alternatives

The roundrobin strategy for the original high-level consumer initially disallowed non-identical subscriptions among members of a consumer group. As discussed in KAFKA-2172 and above this is problematic for rolling configuration changes. When that Jira has been completed and non-identical subscriptions are allowed for the roundrobin strategy, we can document that this leniency may produce significantly unbalanced assignments in some cases. It seems inadequate to address this design flaw in the roundrobin strategy with known-issue documentation only, and inappropriate to redesign the existing roundrobin strategy to be fairness-aware for unbalanced subscriptions.

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