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Kafka clients already provide rebalance-related callbacks, but relying on client-side behavior is operationally fragile. In practice, callback availability and behavior depend on SDK adoption and upgrade cycles, which are difficult to control sometimes. Even when teams implemented the callback with log only. users may reduce or disable verbose the logs by mistake or some other reasons. This creates a common failure mode: when incidents occur, critical rebalance evidence is missing at the client side, making diagnosis slow and uncertain . so that we have to query the log in kafka broker side.

HoweverWhat's more, today server-side observability is limited: coordinator logs can indicate rebalance activity, but logs alone are not a reliable integration surface for automated handling, and existing metrics do not carry the most actionable identifier identifier (consume group . id). As a result, operators can observe aggregate rebalance trends, but cannot lightweightly attach custom processing to specific groups without log parsing or invasive client changes.

This KIP proposes a lightweight broker-side rebalance callback capability to expose key rebalance context  (including including consume group .id) from the coordinatoron kafka broker. 

In short, the . The goal is to provide a stable, centrally controlled extension point for observability and operational automation, without forcing client SDK upgrades and without introducing high-cardinality metric tags. 

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