Eligible Leader Replicas (ELR) in Kafka 4.1: What You Need to Know
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KIP-966 introduces Eligible Leader Replicas (ELR) in Apache Kafka 4.1 to fix the rare but critical 'Last Replica Standing' scenario where committed messages could be lost even with acks=all. ELR creates a middle-ground replica state between ISR and fully untrusted replicas: when the ISR drops below min.insync.replicas, replicas that held data up to the frozen High Watermark are tracked in the ELR list and remain safe leader candidates. The fix also includes a High Watermark advance rule change (HWM only advances when ISR >= min.insync.replicas, regardless of producer acks setting) and improved unclean shutdown detection via CleanShutdownFile epoch checks. Consumers will notice that a degraded ISR now freezes consumption visibility, not just production durability. ELR is experimental in Kafka 4.0 and enabled by default in 4.1 (KRaft only).
