Kafka Error `NOT_ENOUGH_REPLICAS`

Error code 19 · Retriable Broker

Messages are rejected since there are fewer in-sync replicas than required.

Common Causes

One or more brokers are down, shrinking the ISR below 'min.insync.replicas' (common during broker maintenance or unplanned failures)
Slow replica brokers falling out of ISR due to disk I/O bottleneck causing replica fetch lag to exceed 'replica.lag.time.max.ms'
Cluster scaled down (broker decommissioned) without first reassigning partitions, leaving some partitions under-replicated

Solutions

Short-term: temporarily reduce 'min.insync.replicas' on the affected topic to allow writes to proceed during the incident (acknowledge this reduces durability guarantees)
Restore the downed broker or add a replacement broker and use kafka-reassign-partitions.sh to rebalance partitions back to full replication
Monitor ISR size proactively: alert when 'UnderReplicatedPartitions' JMX metric > 0 for more than 60 seconds to catch degradation before min.insync.replicas is breached

Diagnostic Commands

# Check for under-replicated partitions
kafka-topics.sh --describe --bootstrap-server localhost:9092 --under-replicated-partitions

# Check for unavailable partitions
kafka-topics.sh --describe --bootstrap-server localhost:9092 --unavailable-partitions

Related APIs

This error can be returned by: EndTxn · InitProducerId · Produce · WriteTxnMarkers

Related KIPs

KIP-579 · KIP-637

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Kafka Error NOT_ENOUGH_REPLICAS