kafka.replication.under_replicated_partitions

When ISR count drops below the configured replication factor, partitions become under-replicated, indicating broker health issues, network problems, or disk saturation that compromise data durability.

Frequent ISR shrink and expand events indicate replicas repeatedly falling behind and catching up, suggesting intermittent broker issues, network instability, or insufficient resources for replication workload.

When Kafka under-replicated partitions increase (kafka.replication.under_replicated_partitions) or ISR shrinks (kafka.server.ReplicaManager.IsrExpandsPerSec drops), Lambda event source mappings may experience increased fetch latency and offset lag as the cluster struggles to maintain replication consistency.

When ISR count drops below min.insync.replicas, producers configured with acks=all will fail with NotEnoughReplicasException, blocking writes to maintain durability guarantees.