Kafka under-replicated partitions

Hello Jacob Lionel,

In a Kafka cluster, the “under‑replicated partitions” alert after a broker reboot means that replicas are not fully synchronized with the leader. To trigger a reassignment, you use the Kafka partition reassignment tool. First, generate a reassignment plan with kafka-reassign-partitions.sh --generate --zookeeper zkhost:2181 --topics-to-move-json-file topics.json --broker-list <brokerIDs> --output-reassignment-json-file reassignment.json. Then execute it with kafka-reassign-partitions.sh --execute --zookeeper zkhost:2181 --reassignment-json-file reassignment.json. This forces Kafka to redistribute replicas across the specified brokers. You can verify progress with the same script using --verify. On newer versions, the Admin API or kafka-topics.sh --alter can be used instead, but the principle is the same: explicitly instruct the cluster to rebalance replicas. Always confirm that unclean.leader.election.enable is set appropriately to avoid data loss during leader re‑elections.

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Thank you,

HL.