---
title: "Kafka unclean.leader.election.enable: the data loss risk"
description: "What unclean.leader.election.enable does in Kafka: availability over durability when no in-sync replica is left, and the committed records it loses."
url: https://monedula.dev/kafka-simulator/configs/unclean-leader-election-enable/
---

# unclean.leader.election.enable: availability vs data loss

Topic config · default false · Apache Kafka 4.3.

unclean.leader.election.enable lets a replica that is not in the ISR become leader when no in-sync replica is left (default false). The partition comes back online, but every record the new leader never received is lost, including records acknowledged to acks=all producers. With it off, the partition stays offline until an in-sync replica returns.

## At a glance

- Config: `unclean.leader.election.enable` (topic)
- Default: false
- Valid values: true, false
- Also set at: Also a broker setting, unclean.leader.election.enable, which is the default for topics that do not set it.
- Reference: [Apache Kafka documentation](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_unclean.leader.election.enable)

## What it does

Only replicas in the ISR, the in-sync replica set, have every committed record, so normally only they can be elected leader. When the last of them fails, the partition has no leader. A replica that dropped out of the ISR earlier may still be alive, but it is missing records. With unclean election enabled, the controller elects it anyway: the high watermark falls back to its log end, the records past that point are gone for good, and the old replicas truncate their logs to match when they return.

## The trade-off

false picks durability: no acknowledged record is lost, but a partition can be offline until a broker that was in sync comes back, which may be hours. true picks availability: producers and consumers carry on at once, at the price of silently losing committed data, which also breaks exactly-once guarantees and can leave consumers behind offsets that no longer exist.

## Advice

- Leave it false, and avoid the situation with replication.factor=3, min.insync.replicas=2 and acks=all, plus racks spread across failure domains.
- If a partition is offline and you decide its availability matters more than the lost records, trigger a one-off unclean election for that partition with kafka-leader-election.sh --election-type unclean instead of turning the setting on.
- Only consider true for data you can afford to lose, such as metrics.

## See it in the simulator

- [Kafka unclean.leader.election.enable: offline or data loss](https://monedula.dev/kafka-simulator/scenarios/unclean-leader/) (Unclean leader), step 6: Unclean election enabled - broker-3 leads, records lost. broker-3 (LEO 2) is promoted unclean. The high-watermark drops from 5 to 2 - 3 committed records are gone. Availability restored at the cost of durability. (The simulator elects at once; in KRaft a dynamically enabled flag waits for the periodic check, 5 min by default, unless you run kafka-leader-election.sh with the unclean option.) [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=03.2.3&t=5)
- [Kafka unclean leader election: permanent loss of acked data](https://monedula.dev/kafka-simulator/scenarios/unclean-election-data-loss/) (Unclean election: permanent data loss), step 5: broker-2 killed - stale broker-3 promoted UNCLEAN, committed records lost. ISR is empty and the only live replica is the stale broker-3 (LEO 2). Unclean election promotes it anyway: the high-watermark drops from 5 to 2. The three records the producer was told were committed (2-4) are gone - availability bought with a permanent consistency hole. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=03.2.4&t=4)

## Related configs

- [Kafka min.insync.replicas explained (with acks=all)](https://monedula.dev/kafka-simulator/configs/min-insync-replicas/)
- [Kafka acks: 0 vs 1 vs all explained](https://monedula.dev/kafka-simulator/configs/acks/)
- [Kafka enable.idempotence: no duplicates on retry](https://monedula.dev/kafka-simulator/configs/enable-idempotence/)