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Learn about unclean leader election and the availability vs durability trade-off in 10 minutes When no in-sync replicas (ISRs) are available, Kafka has to decide between staying unavailable or promoting an out-of-sync replica to leader. The unclean.leader.election.enable configuration controls this critical trade-off. What you’ll learn:
  • What unclean leader election means
  • When unclean elections cause data loss
  • Use cases where enabling this setting makes sense
  • How to configure unclean leader election

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Clean vs unclean leader election

When the leader for a partition becomes unavailable, one of the in-sync replicas (ISR) becomes the new leader. This is a “clean” election because committed data exists on all ISRs by definition. But what happens when no ISR exists except for the failed leader?
OptionBehaviorRisk
Wait for ISR (default)Topic unavailable until ISR recoversAvailability loss
Unclean electionPromote out-of-sync replicaData loss

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The availability vs durability trade-off

If we allow out-of-sync replicas to become leaders, we will have data loss and data inconsistencies. If we don’t allow them to become leaders, we face lower availability as we have to wait for the original leader to become available before the partition is back online.
Dangerous settingUnclean leader election can cause permanent data loss. Understand the implications fully before enabling it. The default value of false is recommended for most use cases.

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When to enable unclean leader election

Use caseRecommendationReason
Financial transactionsDisableData loss unacceptable
Metrics/logsConsider enablingAvailability more important
Event streamingDepends on criticalityEvaluate trade-off
Audit complianceDisableRegulatory requirements
For an in-depth analysis, see Kafka at Datadog: Unclean Leader Elections.

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Configure unclean leader election

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Enable for a specific topic

kafka-configs --bootstrap-server localhost:9092 --alter \
  --entity-type topics --entity-name configured-topic \
  --add-config unclean.leader.election.enable=true

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Disable (restore default)

kafka-configs --bootstrap-server localhost:9092 --alter \
  --entity-type topics --entity-name configured-topic \
  --delete-config unclean.leader.election.enable

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Verify configuration

kafka-configs --bootstrap-server localhost:9092 --describe \
  --entity-type topics --entity-name configured-topic
Configure unclean leader election per topic rather than cluster-wide. This lets you enable it only for topics where availability is more important than durability.
See it in practice with ConduktorConduktor Console displays topic configurations including unclean leader election status. Monitor ISR counts and leader distribution to understand your replication health.

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Next steps