Lesson 4 of 5

Preferred Leader Election

Each partition has a 'preferred' leader — the first replica in its replica assignment list. After broker restarts, leadership can become unbalanced. Preferred leader election (PLE) moves leadership back to the original brokers, ensuring even load distribution. Kafka can do this automatically (auto.leader.rebalance.enable=true) or manually via CLI.

Key configuration

broker / client configurationproperties

auto.leader.rebalance.enable=true — automatic rebalancing
leader.imbalance.per.broker.percentage=10 — trigger threshold

Try it in the simulator

Guided scenarios for this module — the simulator opens with the exercise already running.

Break a broker, watch the failoverStop a partition leader yourself, watch the election, prove producing still works, then heal the cluster.Intermediate · ~6 min
Shrink the ISR with follower lagDrag a follower's replication lag past the threshold, watch it ejected from the ISR, then let it recover.Intermediate · ~5 min
Trigger NOT_ENOUGH_REPLICASKill two brokers to violate min.insync.replicas and watch Kafka refuse the write rather than risk data loss.Advanced · ~6 min