The Log End Offset (LEO) is the next offset to be written — it advances on every leader append. The High Watermark (HWM) is the highest offset that has been replicated to ALL ISR replicas. Consumers can only read up to the HWM — never past it. This prevents consumers from reading data that might be lost if the leader fails before replication completes.
Lesson 3 of 5
High Watermark (HWM) & LEO
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
- Let a replica fall behind, then catch upTake a broker offline, produce messages it misses, then watch it replicate the backlog before rejoining the ISR.Beginner · ~5 min
- Rebalance leadership with preferred leader electionAfter a failover leaves leadership lopsided, run a preferred leader election to restore the original balance.Intermediate · ~5 min
- Cause data loss with unclean leader electionEngineer the exact conditions where enabling unclean election trades data loss for availability — and watch it happen.Advanced · ~7 min