Kafka: consumer lag on one partition is skew, not a scaling problem

Adding consumers cannot help when a single partition is behind - one partition is consumed by exactly one member of a group. The cause is almost always a hot key, and more instances just leave more of them idle.

• Kafka: linger.ms=0 does not mean every record is sent alone
• Kafka: an eager rebalance stops the entire consumer group
• Kafka: log compaction never removes the last record for a key, including a tombstone
• Kafka: a read_committed consumer stalls behind one open transaction
• Kafka: a non-idempotent producer reorders on retry
• Kafka: adding partitions breaks key ordering permanently

---

Run this yourself in the Online Java Compiler, spin up a live REST API in the API Sandbox, or practise with Java interview questions.

Published 2026-08-25

© Java Coding Hub · About · Contact · Privacy · Terms