🎓 What to try
See lag grow
Raise produce rate above consumers × per-consumer speed. Lag climbs — that's your processing debt, measured in messages.
The skew trap
Set key skew to 70%. One partition's lag explodes while others stay at zero — adding consumers won't help, because a partition belongs to at most ONE consumer. Fix the keys, not the fleet.
Rebalancing isn't free
Add or remove a consumer during high load: ALL consumption pauses while the group rebalances, and lag jumps. This is why flapping consumers are dangerous.
Broker failure
Kill a broker. Its partitions reject writes until a new leader is elected, then everything heals. Replication (implied here) is why data isn't lost.
Idle consumers
With 6 partitions, add a 7th consumer — it sits idle. Partitions are the parallelism ceiling; plan partition counts for future scale.