Kafka: You can add partitions but never remove them

And adding them changes where keys land, so ordering across the change is lost.

Code

kafka-topics.sh --bootstrap-server localhost:9092 \
  --alter --topic orders --partitions 12

Output

order-42 was on partition 3 with 6 partitions.
With 12 it maps to partition 9 - and the old events stay on 3.

Two partitions now hold the history of one key, in the wrong order.

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Published 2026-08-11