Does adding a partition to a topic lose the guarantee that the same key goes to the same partition?

• A. False
• B. Not affected
• C. Depends on the producer
• D. True

Answer: (D)

The mapping of a key to a partition is based on the number of partitions for the topic. Kafka’s default partitioner hashes the key and uses that hash modulo the current number of partitions to pick a partition. If you add more partitions, the modulo base changes, so the same key can end up in a different partition for new messages. In other words, the guarantee that a given key always lands on the same partition holds only while the partition count stays the same; increasing partitions can change where that key goes. Old messages remain in their original partitions, but future ones may be routed elsewhere.