Does increasing the number of partitions affect the guarantee that all messages with the same key go to the same partition?

• A. No
• B. Yes
• C. Only if keys are hashed
• D. It never affects

Answer: (B)

Partitioning in Kafka uses a hash of the key modulo the number of partitions to decide the target partition. When you increase the number of partitions, the modulo base changes, so the partition chosen for a given key can change. As a result, messages with the same key sent after the change may land in a different partition than earlier messages with that key. The guarantee holds only while the partition count stays fixed; once you add partitions, that cross-time guarantee is no longer preserved, though within a single fixed set of partitions the mapping remains deterministic and the same key will always go to the same partition.