If a key is sent, do all messages for that key go to the same partition?

If a key is sent, do all messages for that key go to the same partition?

• A. False
• B. Sometimes depending on partition count
• C. True
• D. Only if the key is null

Answer: (C)

The main idea is that Kafka uses the record key to choose a partition. When a non-null key is present, the producer hashes that key and maps it to one of the topic’s partitions by taking the hash modulo the number of partitions. Because this mapping is deterministic, every record with the same key will be directed to the same partition. That’s why, for a given key, all messages end up in the same partition, which also helps keep their order within that partition. If the key is null, the partitioning behavior changes and messages can land in different partitions (often distributed across partitions in a round-robin or sticky fashion). Also, if the number of partitions changes for the topic, the mapping can change accordingly, so the same key might map to a different partition after a reconfiguration. But with a fixed partition count and a non-null key, the same key will consistently go to the same partition.

The main idea is that Kafka uses the record key to choose a partition. When a non-null key is present, the producer hashes that key and maps it to one of the topic’s partitions by taking the hash modulo the number of partitions. Because this mapping is deterministic, every record with the same key will be directed to the same partition. That’s why, for a given key, all messages end up in the same partition, which also helps keep their order within that partition.

If the key is null, the partitioning behavior changes and messages can land in different partitions (often distributed across partitions in a round-robin or sticky fashion). Also, if the number of partitions changes for the topic, the mapping can change accordingly, so the same key might map to a different partition after a reconfiguration. But with a fixed partition count and a non-null key, the same key will consistently go to the same partition.