Which statement correctly describes keys and partition assignment?

Which statement correctly describes keys and partition assignment?

• A. The producer always writes to the first partition if a key is present
• B. Partitions are chosen randomly and ignored for keys
• C. Keys determine the partition by hashing; if key is null, then round-robin across partitions
• D. Keys are not used by the producer at all

Answer: (C)

Partition assignment relies on the message key: if a key is present, the producer hashes the key and maps the record to a specific partition. This means all records with the same key go to the same partition, which preserves the order of those records within that partition. If the key is null, there’s no key-based mapping, so the producer distributes records across partitions in a round-robin fashion to balance load. That combination—hash-based partitioning when there’s a key, and round-robin distribution when there isn’t—best describes how keys influence partition assignment. The other statements don’t fit: always writing to the first partition isn’t how hashing works, or ignoring keys defeats per-key ordering and load balancing, and saying keys aren’t used at all contradicts the core mechanism described.

Partition assignment relies on the message key: if a key is present, the producer hashes the key and maps the record to a specific partition. This means all records with the same key go to the same partition, which preserves the order of those records within that partition. If the key is null, there’s no key-based mapping, so the producer distributes records across partitions in a round-robin fashion to balance load. That combination—hash-based partitioning when there’s a key, and round-robin distribution when there isn’t—best describes how keys influence partition assignment. The other statements don’t fit: always writing to the first partition isn’t how hashing works, or ignoring keys defeats per-key ordering and load balancing, and saying keys aren’t used at all contradicts the core mechanism described.