Kafka: A null key is not the same as an empty key

Null means "choose a partition for me"; an empty string is a real key that always hashes the same way.

Code

new ProducerRecord<>("orders", null, payload);   // sticky partitioner
new ProducerRecord<>("orders", "", payload);     // murmur2("") - one partition

Output

null: spread across all partitions in batches
""  : every record to the SAME partition - an accidental hot partition

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Published 2026-08-11