Kafka: Ordering is guaranteed per partition, never per topic

Two events land in order only if they share a partition - which means only if they share a key.

Code

// Same key -> same partition -> ordered
producer.send(new ProducerRecord<>("orders", "order-42", "CREATED"));
producer.send(new ProducerRecord<>("orders", "order-42", "PAID"));

// No key -> any partition -> no ordering guarantee at all
producer.send(new ProducerRecord<>("orders", null, "CREATED"));
producer.send(new ProducerRecord<>("orders", null, "PAID"));

Output

Keyed:   partition 3, offset 118
         partition 3, offset 119   <- ordered
Unkeyed: partition 0, offset 55
         partition 5, offset 91    <- consumed in any order

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Published 2026-08-11