Preserving Order

Kafka makes one promise about order: a consumer reads the events of one partition in exactly the order they were written. Because events with the same key go to the same partition, that becomes a promise per key, as long as the key and the partition count stay the same.

Everything else is outside the promise. Kafka does not order events in different partitions, events in different topics, events whose key changed, or events written before and after partitions were added. A retried send can also change the order, unless the producer is configured to prevent it.