Exactly Once in Kafka

Kafka's documentation makes a precise promise about exactly-once delivery. The transactional producer, together with consumers that read only committed data, "can be used generally to provide exactly-once delivery when reading, processing and writing data on Kafka topics." The promise is about Kafka topics: records read from one topic and written to another.

The same page states what is not promised: "Exactly-once delivery for other destination systems generally requires cooperation with such systems