Kafka: Exactly-once needs transactions on producer and consumer

Read-process-write becomes atomic: the output and the offset commit succeed or fail together.

Code

props.put(ProducerConfig.TRANSACTIONAL_ID_CONFIG, "orders-processor-1");
producer.initTransactions();

producer.beginTransaction();
producer.send(outputRecord);
producer.sendOffsetsToTransaction(offsets, consumerGroupMetadata);
producer.commitTransaction();

// consumer side:
props.put(ConsumerConfig.ISOLATION_LEVEL_CONFIG, "read_committed");

Output

Without read_committed the downstream consumer sees aborted records.
Exactly-once holds only INSIDE Kafka - an external HTTP call is still at-least-once.

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Published 2026-08-11