What is exactly-once semantics in Kafka? Does it really provide absolute guarantees?

Exactly-once semantics in Kafka means that each message will be processed exactly once, without duplication or loss, even in case of failures. This is achieved through idempotent producer writes and transactional message processing.

However, in reality, there are no "absolute" guarantees, as external factors such as failures in integrated systems, processing logic errors, and network issues can occur. Kafka provides the closest possible exactly-once processing within its system, but guarantees only apply to operations within Kafka and related transactions. External systems can violate these guarantees.

Example of using transactions in Kafka with Java:

producer.initTransactions();
try {
    producer.beginTransaction();
    producer.send(record1);
    producer.send(record2);
    producer.commitTransaction();
} catch (ProducerFencedException | OutOfOrderSequenceException | AuthorizationException e) {
    producer.close();
} catch (KafkaException e) {
    producer.abortTransaction();
}