What are the message delivery strategies in Kafka and how are they implemented?

In Kafka, there are three main message delivery strategies:

1. At most once

   • A message can be delivered 0 or 1 times.
   • Used when message loss is acceptable.
   • Implemented when the producer does not wait for acknowledgment from the broker (fire-and-forget).

2. At least once

   • A message is guaranteed to be delivered, but may be delivered multiple times (duplicates possible).
   • This is the standard mode of Kafka operation.
   • The producer waits for message write acknowledgment (acks=all), and the consumer commits offsets after processing.

3. Exactly once

   • A message is delivered and processed exactly once.
   • Achieved using Kafka transactions and an idempotent producer.
   • Prevents duplicates even in case of failures.

Example of configuring a producer for "exactly once":

Properties props = new Properties();
props.put("enable.idempotence", "true");
props.put("transactional.id", "my-transactional-id");
KafkaProducer<String, String> producer = new KafkaProducer<>(props);

producer.initTransactions();
try {
    producer.beginTransaction();
    producer.send(new ProducerRecord<>("topic", "key", "value"));
    producer.commitTransaction();
} catch (Exception e) {
    producer.abortTransaction();
}

Thus, the choice of strategy depends on the reliability requirements and the acceptability of duplicates in the system.