Is reliable message delivery in Kafka ensured?

Kafka provides reliable message delivery, but guarantees depend on the application's settings and architecture. The main levels of reliability:

• At most once: messages may be lost but are not duplicated.
• At least once: messages are not lost but may be duplicated.
• Exactly once: messages are delivered without loss or duplication, but require additional configuration (e.g., transactions).

For reliable delivery, it is important to use acknowledgments (acks), topic replication, and properly configure the producer and consumer. For example, setting acks=all and enabling replication increase reliability.

Example of producer configuration for reliable delivery:

Properties props = new Properties();
props.put("acks", "all"); // wait for acknowledgment from all replicas
props.put("retries", 3); // retry on errors
props.put("enable.idempotence", true); // enable idempotence for exactly-once delivery
KafkaProducer<String, String> producer = new KafkaProducer<>(props);

Thus, Kafka can ensure reliable delivery but requires correct configuration and understanding of the delivery guarantee model.