What delivery guarantees have you encountered in Kafka?

There are three main message delivery guarantees in Apache Kafka:

1. At most once: A message may be delivered 0 or 1 times. Loss of messages is possible, but duplicates will not occur.

2. At least once: A message will be delivered at least once, but duplicates may occur if the consumer does not acknowledge receipt in time and the message is resent.

3. Exactly once: A message is delivered and processed exactly once without duplicates. In Kafka, this is achieved using idempotent producers and transactions.

In practice:

• Kafka defaults to "at least once".
• To achieve "exactly once," additional configurations and transaction APIs are required.

Understanding these guarantees is important for building reliable data processing systems where message loss or duplication can have critical consequences.