What guarantees of delivery have you encountered in Kafka?

Kafka has three main message delivery guarantees:

1. At most once — a message may be delivered 0 or 1 times, data loss is possible, but no duplicates.
2. At least once — a message is guaranteed to be delivered, but duplicates may occur.
3. Exactly once — a message is delivered exactly once, with no loss or duplicates.

To implement Exactly once in Kafka, an idempotent producer and transactions are used, allowing atomic writing of messages to multiple partitions.

In real systems, the choice of guarantee depends on reliability and performance requirements.