What message delivery guarantees do you know in brokers?

Message brokers typically distinguish three main delivery guarantees:

1. At most once — the message may be delivered once or may be lost. No retries are performed, reducing duplication but risking data loss.

2. At least once — the message is guaranteed to be delivered, but may be delivered multiple times, requiring idempotent processing on the consumer side.

3. Exactly once — the message is delivered strictly once, with no duplicates or losses. This is the most complex guarantee, requiring sophisticated acknowledgment and transaction mechanisms.

Example: Kafka by default provides "at least once" delivery, but with additional configuration and the use of idempotent producers, it can approach "exactly once".