What are the main message delivery guarantees provided by Kafka according to its theoretical model?

Kafka provides three main message delivery guarantees:

1. At most once — a message may be delivered either once or not at all (for example, in case of a failure).
2. At least once — a message is guaranteed to be delivered, but may be delivered multiple times (duplicates).
3. Exactly once — a message is delivered exactly once without duplicates.

In the theoretical model, Kafka guarantees "at least once" delivery, meaning messages are not lost but may be duplicated in case of failures and retries. To ensure "exactly once" delivery, Kafka uses additional mechanisms such as idempotent producers and transactions.

Thus, the basic Kafka model provides reliable delivery with possible duplicates, while more complex configurations can achieve exactly-once delivery.