Can you explain what is meant by message delivery guarantee in a message broker system?

Message delivery guarantees in a message broker system mean that a message will be delivered to the recipient with a certain level of reliability. Main types of guarantees:

• At most once — the message may be delivered once or lost, without re-sending.
• At least once — the message is guaranteed to be delivered, but may be delivered multiple times (duplicates).
• Exactly once — the message is delivered exactly once, without duplicates or losses.

For example, in RabbitMQ or Kafka, acknowledgments can be configured to ensure "at least once" delivery. For "exactly once," additional processing logic and idempotency on the consumer side are often required.