Which data exchange pattern ensures at-least-once delivery from a message queue, potentially requiring deduplication at the consumer?

Which data exchange pattern ensures at-least-once delivery from a message queue, potentially requiring deduplication at the consumer?

• A. At-least-once
• B. Exactly-once
• C. At-most-once
• D. Fire-and-forget

Answer: (A)

At-least-once delivery means the messaging system guarantees that every message will be delivered to the consumer at least once. Because the broker may retry sending a message when it doesn’t receive a proper acknowledgment or after a failure, duplicates can occur. To handle this safely, the consumer should be designed to deduplicate messages or make processing idempotent—using a unique message ID and ensuring that processing the same ID twice has no adverse effect. This pattern prioritizes reliability and prevents message loss, even if retries happen. In contrast, at-most-once can lose messages if delivery fails, exactly-once is more complex to achieve and often requires coordinated transactions or idempotent processing plus careful state management, and fire-and-forget provides no delivery guarantees.

At-least-once delivery means the messaging system guarantees that every message will be delivered to the consumer at least once. Because the broker may retry sending a message when it doesn’t receive a proper acknowledgment or after a failure, duplicates can occur. To handle this safely, the consumer should be designed to deduplicate messages or make processing idempotent—using a unique message ID and ensuring that processing the same ID twice has no adverse effect. This pattern prioritizes reliability and prevents message loss, even if retries happen.

In contrast, at-most-once can lose messages if delivery fails, exactly-once is more complex to achieve and often requires coordinated transactions or idempotent processing plus careful state management, and fire-and-forget provides no delivery guarantees.