Explain at-most-once, at-least-once and exactly-once delivery.

Summary

At-most-once may lose messages but never duplicates (ack before processing); at-least-once never loses messages but may deliver duplicates (ack after processing); exactly-once is achievable only within limited boundaries (e.g., Kafka transactions within Kafka) and in practice means at-least-once delivery plus idempotent processing.

Longer Answer

Most systems choose at-least-once and make consumers idempotent: deduplicate by message ID, use upserts, or record processed IDs in the same transaction as the side effect.

Key Points

• At-most-once: possible loss
• At-least-once: possible duplicates
• Exactly-once = at-least-once + idempotency
• Ack after processing