At-least-once means every message is delivered, but possibly multiple times. You never lose messages.
How it works:
Process the message, then acknowledge. If processing succeeds but ack fails, message is redelivered.
Implications:
- Your consumer must be idempotent
- Same message may process , , or more times
- Most common default for reliability
Making consumers idempotent: Use message ID to track processed messages. Skip if already seen.