# Redelivery, Ordering and Idempotent Consumers — RabbitMQ

Source: https://www.skillbyai.com/en/rabbitmq/c-idempotent

> Handle duplicate and out-of-order deliveries safely.

## At-least-once means duplicates

With manual acks, RabbitMQ provides **at-least-once delivery**: if a consumer processes a message but its ack is lost (connection dropped just after processing), the message is redelivered with the **`redelivered`** flag set, and may be processed **twice**. Make consumers **idempotent**: record processed **message IDs** (set by the publisher in the `message_id` property) in your database in the same transaction as the business change, and skip IDs you have seen; or design operations that are naturally idempotent, such as "set status to SHIPPED". **Ordering**: a single queue delivers in publish order to one consumer, but with several consumers, requeues and retries, order across messages is not guaranteed. If per-entity order matters, route each entity's messages to the same queue and consumer (for example with the **consistent hash exchange** plugin or **single active consumer**), and carry a version number so consumers can ignore stale updates.

## Deduplicating by message ID

The insert fails for a duplicate, so the business change is skipped.

```python
def handle(ch, method, props, body):
    msg_id = props.message_id          # set by the publisher, stable across retries
    with db.transaction() as tx:
        first_time = tx.execute(
            "INSERT INTO processed_messages (consumer, message_id) VALUES ('billing', %s) "
            "ON CONFLICT DO NOTHING", (msg_id,)
        ).rowcount == 1
        if first_time:
            apply_invoice(tx, json.loads(body))
    ch.basic_ack(delivery_tag=method.delivery_tag)   # ack after commit, duplicate or not
```

## Publishers must set message_id

Deduplication only works if the same logical message keeps the same ID when it is republished. Generate the ID when the event is created (for example in an outbox row), not at each publish attempt.

**Quiz:** Why can a consumer receive the same message twice with manual acknowledgements?

- [ ] RabbitMQ always sends two copies
- [ ] Prefetch duplicates messages
- [x] If the ack is lost after processing, the broker redelivers the message
- [ ] Topic exchanges duplicate every message

*Answer:* If the ack is lost after processing, the broker redelivers the message. At-least-once delivery redelivers messages whose acknowledgements did not arrive.