# Message Queues — System Design: Architecture, Scale and Trade-offs

Source: https://www.skillbyai.com/en/system-design/sd-queues

> Decouple producers from consumers with message queues, and design idempotent consumers and dead-letter queues.

## Why a queue

A queue (Kafka, SQS, RabbitMQ) lets a fast producer hand off work and move on. Consumers process at their own pace, absorbing spikes and surviving restarts.

## Delivery guarantees

Most queues offer **at-least-once** delivery, so consumers must be **idempotent** — processing the same message twice should be safe.

## Add a dead-letter queue

Messages that fail repeatedly go to a DLQ instead of blocking the pipeline. You inspect and replay them later.

**Quiz:** Why must consumers of an at-least-once queue be idempotent?

- [ ] Messages may arrive out of order
- [x] The same message may be delivered more than once
- [ ] Messages can be lost
- [ ] Queues always reorder messages by size

*Answer:* The same message may be delivered more than once. Retries and redeliveries mean duplicates; idempotent handling makes them harmless.