# Quorum Queues — RabbitMQ

Source: https://www.skillbyai.com/en/rabbitmq/q-quorum

> Use replicated quorum queues for safety and understand their behaviour.

## Replicated with Raft

A **quorum queue** replicates its contents across several nodes (typically 3 or 5) using the **Raft** consensus algorithm. One replica is the **leader**; others are **followers**. A message is confirmed to the publisher only after a **majority** of replicas have written it, so the queue survives the loss of a minority of nodes without losing confirmed messages, and a new leader is elected automatically. Quorum queues are always **durable**, are declared with `x-queue-type: quorum`, and are the recommended default for data safety. They track **delivery counts** and enforce a **delivery limit** (default 20 in 4.0), dead-lettering poison messages that keep failing instead of looping forever, and support **at-least-once dead-lettering** to avoid losing messages on their way to the DLX. Trade-offs: higher latency and disk use than a non-replicated queue, and they are designed for data that is consumed reasonably promptly, not for multi-million message backlogs held for days.

## Declaring a quorum queue with sensible settings

Replication, delivery limit and dead-lettering in one declaration.

```python
channel.queue_declare(
    queue="payments.capture",
    durable=True,
    arguments={
        "x-queue-type": "quorum",
        "x-quorum-initial-group-size": 3,         # replicas
        "x-delivery-limit": 5,                     # poison messages dead-letter after 5 attempts
        "x-dead-letter-exchange": "payments.dlx",
        "x-dead-letter-strategy": "at-least-once", # safer dead-lettering
        "x-overflow": "reject-publish",            # required for at-least-once dead-lettering
    },
)
```

## A committee that votes

A quorum queue is a committee of three clerks. A decision (message) is recorded only when at least two have written it in their books. If one clerk falls ill, the other two carry on and nothing confirmed is lost.

**Quiz:** When does a quorum queue confirm a published message?

- [ ] As soon as the leader receives it in memory
- [ ] After every consumer has acknowledged it
- [ ] Only after the queue is empty
- [x] After a majority of replicas have written it

*Answer:* After a majority of replicas have written it. Raft commits a message once a majority of replicas have it, which is what makes it safe.