# Classic Queues — RabbitMQ

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

> Know what classic queues offer and when they are still appropriate.

## The original, single-node queue

**Classic queues** are RabbitMQ's original queue type. Their data lives on **one node**. Modern versions use the **version 2 storage** format, which keeps memory use low by writing messages to disk efficiently rather than holding large backlogs in memory. Classic queues support every feature, including **non-durable** and **exclusive** queues, **priorities** (up to 255 levels, though a few is recommended), and very high throughput for transient data. The old **classic mirrored queues** (high-availability replication of classic queues) were deprecated for years and are **removed in RabbitMQ 4.0**; replicated data now belongs in **quorum queues** or **streams**. Use classic queues for **temporary or non-critical data**: RPC reply queues, exclusive per-connection queues, caches of work that can be regenerated, or very high-throughput transient messages where losing a node's contents is acceptable.

## Three queue types

Classic queues live on one node; quorum queues and streams replicate across nodes.

![Three columns: one single box, one set of three connected boxes with a crown on one, and one long strip with position markers replicated on three nodes.](assets/figures/rabbitmq/section-5-map.svg) — Figure 5.1 — Classic queues, quorum queues and streams.

## When to choose which queue type

A quick decision table.

```text
need                                                    queue type
------------------------------------------------------  ----------------
important work or events that must not be lost           quorum queue
replay, many consumers reading the same data, big logs   stream
temporary RPC reply / exclusive per-connection queue     classic (exclusive / auto-delete)
non-critical, very high-rate transient messages          classic
replicated classic (mirrored) queues                     removed in 4.0 -> use quorum
```

## Plan migrations off mirrored queues

Systems upgraded from RabbitMQ 3.x may still declare mirrored classic queues through policies. Migrate them to quorum queues before upgrading to 4.x, using a new queue name or a blue-green cluster migration.

**Quiz:** What replaced classic mirrored queues for replicated, durable messaging in RabbitMQ 4.x?

- [x] Quorum queues (and streams)
- [ ] Exclusive queues
- [ ] Auto-delete queues
- [ ] Headers exchanges

*Answer:* Quorum queues (and streams). Mirrored classic queues were removed; quorum queues and streams provide replication.