What is the principle of RabbitMQ’s delayed queue?

The principle of RabitMQ’s delayed queue is achieved by using message TTL (Time To Live) and dead letter queue.

The specific principle is as follows:

1. Create a standard queue and set the message TTL in the queue’s attributes, which determines how long a message will survive in the queue.
2. Send delayed messages to this regular queue. Messages will wait in the queue for a period of time until the TTL expires.
3. When the TTL of a message expires, it becomes a “dead letter” and is forwarded to a dead letter queue.
4. You can create a consumer to listen to the dead letter queue and handle delayed messages.

The implementation of delayed queues primarily relies on the RabbitMQ plugin called rabbitmq_delayed_message_exchange. This plugin enables the functionality of delayed queues through custom delay exchanges.

Delay exchange can send messages to a specific queue, automatically forward them to the target queue after a set time, achieving delayed message delivery. It can also forward expired messages to a dead letter queue for further processing.

By utilizing delayed queues, some scenarios that require delayed processing can be achieved, such as delayed task scheduling, message retry mechanisms, and so on.