How to use Java code to implement RabbitMQ delay queue

Introduction to RabbitMQ delay queue

RabbitMQ delay queue means that after the message is sent to the queue, it is not immediately consumed by the consumer. Instead, it waits for a period of time before being consumed by consumers. This kind of queue is usually used to implement scheduled tasks. For example, if an order times out and is not paid, the system cancels the order and releases the occupied inventory.

There are many ways to implement delay queues in RabbitMQ, the more common of which is to use plug-ins or implement it through the DLX (Dead Letter Exchange) mechanism.

Use plug-ins to implement delayed queues

RabbitMQ provides the rabbitmq_delayed_message_exchange plug-in, which can be used to implement delayed queues. The principle of this plug-in is to send the message to a specific Exchange when the message is sent, and then the Exchange will forward the message to the specified queue according to the delay time in the message, thereby realizing the function of the delay queue. .

To use this plug-in, you need to install the plug-in first, then create an Exchange, set the type of the Exchange to x-delayed-message, and then bind the Exchange to the queue.

Use DLX mechanism to implement delay queue

The TTL of a message is the survival time of the message. RabbitMQ can set TTL for queues and messages respectively. The queue setting is the retention time of the queue without consumers connected, and you can also set separate settings for each individual message. After this time, we consider the message dead and call it a dead letter. If the queue is set and the message is set, the smaller value will be used. Therefore, if a message is routed to different queues, the death time of the message may be different (different queue settings). Here we only talk about the TTL of a single message, because it is the key to achieving delayed tasks. You can set the time by setting the expiration field of the message or the x-message-ttl attribute. Both have the same effect.

The DLX mechanism is a message forwarding mechanism provided by RabbitMQ. It can forward messages that cannot be processed to the designated Exchange, thereby achieving delayed processing of messages. The specific implementation steps are as follows:

• Create an ordinary Exchange and Queue and bind them together.

• Create a DLX Exchange and bind a normal Exchange to the DLX Exchange.

• Set the Queue to have a TTL (Time To Live) attribute and set the message expiration time.

• Bind the Queue to the DLX Exchange.

When the message expires, it will be sent to DLX Exchange, and then DLX Exchange will forward the message to the designated Exchange, thereby realizing the delay queue function.

The advantage of using the DLX mechanism to implement a delay queue is that there is no need to install additional plug-ins, but the expiration time of the message needs to be precisely controlled, otherwise the message expiration time may be inaccurate.

Setting the delay queue in Java language

The following are the steps to set the delay queue through RabbitMQ using Java language:

Install the plug-in

First, you need to install it rabbitmq_delayed_message_exchange Plugin. It can be installed through the following command:

rabbitmq-plugins enable rabbitmq_delayed_message_exchange

Create a delay switch

The delay queue requires the use of a delay switch. A delayed switch can be created using the x-delayed-message type. The following is a sample code for creating a delay switch:

Map<String, Object> args = new HashMap<>();
args.put("x-delayed-type", "direct");
channel.exchangeDeclare("delayed-exchange", "x-delayed-message", true, false, args);

Creating a delay queue

When creating a delay queue, you need to bind the queue to the delay switch and set the TTL of the queue ( Time To Live) parameter. The following is a sample code to create a delay queue:

Map<String, Object> args = new HashMap<>();
args.put("x-dead-letter-exchange", "delayed-exchange");
args.put("x-dead-letter-routing-key", "delayed-queue");
args.put("x-message-ttl", 5000);
channel.queueDeclare("delayed-queue", true, false, false, args);
channel.queueBind("delayed-queue", "delayed-exchange", "delayed-queue");

In the above code, the queue is bound to the delay switch, and the TTL parameter of the queue is set to 5000 milliseconds, that is, after the message is sent to the queue , if it is not consumed by the consumer within 5000 milliseconds, it will be forwarded to the delayed-exchange switch and sent to the delayed-queue queue.

Send a delayed message

When sending a delayed message, you need to set the expiration attribute of the message, which indicates the expiration time of the message. The following is a sample code for sending a delayed message:

Map<String, Object> headers = new HashMap<>();
headers.put("x-delay", 5000);
AMQP.BasicProperties properties = new AMQP.BasicProperties.Builder()
        .headers(headers)
        .expiration("5000")
        .build();
channel.basicPublish("delayed-exchange", "delayed-queue", properties, "Hello, delayed queue!".getBytes());

In the above code, the expiration property of the message is set to 5000 milliseconds and the message is sent to delayed-exchange On the switch, the routing key is delayed-queue, and the message content is "Hello, delayed queue!".

Consuming delayed messages

When consuming delayed messages, you need to set the consumer's QOS (Quality of Service) parameters to control the consumer's concurrent processing capabilities. The following is a sample code for consuming delayed messages:

channel.basicQos(1);
channel.basicConsume("delayed-queue", false, (consumerTag, delivery) -> {
    String message = new String(delivery.getBody(), StandardCharsets.UTF_8);
    System.out.println("Received message: " + message);
    channel.basicAck(delivery.getEnvelope().getDeliveryTag(), false);
});

In the above code, the QOS parameter is set to 1, that is, only one message is processed at a time. Then use the basicConsume method to consume the message in the delayed-queue queue, and after the consumption is completed, use the basicAck method to confirm that the message has been consumed.

Through the above steps, you can implement the RabbitMQ delay queue, which is used to implement functions such as scheduled tasks.

RabbitMQ延时队列是一种常见的消息队列应用场景，它可以在消息发送后指定一定的时间后才能被消费者消费，通常用于实现一些延时任务，例如订单超时未支付自动取消等。

RabbitMQ延时队列具体代码

下面是具体代码（附注释）：

import com.rabbitmq.client.*;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeoutException;

public class DelayedQueueExample {
    private static final String EXCHANGE_NAME = "delayed_exchange";
    private static final String QUEUE_NAME = "delayed_queue";
    private static final String ROUTING_KEY = "delayed_routing_key";

    public static void main(String[] args) throws IOException, TimeoutException {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        Connection connection = factory.newConnection();
        Channel channel = connection.createChannel();
        /*
         Exchange.DeclareOk exchangeDeclare(String exchange,
                                              String type,
                                              boolean durable,
                                              boolean autoDelete,
                                              boolean internal,
                                              Map<String, Object> arguments) throws IOException;
                                              */
        // 创建一个支持延时队列的Exchange
        Map<String, Object> arguments = new HashMap<>();
        arguments.put("x-delayed-type", "direct");
        channel.exchangeDeclare(EXCHANGE_NAME, "x-delayed-message", true, false, arguments);

        // 创建一个延时队列，设置x-dead-letter-exchange和x-dead-letter-routing-key参数
        Map<String, Object> queueArguments = new HashMap<>();
        queueArguments.put("x-dead-letter-exchange", "");
        queueArguments.put("x-dead-letter-routing-key", QUEUE_NAME);
        queueArguments.put("x-message-ttl", 5000);
        channel.queueDeclare(QUEUE_NAME, true, false, false, queueArguments);
        channel.queueBind(QUEUE_NAME, EXCHANGE_NAME, ROUTING_KEY);

        // 发送消息到延时队列中，设置expiration参数
        AMQP.BasicProperties properties = new AMQP.BasicProperties.Builder()
                .expiration("10000")
                .build();
        String message = "Hello, delayed queue!";
        channel.basicPublish(EXCHANGE_NAME, ROUTING_KEY, properties, message.getBytes());
        System.out.println("Sent message to delayed queue: " + message);
        channel.close();
        connection.close();
    }
}

在上面的代码中，我们创建了一个支持延时队列的Exchange，并创建了一个延时队列，设置了x-dead-letter-exchange和x-dead-letter-routing-key参数。然后，我们发送了一条消息到延时队列中，设置了expiration参数，表示这条消息延时10秒后才能被消费。

注意，如果我们想要消费延时队列中的消息，需要创建一个消费者，并监听这个队列。当消息被消费时，需要发送ack确认消息已经被消费，否则消息会一直留在队列中。

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