This topic provides the sample code about how to send and subscribe to scheduled messages and delayed messages. In this topic, HTTP client SDK for Java is used as an example.

Background information

  • Delayed messages: A producer sends a message to the Message Queue for Apache RocketMQ broker and expects the message to be delivered to a consumer after a specified period of time. The message is a delayed message.
  • Scheduled messages: A producer sends a message to the Message Queue for Apache RocketMQ broker and expects the message to be delivered to a consumer at a specified point in time in the future. The message is a scheduled message.
The code configurations of scheduled messages and delayed messages are the same. In essence, they are delivered to consumers after a specified period of time based on the attributes in the messages.

For more information, see Scheduled messages and delayed messages.

Prerequisites

Before you begin, make sure that the following operations are complete:

  • SDK for Java is installed. For more information, see Prepare the environment.
  • Resources are created. The resources that you want to specify in the code are created in advance. For example, the instance, topic, and group ID that you want to specify in the code are created in the Message Queue for Apache RocketMQ console in advance. For more information, see Create resources.

Send scheduled messages or delayed messages

The following sample code provides an example on how to send scheduled messages or delayed messages:

import com.aliyun.mq.http.MQClient;
import com.aliyun.mq.http.MQProducer;
import com.aliyun.mq.http.model.TopicMessage;

import java.util.Date;

public class Producer {

    public static void main(String[] args) {
        MQClient mqClient = new MQClient(
                // The HTTP endpoint. To obtain the HTTP endpoint, log on to the Message Queue for Apache RocketMQ console. In the left-side navigation pane, click Instances. On the Instances page, click the name of your instance. On the Instance Details page, scroll to the Basic Information section and view the endpoint on the Endpoints tab. 
                "${HTTP_ENDPOINT}",
                // The AccessKey ID that you created in the Resource Access Management (RAM) console for identity authentication. 
                "${ACCESS_KEY}",
                // The AccessKey secret that you created in the RAM console for identity authentication. 
                "${SECRET_KEY}"
        );

        // The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
        // Each topic can be used to send and subscribe to messages of a specific type. For example, a topic that is used to send and subscribe to normal messages cannot be used to send and subscribe to messages of other types. 
        final String topic = "${TOPIC}";
        // The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
        // If the instance has a namespace, the instance ID must be passed in. If the instance does not have a namespace, pass in null or an empty string. Check whether your instance has a namespace on the Instance Details page in the Message Queue for Apache RocketMQ console.        
        final String instanceId = "${INSTANCE_ID}";

        // Obtain the producer that publishes messages to the topic. 
        MQProducer producer;
        if (instanceId != null && instanceId != "") {
            producer = mqClient.getProducer(instanceId, topic);
        } else {
            producer = mqClient.getProducer(topic);
        }

        try {
            // Cyclically send four messages. 
            for (int i = 0; i < 4; i++) {
                TopicMessage pubMsg;
                pubMsg = new TopicMessage(
                // The content of the message. 
                "hello mq!".getBytes(),
                // The message tag. 
                "A"
                 );
                 // The custom attributes of the message. 
                 pubMsg.getProperties().put("a", String.valueOf(i));
                 // The key of the message. 
                 pubMsg.setMessageKey("MessageKey");
                 // The period of time after which the broker delivers the message. In this example, the broker will deliver the message to the consumer 10 seconds later. The parameter value is a timestamp in milliseconds. 
                 // If the producer sends a scheduled message, set the parameter value to the time difference between the scheduled time and the current time. 
                 pubMsg.setStartDeliverTime(System.currentTimeMillis() + 10 * 1000);             
    
             // Send the message in synchronous mode. If no error occurs, the message is sent. 
             TopicMessage pubResultMsg = producer.publishMessage(pubMsg);

             // Send the message in synchronous mode. If no error occurs, the message is sent. 
             System.out.println(new Date() + " Send mq message success. Topic is:" + topic + ", msgId is: " + pubResultMsg.getMessageId()
                        + ", bodyMD5 is: " + pubResultMsg.getMessageBodyMD5());
            }
        } catch (Throwable e) {
            // Specify the logic to resend or persist the message if the message fails to be sent. 
            System.out.println(new Date() + " Send mq message failed. Topic is:" + topic);
            e.printStackTrace();
        }

        mqClient.close();
    }

}

Subscribe to scheduled messages or delayed messages

The following sample code provides an example on how to subscribe to scheduled messages or delayed messages:

import com.aliyun.mq.http.MQClient;
import com.aliyun.mq.http.MQConsumer;
import com.aliyun.mq.http.common.AckMessageException;
import com.aliyun.mq.http.model.Message;

import java.util.ArrayList;
import java.util.List;

public class Consumer {

    public static void main(String[] args) {
        MQClient mqClient = new MQClient(
                // The HTTP endpoint. To obtain the HTTP endpoint, log on to the Message Queue for Apache RocketMQ console. In the left-side navigation pane, click Instances. On the Instances page, click the name of your instance. On the Instance Details page, scroll to the Basic Information section and view the endpoint on the Endpoints tab. 
                "${HTTP_ENDPOINT}",
                // The AccessKey ID that you created in the RAM console for identity authentication. 
                "${ACCESS_KEY}",
                // The AccessKey secret that you created in the RAM console for identity authentication. 
                "${SECRET_KEY}"
        );

        // The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
        // Each topic can be used to send and subscribe to messages of a specific type. For example, a topic that is used to send and subscribe to normal messages cannot be used to send and subscribe to messages of other types. 
        final String topic = "${TOPIC}";
        // The group ID that you created in the Message Queue for Apache RocketMQ console. 
        final String groupId = "${GROUP_ID}";
        // The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
        final String instanceId = "${INSTANCE_ID}";

        final MQConsumer consumer;
        if (instanceId != null && instanceId != "") {
            consumer = mqClient.getConsumer(instanceId, topic, groupId, null);
        } else {
            consumer = mqClient.getConsumer(topic, groupId);
        }

        // Cyclically consume messages in the current thread. We recommend that you use multiple threads to concurrently consume messages. 
        do {
            List<Message> messages = null;

            try {
                // Consume messages in long polling mode. 
                // In long polling mode, if no message in the topic is available for consumption, the request is suspended on the broker for a specified period of time. If a message is available for consumption within the duration, a response is immediately sent to the consumer. In this example, the duration is 3 seconds. 
                messages = consumer.consumeMessage(
                        3,// The maximum number of messages that can be consumed at a time. In this example, the value is set to 3. The largest value you can set is 16. 
                        3// The duration of a long polling cycle. Unit: seconds. In this example, the value is set to 3. The largest value you can set is 30. 
                );
            } catch (Throwable e) {
                e.printStackTrace();
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e1) {
                    e1.printStackTrace();
                }
            }
            // No messages in the topic are available for consumption. 
            if (messages == null || messages.isEmpty()) {
                System.out.println(Thread.currentThread().getName() + ": no new message, continue!");
                continue;
            }

            // Specify the business processing logic. 
            for (Message message : messages) {
                System.out.println("Receive message: " + message);
            }

            // If the broker does not receive an acknowledgment (ACK) from the consumer before the delivery retry interval elapses, the message will be consumed again. 
            // A unique timestamp is specified for the handle of a message each time the message is consumed. 
            {
                List<String> handles = new ArrayList<String>();
                for (Message message : messages) {
                    handles.add(message.getReceiptHandle());
                }

                try {
                    consumer.ackMessage(handles);
                } catch (Throwable e) {
                    // The broker may fail to receive an ACK for a message from the consumer if the handle of the message times out. 
                    if (e instanceof AckMessageException) {
                        AckMessageException errors = (AckMessageException) e;
                        System.out.println("Ack message fail, requestId is:" + errors.getRequestId() + ", fail handles:");
                        if (errors.getErrorMessages() != null) {
                            for (String errorHandle :errors.getErrorMessages().keySet()) {
                                System.out.println("Handle:" + errorHandle + ", ErrorCode:" + errors.getErrorMessages().get(errorHandle).getErrorCode()
                                        + ", ErrorMsg:" + errors.getErrorMessages().get(errorHandle).getErrorMessage());
                            }
                        }
                        continue;
                    }
                    e.printStackTrace();
                }
            }
        } while (true);
    }
}