Ordered messages, also known as first-in-first-out (FIFO) messages, are a type of message provided by Message Queue for Apache RocketMQ. Such messages are published and consumed in a strict order. This topic provides the sample code about how to send and subscribe to ordered messages. In this topic, HTTP client SDK for Java is used as an example.

Background information

Ordered messages are classified into the following types:
  • Globally ordered messages: All messages of a specified topic are published and consumed in first-in-first-out (FIFO) order.
  • Partitionally ordered messages: All messages of a specified topic are distributed to different partitions by using partition keys. The messages in each partition are published and consumed in FIFO order. A partition key is a key field that is used for ordered messages to distinguish among different partitions. It is different from the key of a normal message.

For more information, see Ordered 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 ordered messages

The following sample code provides an example on how to send ordered 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 OrderProducer {

    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 eight messages. 
            for (int i = 0; i < 8; i++) {
                TopicMessage pubMsg = new TopicMessage(
                        // The content of the message. 
                        "hello mq!".getBytes(),
                        // The message tag. 
                        "A"
                );
                // The partition key that is used to distribute ordered messages to different partitions in a topic. A partition key is different from a message key. 
                pubMsg.setShardingKey(String.valueOf(i % 2));
                pubMsg.getProperties().put("a", String.valueOf(i));
                // 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 ordered messages

The following sample code provides an example on how to subscribe to ordered 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 OrderConsumer {

    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. 
        // 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}";

        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. The consumer may pull partitionally ordered messages from multiple partitions. The consumer consumes the messages in the same partition in the order that the messages are sent. 
                // Assume that the consumer pulls partitionally ordered messages. If the broker does not receive an ACK for a message in a partition from the consumer, the broker delivers the same message in the partition to the consumer next time. 
                // The consumer can consume the next batch of messages from a partition only after all the messages pulled from the partition in the previous batch are consumed. 
                // 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.consumeMessageOrderly(
                        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. 
            System.out.println("Receive " + messages.size() + " messages:");
            for (Message message : messages) {
                System.out.println(message);
                System.out.println("ShardingKey: " + message.getShardingKey() + ", a:" + message.getProperties().get("a"));
            }

            // 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);
    }
}