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 by using HTTP client SDK for C#.

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 C# 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:

using System;
using System.Collections.Generic;
using System.Threading;
using Aliyun.MQ.Model;
using Aliyun.MQ.Model.Exp;
using Aliyun.MQ.Util;

namespace Aliyun.MQ.Sample
{
    public class OrderProducerSample
    {
        // 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. 
        private const string _endpoint = "${HTTP_ENDPOINT}";
        // The AccessKey ID that you created in the Resource Access Management (RAM) console for identity authentication. 
        private const string _accessKeyId = "${ACCESS_KEY}";
        // The AccessKey secret that you created in the RAM console for identity authentication. 
        private const string _secretAccessKey = "${SECRET_KEY}";
        // The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
        private const string _topicName = "${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.
        private const string _instanceId = "${INSTANCE_ID}";

        private static MQClient _client = new Aliyun.MQ.MQClient(_accessKeyId, _secretAccessKey, _endpoint);

        static MQProducer producer = _client.GetProducer(_instanceId, _topicName);

        static void Main(string[] args)
        {
            try
            {
                // Cyclically send eight messages. 
                for (int i = 0; i < 8; i++)
                {
                    // The content and tag of the message. 
                    TopicMessage sendMsg = new TopicMessage("hello mq", "tag");
                    // The custom attributes of the message. 
                    sendMsg.PutProperty("a", i.ToString());
                    // The partition key that is used to distribute ordered messages to a specific partition. The partition key distinguishes the partition from other partitions. A partition key is different from a message key. 
                    sendMsg.ShardingKey = (i % 2).ToString();
                    TopicMessage result = producer.PublishMessage(sendMsg);
                    Console.WriteLine("publis message success:" + result);
                }
            }
            catch (Exception ex)
            {
                Console.Write(ex);
            }
        }
    }
}

Subscribe to ordered messages

The following sample code provides an example on how to subscribe to ordered messages:

using System;
using System.Collections.Generic;
using System.Threading;
using Aliyun.MQ.Model;
using Aliyun.MQ.Model.Exp;
using Aliyun.MQ;

namespace Aliyun.MQ.Sample
{
    public class OrderConsumerSample
{
        // 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. 
        private const string _endpoint = "${HTTP_ENDPOINT}";
        // The AccessKey ID that you created in the RAM console for identity authentication. 
        private const string _accessKeyId = "${ACCESS_KEY}";
        // The AccessKey secret that you created in the RAM console for identity authentication. 
        private const string _secretAccessKey = "${SECRET_KEY}";
        // The topic of the message. The topic must have been created in the Message Queue for Apache RocketMQ console. 
        private const string _topicName = "${TOPIC}";
        // The group ID that you created in the Message Queue for Apache RocketMQ console. 
        private const string _groupId = "${GROUP_ID}";
        // The ID of the instance to which the topic belongs. The instance must have been created in the Message Queue for Apache RocketMQ console. 
        // If the instance has a namespace, the instance ID must be passed in. If the instance does not have a namespace, pass in null or an empty string. Check whether your instance has a namespace on the Instance Details page in the Message Queue for Apache RocketMQ console.
        private const string _instanceId = "${INSTANCE_ID}";
        
        private static MQClient _client = new Aliyun.MQ.MQClient(_accessKeyId, _secretAccessKey, _endpoint);
        static MQConsumer consumer = _client.GetConsumer(_instanceId, _topicName, _groupId, null);

        static void Main(string[] args)
        {
            // Cyclically consume messages in the current thread. We recommend that you use multiple threads to concurrently consume messages. 
            while (true)
            {
                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 a message in one partition is not consumed by the consumer, the consumer consumes the unconsumed message in the partition 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. 
                    List<Message> messages = null;

                    try
                    {
                        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 (Exception exp1)
                    {
                        if (exp1 is MessageNotExistException)
                        {
                            Console.WriteLine(Thread.CurrentThread.Name + " No new message, " + ((MessageNotExistException)exp1).RequestId);
                            continue;
                        }
                        Console.WriteLine(exp1);
                        Thread.Sleep(2000);
                    }

                    if (messages == null)
                    {
                        continue;
                    }

                    List<string> handlers = new List<string>();
                    Console.WriteLine(Thread.CurrentThread.Name + " Receive Messages:");
                    // Specify the business processing logic. 
                    foreach (Message message in messages)
                    {
                        Console.WriteLine(message);
                        Console.WriteLine("Property a is:" + message.GetProperty("a"));
                        handlers.Add(message.ReceiptHandle);
                    }
                    // If the broker does not receive an ACK from the consumer before the period of time specified by the Message.nextConsumeTime parameter elapses, the message will be consumed again. 
                    // A unique timestamp is specified for the handle of a message each time the message is consumed. 
                    try
                    {
                        consumer.AckMessage(handlers);
                        Console.WriteLine("Ack message success:");
                        foreach (string handle in handlers)
                        {
                            Console.Write("\t" + handle);
                        }
                        Console.WriteLine();
                    }
                    catch (Exception exp2)
                    {
                        // The broker may fail to receive an ACK for a message from the consumer if the handle of the message times out. 
                        if (exp2 is AckMessageException)
                        {
                            AckMessageException ackExp = (AckMessageException)exp2;
                            Console.WriteLine("Ack message fail, RequestId:" + ackExp.RequestId);
                            foreach (AckMessageErrorItem errorItem in ackExp.ErrorItems)
                            {
                                Console.WriteLine("\tErrorHandle:" + errorItem.ReceiptHandle + ",ErrorCode:" + errorItem.ErrorCode + ",ErrorMsg:" + errorItem.ErrorMessage);
                            }
                        }
                    }
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex);
                    Thread.Sleep(2000);
                }
            }
        }
    }
}