一、引言

在具體業務中可能會遇到一些要提前處理的訊息，比如普通客戶的訊息按先進先出的順序處理，Vip客戶的訊息要提前處理。在RabbitMQ中，訊息優先順序的實現方式是：在宣告queue時設定佇列的x-max-priority屬性，然後在publish訊息時，設定訊息的優先順序即可。

RabbitMQ優先順序佇列注意事項：

1）RabbitMQ3.5以後才支援優先順序佇列。

2）只有當消費者不足，不能及時進行消費的情況下，優先順序佇列才會生效。

3）優先順序取值範圍在0~9之間，數值越大則優先順序越高。

二、示例

2.1、傳送端(生產端)

新建一個控制檯專案Send，並新增一個類RabbitMQConfig。

    class RabbitMQConfig
    {
        public static string Host { get; set; }

        public static string VirtualHost { get; set; }

        public static string UserName { get; set; }

        public static string Password { get
 
; set; }

        public static int Port { get; set; }

        static RabbitMQConfig()
        {
            Host = "192.168.2.242";
            VirtualHost = "/";
            UserName = "hello";
            Password = "world";
            Port = 5672;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("按任意鍵開始生產。");
            Console.ReadLine();
            PriorityMessagePublish();
            Console.ReadLine();
        }

        private static void PriorityMessagePublish()
        {
            const string MessagePrefix = "message_";
            const int PublishMessageCount = 6;
            byte messagePriority = 0;

            var factory = new ConnectionFactory()
            {
                HostName = RabbitMQConfig.Host,Port = RabbitMQConfig.Port,VirtualHost = RabbitMQConfig.VirtualHost,UserName = RabbitMQConfig.UserName,Password = RabbitMQConfig.Password,Protocol = Protocols.DefaultProtocol
            };

            using (var connection = factory.CreateConnection())
            {
                using (var channel = connection.CreateModel())
                {
                    //設定佇列優先順序，取值範圍在0~255之間。
                    Dictionary<string,object> dict = new Dictionary<string,object>
                    {
                        { "x-max-priority",255 }
                    };

                    //宣告佇列
                    channel.QueueDeclare(queue: "priority",durable: true,exclusive: false,autoDelete: false,arguments: dict);


                    //向該訊息佇列傳送訊息message
                    Random random = new Random();
                    for (int i = 0; i < PublishMessageCount; i++)
                    {
                        var properties = channel.CreateBasicProperties();
                        messagePriority = (byte)random.Next(0,9);
                        properties.Priority = messagePriority;//設定訊息優先順序，取值範圍在0~9之間。
                        var message = MessagePrefix + i.ToString();
                        var body = Encoding.UTF8.GetBytes(message);
                        channel.BasicPublish(exchange: "",routingKey: "priority",basicProperties: properties,body: body);
                        Console.WriteLine($"{DateTime.Now.ToString()} Send {message},Priority {messagePriority}");
                    }
                }
            }
        }
    }

2.2、接收端(消費端)

新建一個控制檯專案Receive，按住Alt鍵，將傳送端RabbitMQConfig類拖一個快捷方式到Receive專案中。

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("按任意鍵開始消費。");
            Console.ReadLine();
            PriorityMessageSubscribe();
        }

        public static void PriorityMessageSubscribe()
        {
            var factory = new ConnectionFactory()
            {
                HostName = RabbitMQConfig.Host,Password = RabbitMQConfig.Password
            };

            using (var connection = factory.CreateConnection())
            {
                using (var channel = connection.CreateModel())
                {
                    channel.BasicQos(prefetchSize: 0,prefetchCount: 1,global: false);
                    var consumer = new EventingBasicConsumer(channel);
                    consumer.Received += async (model,ea) =>
                    {
                        await Task.Run(() =>
                        {
                            var message = Encoding.UTF8.GetString(ea.Body);
                            Thread.Sleep(1000 * 2);
                            channel.BasicAck(deliveryTag: ea.DeliveryTag,multiple: false);//手動訊息確認
                            Console.WriteLine($"{DateTime.Now.ToString()} Received {message}");
                        });
                    };
                    channel.BasicConsume(queue: "priority",noAck: false,consumer: consumer);//需要啟用訊息響應，否則priority無效。
                    Console.ReadKey();
                }
            }
        }
    }

2.3、執行結果

從消費情況可以看出，message_2及message_3由於priority優先順序最高都是7，所以它們會被最早消費，而message_5的priority是0，所以最後才被消費。