订阅者未主动接收已发布的消息

Question

I have successfully connected MQTT to my broker both publisher and subscriber are working fine but there is a problem my subscriber is not actively receiving the message, the function mqtt_publishedRecieved triggers only one time,when I restart the subscriber app and only one message receives at a time. in order to get another message, I have to restart my app again. Well, as per my understanding it is because I called the config method of my class at startup. so therefore It checks for the subscribed topic only at startup. But I really want that functionality in my subscriber class to immediately receive the message once it is published by the receiver, which means both should work parallel. My requirement is => if subscriber is connected, receives the message as soon as publisher fires. => if the subscriber is disconnected, message should be queued and later on all message will receive when subscriber connected again.

I do a research mqtt clean session set to false will ensure the persistent session so I set that flag to false to occupy persistent session, but it won't work for me.

it seems to me that I should add functionality to receive a message, for example, a button, so when the button is clicked it starts to get a message, but I can't set trigger/callback/button to receive the message for the subscribed topic. My app should start receiving all the published messages when service is started and stopped when it is disconnected.

below is the code of the publisher class.

public class publisher : IDatabaseSubscription
    {
public publisher()
        {

                    _mqttClient = new MqttClient("127.0.0.1");
                    _mqttClient.Connect("clientId", null, null, false, 60);
                
              
            }
 private void MQTT_OnChanged(object sender, RecordChangedEventArgs<EventDto> e)
        {
           
                                    if (_mqttClient != null && _mqttClient.IsConnected)
                                    {
                                      var message = System.Text.Encoding.UTF8.GetBytes("Hello from app 1");
                                        var statusCode = _mqttClient.Publish("Message1",message , MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE, true);
                                        
                                }
                            }
    }

below is the code of subscriber class

 public class subscriber
    {
        private MqttClient _mqttClient;
        public subscriber()
        {
           
           
        }
        public void configure ()
        {
            _mqttClient = new MqttClient("127.0.0.1");
            _mqttClient.MqttMsgPublishReceived += client_MqttMsgPublishReceived;
            var status = _mqttClient.Subscribe(new string[] { "Message1" }, new byte[] { MqttMsgBase.QOS_LEVEL_EXACTLY_ONCE });
            _mqttClient.Connect("clientId", null, null , false, 60) ;
        }

        private void client_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
        {
            var message = System.Text.Encoding.UTF8.GetString(e.Message);
        }
}

below is the code in startup

if(IsSubscriptionEnabled())
            {
                var service = _container.GetInstance<subscription>();
                service.configure();
            }
        
    

Answer 1

这是由于发布者和订阅者中的客户端 ID 相同，如果新客户端连接到相同的客户端 ID，则代理会启动客户端。