MQTT PUBACK web sockets

Question

I'm working on HiveMQ Websocket Client and I'm facing some issues with the message delivery. so, I've come across the word PUBACK

let me explain you about my understanding and then I will ask my question.

whenever we send a message with QOS1, the hivemq server will acknowledge the sender with a PUBACK callback.

Now, I'm planning to subscibe to onPubackReceived event in my websockets, but the event is not firing after sending the message.

My Code:

var clientId = ClientIdentifier;


    mqtt = new Messaging.Client(
                    host,
                    port,
                    clientId);
    var options = {
        timeout: 3,
        keepAliveInterval: 60,
        useSSL: useTLS,
        cleanSession: cleansession,
        onSuccess: onConnect,
        onFailure: function (message) {
            connected = false;            
            setTimeout(MQTTconnect, reconnectTimeout);
        }
    };

    mqtt.onConnectionLost = onConnectionLost;
    mqtt.onMessageArrived = onMessageArrived;
    mqtt.onPubackReceived = OnPubackReceived;

Both the onConnectionLost and onMessageArrived are firing properly when a connection lost and message arrived, but the onPubackReceived is not firing.

please let me know, if I have understood it correctly or if I'm doing some mistake?

Answer 1

This not a HiveMQ issue.

My assumption is, that you used the HiveMQ Websocket Client as a starting point for your implementation.

In any case a Paho MQTT Client does not have a onPubackReceived field. If you provide more details about your use case or what's your issue with message delivery, I might be able to point you into the right direction.

EDIT: What you are describing is called Quality of Service 1 in MQTT. It is a guarantee, that a message is received at least once. It is the client implementation's job to keep this guarantees and therefor resend a message, should a PUBACK not be received. Manually interfering with this behaviour in your application would result in inconsistency regarding the client's persistence. For clarification: Simply setting duplicate=truewill not result in a message being recognised as a duplicate. It will also have to have the the same messageID as the original. I was not able to actually find any documentation about paho.jskeeping the Quality of Service = 1. However, MQTT.js does.

QoS 1 : received at least once : The packet is sent and stored as long as the client has not received a confirmation from the server. MQTT ensures that it will be received, but there can be duplicates.

To sum things up:

• Resending of messages, no PUBACK was received on, is the client Object's job. This is part of the MQTT specification.
• Using the MQTT.js works over Websockets and ensures to keep QoS levels

Hope this helps.