Azure node IOT SDK how to set callbacks for MQTT exceptions

Question

I have used the Azure IOT SDK for C in several projects. There's pretty good documentation and it's clear how to register a callback to be notified if a message cannot be sent, or if the MQTT connection drops. Looking over the javascript SDK for node, I can't figure out how to register similar callbacks. For example, after creating the client I can call client.open(onConnect)and the onConnect method is called when the connection is established. I can also call client.sendEvent(message,...) to send a message, and if successful am notified that the message is queued. However:

• How do I register a callback to be notified if the mqtt connection is interrupted?
• How do I register a callback to be told if a packet is successfully received (QOS 1 or 2) or fails to send?
• How do I specify level of QOS that I would like?

Thanks

Answer 1

Firstly, few points to note regarding MQTT QoS support in IoT hub:

• IoT Hub does not support QoS 2 messages. If a device app publishes a message with QoS 2, IoT Hub closes the network connection. When a device app subscribes to a topic with QoS 2, IoT Hub grants maximum QoS level 1 in the SUBACK packet. After that, IoT Hub delivers messages to the device using QoS 1.
• By default, the device SDKs connect to an IoT Hub with QoS 1 for message exchange with the IoT hub.

For details, refer Communicate with your IoT hub using the MQTT protocol.

Now coming to the question, you have 2 options:

Using Azure IoT SDK:

If you want to use IoT hub device sdk, below is one reference from official sample which handles disconnection scenario. Regarding QoS 1 related exchange (like PUBACK confirmation), it does not allow at lower level, but handles internally to ensure guaranteed message delivery. For directly using MQTT, refer later part of my answer.

// Copyright (c) Microsoft. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

'use strict';

const Protocol = require('azure-iot-device-mqtt').Mqtt;
// Uncomment one of these transports and then change it in fromConnectionString to test other transports
// const Protocol = require('azure-iot-device-amqp').AmqpWs;
// const Protocol = require('azure-iot-device-http').Http;
// const Protocol = require('azure-iot-device-amqp').Amqp;
// const Protocol = require('azure-iot-device-mqtt').MqttWs;
const Client = require('azure-iot-device').Client;
const Message = require('azure-iot-device').Message;

// String containing Hostname, Device Id & Device Key in the following formats:
//  "HostName=<iothub_host_name>;DeviceId=<device_id>;SharedAccessKey=<device_key>"
const deviceConnectionString = process.env.DEVICE_CONNECTION_STRING;
let sendInterval;

function disconnectHandler () {
  clearInterval(sendInterval);
  client.open().catch((err) => {
    console.error(err.message);
  });
}

// The AMQP and HTTP transports have the notion of completing, rejecting or abandoning the message.
// For example, this is only functional in AMQP and HTTP:
// client.complete(msg, printResultFor('completed'));
// If using MQTT calls to complete, reject, or abandon are no-ops.
// When completing a message, the service that sent the C2D message is notified that the message has been processed.
// When rejecting a message, the service that sent the C2D message is notified that the message won't be processed by the device. the method to use is client.reject(msg, callback).
// When abandoning the message, IoT Hub will immediately try to resend it. The method to use is client.abandon(msg, callback).
// MQTT is simpler: it accepts the message by default, and doesn't support rejecting or abandoning a message.
function messageHandler (msg) {
  console.log('Id: ' + msg.messageId + ' Body: ' + msg.data);
  client.complete(msg, printResultFor('completed'));
}

function generateMessage () {
  const windSpeed = 10 + (Math.random() * 4); // range: [10, 14]
  const temperature = 20 + (Math.random() * 10); // range: [20, 30]
  const humidity = 60 + (Math.random() * 20); // range: [60, 80]
  const data = JSON.stringify({ deviceId: 'myFirstDevice', windSpeed: windSpeed, temperature: temperature, humidity: humidity });
  const message = new Message(data);
  message.properties.add('temperatureAlert', (temperature > 28) ? 'true' : 'false');
  return message;
}

function errorCallback (err) {
  console.error(err.message);
}

function connectCallback () {
  console.log('Client connected');
  // Create a message and send it to the IoT Hub every two seconds
  sendInterval = setInterval(() => {
    const message = generateMessage();
    console.log('Sending message: ' + message.getData());
    client.sendEvent(message, printResultFor('send'));
  }, 2000);

}

// fromConnectionString must specify a transport constructor, coming from any transport package.
let client = Client.fromConnectionString(deviceConnectionString, Protocol);

client.on('connect', connectCallback);
client.on('error', errorCallback);
client.on('disconnect', disconnectHandler);
client.on('message', messageHandler);

client.open()
.catch(err => {
  console.error('Could not connect: ' + err.message);
});

// Helper function to print results in the console
function printResultFor(op) {
  return function printResult(err, res) {
    if (err) console.log(op + ' error: ' + err.toString());
    if (res) console.log(op + ' status: ' + res.constructor.name);
  };
}

Using MQTT Directly:

As mentioned in Using the MQTT protocol directly (as a device), you can also connect to the public device endpoints using the MQTT protocol on port 8883. For that you can use any standard MQTT library available (for example https://www.npmjs.com/package/mqtt). However, note the below for some special consideration:

In the CONNECT packet, the device should use the following values:

• For the ClientId field, use the deviceId.
• For the Username field, use {iothubhostname}/{device_id}/?api-version=2018-06-30, where {iothubhostname} is the full CName of the IoT hub. For example, if the name of your IoT hub is contoso.azure-devices.net and if the name of your device is MyDevice01, the full Username field should contain: contoso.azure-devices.net/MyDevice01/?api-version=2018-06-30
• For the Password field, use a SAS token. The format of the SAS token is the same as for both the HTTPS and AMQP protocols: SharedAccessSignature sig={signature-string}&se={expiry}&sr={URL-encoded-resourceURI}

Note

If you use X.509 certificate authentication, SAS token passwords are not required. For more information, see Set up X.509 security in your Azure IoT Hub and follow code instructions in the TLS/SSL configuration section.

For more information about how to generate SAS tokens, see the device section of Using IoT Hub security tokens.