Node.Js MQTT

Compare

How to Use MQTT with Node.js

Now that you understand the basics of MQTT and Node.js, let’s move on to the hands-on tutorial.

Prerequisites

• Basic experience in Node.js programming
• Familiarity with MQTT will help (which we covered above!)

Download Node.js if you don’t already have it. If you are not sure whether Node.js is installed on your machine, open up a terminal window and enter the following command:

node --version

If it provides you with a version number, you are good to go.

Install the MQTT.js package

We’ll be using the MQTT.js package in this tutorial. You can find the source code here. Run this npm command to install it:

npm install mqtt

Once installed, you can import the package in your code as follows:

const mqtt = require('mqtt')

Connecting to a broker

If you see the API reference, there are two ways to connect to a client: pass in a client URL or add an options JSON. Let’s see examples with both.

Using URL:

const client  = mqtt.connect('mqtt://test.mosquitto.org');

Using options:

const options = {
    port: 1883,
    host: 'broker.hivemq.com',
    clientId: 'myclient',
};
const client = mqtt.connect(options);

Here, we define the clientID (which is an identifier for the broker), and we connect to the host (broker) over the specified port. Note that the client ID needs to be unique, i.e., two clients sharing the same client ID cannot communicate with the broker at the same time.

These aren’t the only settings you can specify in the options JSON. You can find the exhaustive list in the MQTT readme. You can add all the relevant settings (keep-alive time, last-will, etc.) that you require in the JSON. We will see a more detailed example in the tutorial below.

Once the connection is successful, a ‘connect’ event will be triggered. You can take relevant actions, like subscribing to topics, or publishing to topics whenever the event is triggered.

client.on('connect', function () {
  //Do whatever you want to do on connection
});

Publishing and Subscribing

For publishing, the name of the topic and the message are required fields. Optional settings, like QoS level, whether the message should be retained, etc., can be specified in the options json. You can also optionally specify a callback function to be executed when the QoS handling is complete, or, in the case of QoS 0, at the next tick.

let pub_topic = 'ys/pub';
let message = 'Greetings from client';
let pub_options = {qos:0, retain:true};
client.publish(pub_topic, message, pub_options, function(err) {
            if(err) {
                console.log("An error occurred during publish")
            } else {
                console.log("Published successfully")
            }
        });

You can read more about the publishing options in this section of the MQTT readme](https://github.com/mqttjs/MQTT.js#mqttclientpublishtopic-message-options-callback). The subscribe syntax is similar. The only differences are that there is no message argument, and instead of a single topic, you can specify an array of topics to subscribe, or even a object, with different QoS levels for each topic, like {‘topic1’:{‘qos’:0},’topic2’:{‘qos’:2}}. An example snippet is given below:

let sub_topic = 'ys/sub';
let sub_options = {qos:0};
client.subscribe(sub_topic, sub_options, function (err) {
        if (err) {
            console.log("An error occurred while subscribing");
        } else {
            console.log("Subscribed successfully");
        }
    });

Apart from publish and subscribe, there are other methods and properties of the Client class that you should be aware of. Here is a brief overview of the important methods and properties:

Client class methods and properties

Events

In the ‘Connecting to a broker’ section above, we saw the example of one event: ‘connect’. There are several other events as well (you can find the API reference in this section of the MQTT readme). A brief description of the frequently used events is below. You can trigger relevant actions for each event.

MQTT Events

Preparing the code

const mqtt = require('mqtt');

// Connection variables
let broker_host = 'broker.hivemq.com';
let broker_port = 1883;
let client_id = 'ys_client';

// Publish variables
let pub_topic = 'ys/pub';
let message = 'Greetings from ' + client_id.toString();
let pub_options = {qos: 0, retain: false};

// Subscribe variables
let sub_topic = 'ys/pub';
let sub_options = {qos: 0};


const connection_options = {
    port: broker_port,
    host: broker_host,
    clientId: client_id,
    reconnectPeriod: 5000 // Try reconnecting in 5 seconds if connection is lost
};

const client = mqtt.connect(connection_options);

client.on('message', function (topic, message) {
    console.log("Received message " + message.toString() + " on topic: " + topic.toString());
})

client.on('connect', async function () {
    console.log('Connection successful');
    client.subscribe(sub_topic, sub_options, function (err) {
        if (err) {
            console.log("An error occurred while subscribing")
        } else {
            console.log("Subscribed successfully to " + sub_topic.toString())
        }
    });

    while (client.connected) {
        client.publish(pub_topic, message, pub_options, function (err) {
            if (err) {
                console.log("An error occurred during publish")
            } else {
                console.log("Published successfully to " + pub_topic.toString())
            }
        });

        // Delay of 5 seconds
        await new Promise(resolve => setTimeout(resolve, 5000));
    }
})

// Handle errors
client.on("error", function (error) {
    console.log("Error occurred: " + error);
});

// Notify reconnection
client.on("reconnect", function () {
    console.log("Reconnection starting");
});

// Notify offline status
client.on("offline", function () {
    console.log("Currently offline. Please check internet!");
});

If the connection is successful, we publish to the broker on the specified topic every 5 seconds. If the connection is lost, we attempt re-connection every 5 seconds (specified by the reconnectPeriod setting in connection_options JSON). We log all events on the console, including error, reconnection, and offline.

Testing it locally

Open a terminal window and navigate to the directory containing the file you just saved. Enter the following command:

node <file_name> .js

You should be able to see all the events logs in the terminal window. Try turning off the internet on your machine briefly, and then turning it back ON again. You should be able to see the system auto-recover.

This content was produced by Inbound Square