MQTT with ExpressJs

This documentation covers what I had learnt while setting up MQTT with an Express server built using NodeJS. For this project we had written a script to detect a bot (hardware) and continuously track it using YOLO.

Python Code

The camera will keep detecting the bot and track it, when the bot reaches its sticker the camera should run a post request in Python.

The script for the python script is as follows -

import requests
endpoint = 'http://localhost:3000/send-mqtt'
data = {
    'message': 'Right'
}
r = requests.post(url=endpoint, data=data)

If one is connecting via another device they have to simply replace localhost with the IP address of the device the server is being run on.

IP of the server can be found by typing in ipconfig in Windows and scrolling to the IPv4 section.

Note - If the script gives an error saying the module requests is not found just run pip install requests.

The Server

The broker used for sending messages is mosquitto. The url is as follows: mqtt://test.mosquitto.org. The following server is open to anyone hence sensitive messages should be avoided.

Using NodeJs

The first steps involve installing Node itself from their website here.

After Node is setup in a new folder run the following line of code: npm init

Fill in all the details or just keep hitting enter till a package.json file is formed.

Proceed to install the express, body-parser and mqtt packages by using the following command: npm i express body-parser mqtt

If somehow this command fails install each separately in the following format: npm i <package name>

Server Code

var express = require("express");
var bodyParser = require("body-parser");
var app = express();
var mqttHandler = require("./mqtt_handler");

app.use(express.json());
app.use(express.urlencoded({ extended: true }));

var mqttClient = new mqttHandler();
mqttClient.connect();

// Routing
app.post("/send-mqtt", function (req, res) {
  mqttClient.sendMessage(req.body.message);
  res.status(200).send("Message sent to mqtt");
});

var server = app.listen(3000, function () {
  console.log("App running on port.", server.address().port);
});

MQTT Handler

const mqtt = require("mqtt");

class MqttHandler {
  constructor() {
    this.mqttClient = null;
    this.host = "mqtt://test.mosquitto.org";
    this.username = "YOUR_USER"; // mqtt credentials if these are needed to connect
    this.password = "YOUR_PASSWORD";
  }

  connect() {
    // Connect mqtt with credentials (in case of needed, otherwise we can omit 2nd param)
    this.mqttClient = mqtt.connect(this.host);
    // , { username: this.username, password: this.password }
    // Mqtt error calback
    this.mqttClient.on("error", (err) => {
      console.log("An error has occured: ", err);
      this.mqttClient.end();
    });

    // Connection on success
    this.mqttClient.on("connect", () => {
      console.log(`Mqtt client connected`);
    });

    // Mqtt subscriptions
    this.mqttClient.subscribe("mytopic", { qos: 0 });

    // Logging a message when it arives
    this.mqttClient.on("message", function (topic, message) {
      if (!message.toString().startsWith("32333")) {
        console.log(message.toString());
      }
    });

    this.mqttClient.on("close", () => {
      console.log(`mqtt client disconnected`);
    });
  }

  // Sends a mqtt message to topic: mytopic
  sendMessage(message) {
    this.mqttClient.publish("mytopic", message);
  }
}

module.exports = MqttHandler;