How to send MQTT "message" event back to REST body? - node.js

Question

I'm currently having problems figuring out how to capture my MQTT message event back to my REST API body which is written in NodeJS. My current setup is App -> NodeJS REST API -> MQTT broker inside RPi 3.
This is my MQTTHandler.js class where I have put all my reusable MQTT functions
const mqtt = require('mqtt')
class MQTTHandler {
constructor (host) {
this.client = null
this.host = host
}
connect () {
this.client = mqtt.connect(this.host)
this.client.on('error', function (err) {
console.log(err)
this.client.end()
})
this.client.on('connect', function () {
console.log('MQTT client connected...')
})
// I need this to send message back to app.js
this.client.on('message', function (topic, message) {
if (!message.toString()) message = 'null'
console.log(JSON.parse(message.toString()))
})
this.client.on('close', function () {
console.log('MQTT client disconnected...')
})
}
subscribeTopic (topic) {
this.client.subscribe(topic)
}
unsubscribeTopic (topic) {
this.client.unsubscribe(topic)
}
sendMessage (topic, message) {
this.client.publish(topic, message)
}
}
module.exports = MQTTHandler
And below is a short snippet of my app.js
const MQTTHandler = require('./mqtt.handler')
...
var mqttClient = new MQTTHandler('mqtt://127.0.0.1')
mqttClient.connect()
app.get('/hello', function (req, res) {
mqttClient.subscribeTopic('topic')
mqttClient.sendMessage('topic', 'hello world')
// I need to return the MQTT message event here
// res.json(<mqtt message here>)
res.end()
})
I have already tried using NodeJS' event emitter but it doesn't seem to work. Any help or suggestions would be much appreciated, thank you!

Answer 1

You are trying to mix a synchronous protocol (HTTP) with and asynchronous protocol (MQTT). These 2 paradigm don't easily mix.
When you publish an MQTT message you have no idea how many clients may be subscribed to that topic, it could be zero, it could be many. There is also no guarantee that any of them will send a reply so you need to include a timeout. (You also need to include a request id in the payload so you can coordinate any response with the request as you can't say anything about what order responses may come in.)
Your example code is only using 1 topic, this is very bad as you will end up needing to filter out request messages from response messages. Better to use 2 different topics (MQTT v5 even has a msg header to specify the topic the response should be sent on).
Having said all that it is possible to build something that will work (I will use request and reply topics.
var inflightRequests = {};
// interval to clear out requests waiting for a response
// after 3 seconds
var timer = setInterval(function() {
var now = new Date.now();
var keys = Object.keys(inflightRequests);
for (var key in keys) {
var waiting = inflightRequests[keys[key]];
var diff = now = waiting.timestamp;
// 3 second timeout
if (diff > 3000) {
waiting.resp.status(408).send({});
delete(inflightRequests[keys[key]]);
}
}
},500);
// on message handler to reply to the HTTP request
client.on('message', function(topic, msg){
if (topic.equals('reply')) {
var payload = JSON.parse(msg);
var waiting = inflightRequest[payload.requestId];
if (waiting) {
waiting.res.send(payload.body);
delete(inflightRequests[payload.requestId]);
} else {
// response arrived too late
}
}
});
// HTTP route handler.
app.get('/hello', function(req, res) {
//using timestamp as request Id as don't have anything better in this example.
var reqId = Date.now();
var waiting = {
timeStamp: reqId,
res: res
}
inflightRequests[reqId] = waiting;
var message = {
requestId: reqId,
payload: 'hello world'
}
client.publish('request',JSON.stringify(message));
});