What is MQTT and How It Works

This article is an introduction to the MQTT protocol. MQTT stands for Message Queuing Telemetry Transport, a simple messaging protocol suitable for communication between IoT devices.

What is MQTT and How it Works

Updated 16 December 2021

What is MQTT?

MQTT stands for Message Queuing Telemetry Transport. MQTT is a simple messaging protocol, designed for constrained devices with low bandwidth. So, it’s the perfect solution to exchange data between multiple IoT devices.

MQTT communication works as a publish and subscribe system. Devices publish messages on a specific topic. All devices that are subscribed to that topic receive the message.

MQTT Publish Subscribe

Its main applications include sending messages to control outputs, read and publish data from sensor nodes and much more.

MQTT Basic Concepts

In MQTT there are a few basic concepts that you need to understand:

MQTT – Publish/Subscribe

The first concept is the publish and subscribe system. In a publish and subscribe system, a device can publish a message on a topic, or it can be subscribed to a particular topic to receive messages

Publish Subscribe Topic MQTT
  • For example Device 1 publishes on a topic.
  • Device 2 is subscribed to the same topic that device 1 is publishing in.
  • So, device 2 receives the message.

MQTT – Messages

Messages are the information that you want to exchange between your devices. It can be a message like a command or data like sensor readings, for example.

MQTT – Topics

Another important concept is the topics. Topics are the way you register interest for incoming messages or how you specify where you want to publish the message.

Topics are represented with strings separated by a forward slash. Each forward slash indicates a topic level. Here’s an example of how you would create a topic for a lamp in your home office:

Note: topics are case-sensitive, which makes these two topics different:

If you would like to turn on a lamp in your home office using MQTT you can imagine the following scenario:

MQTT Publish Subscribe Example
  1. A device publishes “on” and “off” messages on the home/office/lamp topic.
  2. You have a device that controls a lamp (it can be an ESP32, ESP8266, or any other board or device). The ESP32 that controls your lamp, is subscribed to that same topic: home/office/lamp.
  3. So, when a new message is published on that topic, the ESP32 receives the “on” or “off” messages and turns the lamp on or off.

The device that is publishing the messages can be an ESP32, an ESP8266, or an Home Automation controller platform with MQTT support like Node-RED, Home Assistant, Domoticz, or OpenHAB, for example.

mqtt-device

MQTT – Broker

Finally, another important concept is the broker.

The MQTT broker is responsible for receiving all messages, filtering the messages, deciding who is interested in them, and then publishing the message to all subscribed clients.

mqtt-broker

There are several brokers you can use. In home automation projects, we use the Mosquitto Broker installed on a Raspberry Pi. You can also install the Mosquitto broker on your PC (which is not as convenient as using a Raspberry Pi board, because you have to keep your computer running all the time to keep the MQTT connection between your devices).

Having the Mosquitto broker installed on a Raspberry Pi on your local network allows you to exchange data between your IoT devices that are also connected to that same network.

To install Mosquitto broker on the Raspberry Pi follow our tutorial:

You can also run Mosquitto MQTT broker in the cloud. Running the MQTT Mosquitto Broker in the cloud allows you to connect several IoT devices from anywhere using different networks as long as they have an Internet connection. Check the tutorial below:

How to Use MQTT in Home Automation and IoT Projects

MQTT is great for home automation and internet of things projects. Here’s an example of how it can be used in a Home Automation System built with low-cost development boards like a Raspberry Pi, ESP32, ESP8266, and Arduino.

Home Automation System Example
  • A Raspberry Pi runs the Mosquitto broker, which is essential for MQTT protocol.
  • The same Raspberry Pi runs Node-RED, which is a Home Automation Platform with MQTT support—this means it can subscribe to topics to receive messages from the other IoT devices, and publish messages on specific topics to send messages to other devices.
  • Node-RED also allows you to build an User Interface with buttons to control outputs and charts to display sensor readings.
  • The Arduino, the ESP32 and ESP8266 can act as MQTT clients that publish and subscribe to topics.
  • These boards are connected to actuators like LEDs or relays, and sensors like temperature, humidity, smoke sensors, etc..
  • These boars can publish data about the sensor’s state on a specific topic, that Node-RED is also subscribed to. This way, Node-RED receives the sensor readings and can display them on the user interface.
  • On the other side, Node-RED can publish data on a specific topic to control outputs when you use the buttons on the interface. The other boards are also subscribed to that topic and control the outputs accordingly.

The following image shows an example of a Node-RED UI that allows you to control one output and displays temperature and humidity readings:

Here’s a quick summary of the steps you should follow to build something as described previously:

  1. Set up your Raspberry Pi. Follow our Getting Started Guide with Raspberry Pi.
  2. Enable and Connect your Raspberry Pi with SSH.
  3. You need Node-RED installed on your Pi and Node-RED Dashboard.
  4. Install the Mosquitto broker on the Raspberry Pi.
  5. Add the ESP8266 or the ESP32 to this system. You can follow the next MQTT tutorials:

If you want to learn more about these subjects, we have a dedicated course on how to create your own Home Automation System using Raspberry Pi, ESP8266, Arduino, and Node-RED. Just click the following link.

>> Enroll in Build a Home Automation System for $100 <<

Wrapping Up

MQTT is a communication protocol based on a publish and subscribe system. Devices can subscribe to a topic or publish data on a topic. Devices receive messages that are published on topics they are subscribed to.

MQTT is simple to use and it is great for Internet of Things and Home Automation projects. You can check all our MQTT-related tutorials here.

We hope you’ve found this tutorial useful and you now understand what is MQTT and how it works.

Thanks for reading. If you like this article, please support our work by subscribing to my blog.

Watch the video tutorial below



Build Web Server projects with the ESP32 and ESP8266 boards to control outputs and monitor sensors remotely. Learn HTML, CSS, JavaScript and client-server communication protocols DOWNLOAD »

Build Web Server projects with the ESP32 and ESP8266 boards to control outputs and monitor sensors remotely. Learn HTML, CSS, JavaScript and client-server communication protocols DOWNLOAD »


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35 thoughts on “What is MQTT and How It Works”

  1. This is good for many because of the visual aspect. IMHO it’s not quite technical enough, as there are a couple key points such as NOT using a leading “/” in the topic that are omitted, which many folks assume is required due to typical file structure convention, but not for MQTT, so a little embellishment in those lines would make it even better.

    Reply
    • Hi Rick, I totally agree that I could explore in depth those conventions and other aspects. I’ve tried to keep it as simple as possible, because MQTT is still a bit confusing to a lot of people and there are other resources that go in more depth. Thanks for your feedback!
      Rui

      Reply
      • Great article, Rui!

        A Raspberry Pi Zero at under $20 runs an MQTT broker just fine, and it’s a great way to get started with these computers. Or, you can run MQTT on Windows if Linux isn’t your cup of tea (steves-internet-guide.com/install-mosquitto-broker/).

        Reply
  2. Hi. I have an mqtt server at home and one at work but I find it difficult to keep them in sync. Is there a service out “there” that provides free mqtt broker service? Thanks.

    Reply
  3. Hi,
    nice project, but my dht22 doesn’t work ! So I switched to Webserver with same Pin D1 connected to DHT22 and this sample works fine but not reliable ! Pullup on D1pin 4,7kOhm, could I do anything to get a more reliable solution for this topic .
    Mosquitto and node red works fine !!
    Thanks Martin

    Reply
  4. Hi Rui, great explanation of this protocol. I have a question, what are the advantages for preferring mqtt instead of a classical http+mysql database (that are quite lightweight for sending data from few sensors). Actually using mqtt I can’t store the data, but I may at most download after the visualization. So is it more suitable for multiple iot devices? What’s your opinion?
    Thanks
    Paolo

    Reply
  5. I think I’m beginning to get this understood. Please correct me if I’m wrong.

    MQTT must be run on something (R. Pi is a good system) will be the intermediary that passes requests and actions to the clients.

    Node Red isn’t required unless you want to add an interface that you can interact with or that displays data.

    Have I got this right? I was wondering about the difference between Node Red and MQTT.

    Thank you
    Bill

    Reply
    • Hi Bill.

      That’s right. To use MQTT, you need an MQTT broker—which is responsible for getting the messages and sending them to the right subscribers. We like to install the MQTT broker on a Raspberry Pi for MQTT on the local network. You can also install MQTT on your computer. Alternatively, you can also install it on the cloud.

      Node-RED is a Home Automation platform that happens to have support for MQTT (send messages and receive messages from the broker). Node-RED is not essential. You can use it if you want to have an interface, but there are many other different ways to create an interface.

      I hope this is clear now.

      Regards,
      Sara

      Reply
      • One more question: Can the broker and clients all work if none are connected to the internet? I noticed you mentioned “local network” and I’m wondering if this separate system of broker and clients can all work together and without any internet connection.

        Thanks for the education!
        Bill

        Reply
        • Hi.
          Regarding your previous comments, all instructions were tested before publishing and they were working (two days ago). So, I think everything should be fine now.
          All devices need to be connected to the internet.
          Regards,
          Sara

          Reply
  6. I have a few questions
    – Is there any limit to add number of clients in MQTT?
    – MQTT data is passing in which format?
    Btw nice explanation…

    regards,
    Abhi

    Reply

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