Book Image

MQTT Essentials - A Lightweight IoT Protocol

5 (1)
Book Image

MQTT Essentials - A Lightweight IoT Protocol

5 (1)

Overview of this book

This step-by-step guide will help you gain a deep understanding of the lightweight MQTT protocol. We’ll begin with the specific vocabulary of MQTT and its working modes, followed by installing a Mosquitto MQTT broker. Then, you will use best practices to secure the MQTT Mosquitto broker to ensure that only authorized clients are able to publish and receive messages. Once you have secured the broker with the appropriate configuration, you will develop a solution that controls a drone with Python. Further on, you will use Python on a Raspberry Pi 3 board to process commands and Python on Intel Boards (Joule, Edison and Galileo). You will then connect to the MQTT broker, subscribe to topics, send messages, and receive messages in Python. You will also develop a solution that interacts with sensors in Java by working with MQTT messages. Moving forward, you will work with an asynchronous API with callbacks to make the sensors interact with MQTT messages. Following the same process, you will develop an iOS app with Swift 3, build a website that uses WebSockets to connect to the MQTT broker, and control home automation devices with HTML5, JavaScript code, Node.js and MQTT messages
Table of Contents (16 chapters)
MQTT Essentials - A Lightweight IoT Protocol
About the Author
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Working with the publish-subscribe pattern

Before we dive deep into MQTT, we must understand the publish-subscribe pattern, also known as the pub-sub pattern. In the publish-subscribe pattern, a client that publishes a message is decoupled from the other client or clients that receive the message. The clients don't know about the existence of the other clients. A client can publish messages of a specific type and only the clients that are interested in specific types of messages will receive the published messages.

The publish-subscribe pattern requires a broker, also known as server. All the clients establish a connection with the broker. The client that sends a message through the broker is known as the publisher. The broker filters the incoming messages and distributes them to the clients that are interested in the type of received messages. The clients that register to the broker as interested in specific types of messages are known as subscribers. Hence, both publishers and subscribers establish a connection with the broker.

It is easy to understand how things work with a simple diagram. The following diagram shows one publisher and two subscribers connected to a broker:

A Raspberry Pi 3 board with an altitude sensor wired to it is a publisher that establishes a connection with the broker. An iOS smartphone and an Android tablet are two subscribers that establish a connection with the broker.

The iOS smartphone indicates to the broker that it wants to subscribe to all the messages that belong to the sensor1/altitude topic. The Android tablet indicates the same to the broker. Hence, both the iOS smartphone and the Android tablet are subscribed to the sensor1/altitude topic.


A topic is a named logical channel and it is also referred to as a channel or subject. The broker will send publishers only the messages published to topics to which they are subscribed.

The Raspberry Pi 3 board publishes a message with 100 feet as the payload and sensor1/altitude as the topic. The board, that is, the publisher, sends the publish request to the broker.


The data for a message is known as payload. A message includes the topic to which it belongs and the payload.

The broker distributes the message to the two clients that are subscribed to the sensor1/altitude topic: the iOS smartphone and the Android tablet.

Publishers and subscribers are decoupled in space because they don't know each other. Publishers and subscribers don't have to run at the same time. The publisher can publish a message and the subscriber can receive it later. In addition, the publish operation isn't synchronized with the receive operation. A publisher requests the broker to publish a message and the different clients that have subscribed to the appropriate topic can receive the message at different times. The publisher can send messages as an asynchronous operation to avoid being blocked until the clients receive the messages. However, it is also possible to send a message to the broker as a synchronous operation with the broker and to continue the execution only after the operation was successful. In most cases, we will want to take advantage of asynchronous operations.

A publisher that requires sending a message to hundreds of clients can do it with a single publish operation to a broker. The broker is responsible for sending the published message to all the clients that have subscribed to the appropriate topic. Because publishers and subscribers are decoupled, the publisher doesn't know whether there is any subscriber that is going to listen to the messages it is going to send. Hence, sometimes it is necessary to make the subscriber become a publisher too and to publish a message indicating that it has received and processed a message. The specific requirements depend on the kind of solution we are building. MQTT offers many features that make our lives easier in many of the scenarios we have been analyzing.