Chapter 1, Lightweight Messaging with MQTT 3.1.1 and Mosquitto, starts our journey towards understanding the MQTT protocol. You will learn convenient scenarios for this protocol, the details of the publish-subscribe pattern, and message filtering. You will learn basic concepts related to MQTT and understand the different components: clients, brokers or servers, and connections. You will learn to install a Mosquitto broker or server on Linux, macOS, and Windows. You will learn best practices related to topics and single-level and multilevel wildcards. We will study in detail the different Quality of Service levels supported by MQTT and when it is convenient to use each of them. We will analyze their advantages and disadvantages.

Chapter 2, Securing an MQTT Mosquitto Server, covers generating a private certificate authority, a server certificate, and client certificates to enable TLS transport security and TLS client authentication with Mosquitto. Communications between the MQTT clients and the MQTT server will be encrypted. We will work with OpenSSL to generate the digital certificates. We will test the MQTT TLS configuration with MQTT.fx, MQTT-spy, and the Mosquitto command-line utilities. We will force Mosquitto to use only a specific TLS version. This way, we will understand how to encrypt communications between the MQTT clients and the Mosquitto MQTT server.

Chapter 3, Controling a Drone with Python, analyzes the requirements for controlling a drone with MQTT messages. We will define the topics that we will use and the commands that will be part of the messages’ payloads to control a drone. Then, we will work with the Paho Python Client to write Python code that connects an MQTT client to the MQTT server. We will understand the methods we need to call for the Paho Python Client and their parameters. We will analyze how callbacks work and we will be able to publish messages to topics, subscribe to topic filters, and receive and process messages. We will write code that processes commands for a drone with Python. The code will be able to run on different IoT platforms, including Intel Joule boards and Raspberry Pi 3, and also on any computer that is capable of executing Python 2.7.x or Python 3.x. We will work with the network loop for the MQTT client in Python. We will work with a GUI utility, command-line-based utilities, and, finally, with Python code to control the drone and display the feedback provided by the drone.

Chapter 4, Interacting with Sensors in Java, analyzes the requirements for controlling sensors wired to different IoT boards with MQTT messages and Java code. We will define the topics that we will use and the commands that will be part of the messages’ payloads to control sensors. We will take advantage of the last will and testament feature of MQTT with Java. We will work with the Eclipse Paho Java Client to write Java code that will connect a non-blocking (asynchronous) MQTT client to the MQTT server. We will create a security helper class to establish a TLS secured connection, and we will work with asynchronous notifications and callbacks in Java. We will subscribe to topics, understand how notification events work, and we will write code that processes commands that interact with sensors. Finally, we will work with a GUI utility to interact with our Java application through MQTT messages. We will be able to turn on and turn off the sensors and retrieve data from them.

Chapter 5, Controlling Home Automation in JavaScript, analyzes the requirements to control LEDs wired to different IoT boards with MQTT messages over WebSockets. We will define the topics that we will use and the commands that will be part of the message’s payloads to set the colors for LEDs. Then, we will create an HTML 5 web page combined with JavaScript and the Eclipse Paho JavaScript asynchronous client to control LEDs. We will write code to publish messages to target remote devices in JavaScript and we will be able to process received messages to provide feedback to the user. We will work with MQTT.js to write a Node.js script to process commands that simulate the interaction with LEDs on different IoT boards. We will be able to run the home automation web application and understand how all the pieces work together by exchanging MQTT over WebSockets messages. Finally, we will understand the necessary steps to work with MQTT over WebSockets with TLS.

Chapter 6, Interacting with Actuators in Swift, analyzes the requirements for controlling actuators wired to different IoT boards with MQTT messages. We will define the topics that we will use and the commands that will be part of the message’s payloads to turn on and turn off motors. Then, we will create an iOS app with Xcode and Swift 3, combined with the CocoaMQTT client, to control a motor. We will write code to publish messages to target remote devices in Swift 3 and we will be able to process received messages to provide feedback to the user. We will work with MQTT.js to write a Node.js script to process commands that simulate the interaction with motors on the different IoT boards. We will be able to run the iOS app and the Node.js script and understand how all the pieces work together by exchanging MQTT messages.

You will need any computer or device capable of running Python 3.x or greater, Chrome 56.x or greater, JDK 8 or greater, and Node.js 6.7.x or greater.

In order to work with Mosquitto 1.4.11 or greater, you will need a computer with any of the operating systems supported by this version of Mosquitto. The list is available at https://mosquitto.org/download/.

In order to work with Xcode 8.x, you will need a Mac computer capable of running OS X 10.11.5 or later, with 8 GB RAM.

This book is a great resource for developers who want to learn more about the MQTT protocol to apply it to their individual IoT projects. Prior knowledge of working with IoT devices is essential.

In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "If a client specifies and empty value as the ClientId."

A block of code is set as follows:

# Replace localhost with the IP for the Mosquitto
# or other MQTT server
mqtt_server_host = "localhost"
mqtt_server_port = 8883
mqtt_keepalive = 60

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

<div id="led-2">
<span>
Select color for LED #2
</span>

Any command-line input or output is written as follows:

sudo apt-get update

New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "In order to download new modules, we will go to Files | Settings | Project Name | Project Interpreter."

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