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TEACHING-MATERIALS/02 MQTT CLI.md

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Publish Subscribe 1 Set Up 1.1 MacOS 1.2 Ubuntu 1.3 Windows 10 2 The MQTT Protocol 2.1 Test Your Understanding
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Publish Subscribe

Up to this point all the activities have been using the HTTP Protocol, which uses a request-response process (the client requests a resource and the server responds with this resource). If this seems unfamiliar you should work through the HTTP Protocol worksheet.

Whilst this approach works fine for delivering content to a web browser it is not a useful approach for certain applications. Imagine a chat room where you had to refresh the page to view new messages.

In this worksheet you will learn how to use a new HTML5 websocket protocol that allows a full duplex (2 way) communication over a single TCP connection. We will then explore the MQTT protocol which can be run over websockets and is used to implement a push message system, technically called publish-subscribe.

1 Set Up

Start by installing the Mosquitto Tools.

1.1 MacOS

If you are using MacOS you should install the Brew Package Manager and use this to install Mosquitto using brew install mosquitto.

1.2 Ubuntu

If you are using Ubuntu you can install using the following commands:

sudo apt-add-repository ppa:mosquitto-dev/mosquitto-ppa
sudo apt update
sudo apt install mosquitto-clients

1.3 Windows 10

If you are using Windows 10 you can download the 64 bit Binary exe and install. When you try running the commands you may need to include the full path to the executable.

If (when) you encounter issues using Windows we recommend you either dual boot your computer or install Virtual Box and use this to install Ubuntu. There are detailed instructions online.

2 The MQTT Protocol

Now we have the tools installed we can start using the protocol. You have installed 2 tools, mosquitto_pub is used to publish messages and mosquitto_sub subscribes to a channel. We will use the test.mosquitto.org broker.

Before connecting you will need to download the public key certificate that enables encryption between your computer and the broker. You can find this in the exercises/MQTT directory. Since the mqtt.crt file contains text this will display in the browser when selected rather than being downloaded. To solve this problem you should download the mqtt.crt.zip file and then unzip this to extract the key. Place this in a sensible directory on your computer such as documents.

Nowpening two terminal windows and in both of these navigate to the directory you placed the certificate file in:

In the first window we will run the mosquitto_sub command and subscribe to a topic called 302CEM/XXX where XXX is your university username. Make sure you are running the commands in the same directory as the mqtt.crt. The correct password will be given out in the lab session. The topic should include your team name and your own username (replace the xxx). Note that on Windows computers you will need to include the full path to the mosquitto_sub command.

$ mosquitto_sub -v -h mqtt.coventry.ac.uk -p 8883 --cafile mqtt.crt -u 302CEM -P xxx -t 302CEM/elephant/xxx
  1. The -v is the verbose flag and this will force the program to display both the topic and message (if this is omitted it will only print the message).
  2. The -h flag allows us to specify the host, in this case test.mosquitto.org.
  3. The -p flag allows us to set the port (1883 for non-secure connections and 8883 if the connection is encrypted).
  4. The --cafile flag is where we tell the tool where the server's public key is located.
  5. The -u and -P flags are used to supply the username and password needed to validate the subscription.
  6. The -t flag allows us to specify the topic, in this case 302CEM/elephant/XXX (remember to substitute your team nane and username)

The terminal will sit there, subscribed to the chosen topic, waiting to be sent some data which we will send using the second terminal window.

In the second terminal we will run the mosquitto_pub command to publish messages to our topic.

$ mosquitto_pub -h mqtt.coventry.ac.uk -p 8883 --cafile mqtt.crt -u 302CEM -P xxx -t 302CEM/elephant/xxx  -m "hello world"
  1. The -m flag allows us to specify the message, in this case hello world.

If you look at the first terminal window (running mosquitto_sub) you should see your message displayed.

The first terminal will continue to wait for messages until the command is exited by pressing ctrl+c.

2.1 Test Your Understanding

Working in small groups of between 2 and 4 people:

  1. Each member of the team should choose the same topic name 302CEM/elephant, substituting your team name.
  2. Everyone runs the mosquitto_sub tool and subscribes to this same topic.
  3. Each person launches a new terminal in a new pane (so you can see both terminal windows).
  4. use the mosquitto_pub tool to send a message to your chosen topic name.
  5. Look at the output of your mosquitto_sub command (in the first terminal window).
  6. What happens if you subscribe to the 302CEM/ topic?

What have you produced? Can you think of any application for this...