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340CT-TEACHINGMATERIALS/02 Lab 3 MQTT.md

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Publish Subscribe 2 Set Up 1 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.

2 Set Up

Start by installing the Mosquitto Tools.

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

If you are using Ubuntu you can install usingsudo apt install mosquitto. This also works for most online IDEs.

mosquitto_sub -h test.mosquitto.org -t "#" -v

If you are using Windows 10 you can download the 64 bit Binary exe and install.

1 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.

Start by opening two terminal windows:

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.

$ mosquitto_sub -h test.mosquitto.org -t 302CEM/XXX
  1. The -h flag allows us to specify the host, in this case test.mosquitto.org.
  2. The -t flag allows us to specify the topic, in this case 205CDE/XXX (remember to substitute your username)

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

$ mosquitto_pub -h test.mosquitto.org -t 302CEM/XXX  -m 'hello world'
  1. The -h flag allows us to specify the host, in this case test.mosquitto.org.
  2. The -t flag allows us to specify the topic, in this case 205CDE/XXX (remember to substitute your username)
  3. 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.

2.1 Test Your Understanding

Working in small groups of between 2 and 4 people:

  1. Decide on the topic name you will use.
  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).

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