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An hands-on introduction to MQTT

This lab aims to offer you an hands-on experience with MQTT. You will perform experiments that will allow you to learn how to "publish" and "subscribe" to data. To this end you will use:

  1. your own broker
  2. a "sandbox" external broker

You will learn how to:

  • install and configure an MQTT broker
  • interchange data using MQTT clients based
  • use MQTT to feed data to your own mobile

Hardware

Each student will use your own computer and mobile.

Block 0: Installing the MQTT broker on your computer

For our experiments we will use Mosquitto, which is part of the Eclipse Foundation and is an iot.eclipse.org project. The manual page can be found here man page.

Detailed installation indications can be found here: https://mosquitto.org/download/
As a quick guide:

  • Linux distros like Debian/UBUNTU/Raspian already have Mosquitto in their repositories… so it's enough with:
sudo apt-get update
sudo apt-get install mosquitto mosquitto-clients

  • with Ubuntu MATE maybe you'll need to add this before:
    sudo apt-add-repository ppa:mosquitto-dev/mosquitto-ppa

  • with Macs, Mosquitto can be installed from the homebrew project. See http://brew.sh/ and then use “brew install mosquitto”

Managing the broker

… with Debian/UBUNTU/Raspian

To start and stop its execution use:

sudo /etc/init.d/mosquitto start/stop

At thist moment, to run the broker execute:

sudo mosquitto –v

note: "-v" stands for "verbose mode" and can be useful at the beginning to see what is going on in the broker. Can be convenient to use a dedicated terminal for the broker to execute in, if the "-v" option is used.

To check if the broker is running you can use the command:

sudo netstat -tanlp | grep 1883

note: "-tanlp" stands for: tcp, all, numeric, listening, program

alternatively use:

ps -ef | grep mosquitto

… with Mac OS

To start and stop its execution use:

/usr/local/sbin/mosquitto -v

note: "-v" stands for "verbose mode" and can be useful at the beginning to see what is going on in the broker. Can be convenient to use a dedicated terminal for the broker to execute in, if the "-v" option is used.

or:

/usr/local/sbin/mosquitto -c /usr/local/etc/mosquitto/mosquitto.conf

or:

brew services start/stop mosquitto

To check if the broker is running you can use the command:

sudo lsof -i -n -P | grep 1883

or:

ps -ef | grep mosquitto

Clients for testing

The broker comes with a couple of useful commands to quickly publish and subscribe to some topic. Their basic syntax is the following.

mosquitto_sub -h HOSTNAME -t TOPIC
mosquitto_pub -h HOSTNAME -t TOPIC -m MSG

More information can be found:

Block 1: some basic example.

Set-up

Open three terminals (e.g., xterm) in your computer, more or less like this:


The biggest terminal on the right will be used to see the execution of the broker, the two smaller terminals will be used to execute the publisher and the subscriber, respectively.

Now, run the broker with the -v flag in the biggest terminal.

Exercises

Let's start with a easy one. In one of the small terminals write:

mosquitto_sub -t i/LOVE/Python

the broker terminal should show something like:

the broker registered the subscription request of the new client. Now in the other small terminal, execute:

mosquitto_pub -t i/LOVE/Python -m "Very well."

in the broker terminal, after the new registration messages, you'll also see something like:

meaning that the broker received the published message and that it forwarded it to the subscribed client. In the terminal where mosquitto_sub is executing you'll see the actual message appear.

Try now:

mosquitto_pub -t i/love/python -m "Not so well"

What happened? Are topics case-sensitive?

Another useful option of mosquitto_pub is -l. Execute the following command:

mosquitto_pub -t i/LOVE/Python -l

and start typing some line of text. It sends messages read from stdin, splitting separate lines into separate messages. Note that blank lines won't be sent. You basically obtained a MQTT based "unidirectional chat" channel…

… about Keepalive

By the way, if you kept the broker running with the -v option until now in a separate window, you can see various lines like:

1524673958: Sending PINGRESP to mosqpub|3592-iMac-de-Pi
1524673985: Received PINGREQ from mosqsub|3587-iMac-de-Pi

this simply shows that the broker and the client are interchanging these special messages to know whether they are still alive.

QoS (Quality of Service):

Adding the -q option, for example to the mosquitto_pub you'll see the extra message that are now interchanged with the broker. For example, doing:

mosquitto_pub -t i/LOVE/Python -q 2 -m testing

you'll get:

compare this sequence of messages with the one obtained with -q 0 or with -q 1.

Retained messages:

Normally if a publisher publishes a message to a topic, and no one is subscribed to that topic the message is simply discarded by the broker. If you want your broker to remember the last published message, you'll have to use the retain option. Only one message is retained per topic. The next message published on that topic replaces the retained message for that topic.

To set the retain message flag you have to add -r using the Mosquitto clients.

So try the following cases, but remember now to always execute, for each test, the subscriber after the publisher:

  1. Publish a message with the retain message flag not set, like we did before. What happens?
  2. Publish a message with the retain message flag set (-r). What happens?
  3. Publish several (different) messages with the retain message flag set before starting the subscriber. What happens?
  4. Publish a message with the retain message flag not set again. What happens?

Finally, how do I remove or delete a retained message? You have to publish a blank message(-m "") with the retain flag set to true which clears the retained message. Try it.

Public brokers

There are also various public brokers in Internet, also called sandboxes. For example:

we will always access them through port 1883.

Repeat some of the exercise above with one of these sandboxes (remember to use the -h option). Any difference?

Block 2: A testbed on my mobile

For this demo you have to use your own mobile and get installed two apps: A MQTT broker (any) and MQTT Dash.

Enabling the MQTT (Broker) Server

You have to enable the MQTT Broker you have installed in your mobile

Connecting MQTT Dash

Now you have to connect the MQTT Dash to the Broker that is running in your own mobile. You can either run the MQTT Dash in the same mobile or run it in another one. After getting the MQTT connected to the Broker you have to add some elements to the MQTT Dash environment and try to interact with the element you have created either via a terminal in your computer or via another mobile.

Material adapted from: https://hackmd.io/@pmanzoni/BJ9hwSfhG#

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