Build an MQTT Server Using Raspberry Pi

1. Install the mosquitto MQTT Broker
mosquitto is a popular MQTT broker that is well-supported on Debian-based Linux platforms such as Raspbian. It’s easy to install using apt:-
sudo apt install mosquitto mosquitto-clients
You’ll need to enter your password the first time you run sudo.
You don’t strictly need the mosquitto-clients package for running the broker, but installing it allows you to run the MQTT client code locally which is great for testing.
It also means you can use the Raspberry Pi as a proper MQTT client as well as a broker. This means you could, for example, add a user interface to control other MQTT clients around your home directly from the Raspberry Pi.
2. Enable the mosquitto broker
Enable the broker and allow it to auto-start after reboot using the following command:-
sudo systemctl enable mosquitto
The broker should now be running. You can confirm by checking the systemd service status:-
sudo systemctl status mosquitto
This should produce an output similar to:-
● mosquitto.service - LSB: mosquitto MQTT v3.1 message broker
   Loaded: loaded (/etc/init.d/mosquitto; generated; vendor preset: enabled)
   Active: active (running) since Sat 2018-12-29 16:27:56 GMT; 22h ago
     Docs: man:systemd-sysv-generator(8)
   CGroup: /system.slice/mosquitto.service
           └─1685 /usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.conf
​
Dec 29 16:27:56 raspberrypi systemd[1]: Starting LSB: mosquitto MQTT v3.1 message broker...
Dec 29 16:27:56 raspberrypi mosquitto[1679]: Starting network daemon:: mosquitto.
Dec 29 16:27:56 raspberrypi systemd[1]: Started LSB: mosquitto MQTT v3.1 message broker.
3. Subscribe to the MQTT Topic Locally
In order to test the broker, you need to subscribe to an MQTT topic.
A topic is simply a string that looks like a file system path. It has the general form:-
a/b/c/...
There are no restrictions on the number of elements. There are some special characters: apart from the slash / itself, the characters + and # are wildcards, which you don’t really need to know about yet, other than not to use them in a topic name.
The great thing about MQTT is that you can just make up topics to suit your needs. You don’t need to register them anywhere. For the sake of this test, you can use a topic called test/message.
In the existing terminal, subscribe to the test/message topic:-
mosquitto_sub -h localhost -t "test/message"
This will send a subscription message to the MQTT broker which is currently running on the same system (as specified by the -h localhost option). But it could be running somewhere else, as you’ll see later.
So long as the mosquitto_sub programme is running you’re listening to the test/message topic as an MQTT client.
4. Publish to the MQTT Topic Locally
Because your current terminal is occupied listening to the topic, you’ll need to open another terminal. You can do this using another SSH session or on the Raspbian GUI, depending how your system is configured.
Once open, publish message to the test/message topic like this:-
mosquitto_pub -h localhost -t "test/message" -m "Hello, world"
If you look back at the first terminal now you should see this:-
Hello, world
Congratulations, you have just published your first MQTT message!

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