In this post you’re going to test the Mosquitto Broker and Client on a Raspberry Pi by subscribing to an MQTT topic and publishing sample messages.
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Testing MQTT Broker Installation
After installing MQTT Broker, I recommend installing an MQTT Cllient to test the Broker installation and publish sample messages.
The next command shows how to install MQTT Mosquitto Client:
pi@raspberry:~ $ sudo apt-get install mosquitto-clients
You’ll have to type Y and press Enter to confirm the installation.
Run Mosquitto on background as a daemon:
pi@raspberry:~ $ mosquitto -d
Subscribing to testTopic Topic
To subscribe to an MQTT topic with Mosquitto Client open a terminal Window #1 and enter the command:
pi@raspberry:~ $ mosquitto_sub -d -t testTopic
You’re now subscribed to a topic called testTopic.
Publishing “Hello World!” Message to testTopic Topic
To publish a sample message to testTopic, open a terminal Window #2 and run this command:
pi@raspberry:~ $ mosquitto_pub -d -t testTopic -m "Hello world!"
The message “Hello World!” is received in Window #1 as illustrated in the figure above.
Publishing a Message to Multiple Clients
Having Window #1 still subscribed to topic testTopic, open a new terminal Window #3 and run this command to subscribe to testTopic topic:
pi@raspberry:~ $ mosquitto_sub -d -t testTopic
On Window #2 publish the “Hello World!” message:
pi@raspberry:~ $ mosquitto_pub -d -t testTopic -m "Hello world!"
Since two clients are subscribed to testTopic topic, they will both receive “Hello world!” message.
This simple example shows how MQTT works and how your devices (for example: ESP8266) could be subscribed to the same topic to receive messages or a device could publish messages to multiple devices. We’ll explore this concept further in future blog posts.
Wrapping up
In the next blog post, we will experiment with MQTT and ESP8266 to see how everything works with practical examples.
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