In-Depth Exploration of Lightweight MQTT Protocol

MQTT was developed around 1999, primarily to create a protocol suitable for bandwidth-constrained environments. Additionally, its low power consumption is very attractive for IoT.

This protocol uses a publish-subscribe paradigm, contrasting with the request/response paradigm primarily oriented towards HTTP. It uses binary messages to exchange information, which is very low overhead. It is simpler to implement, superior, and open. Another interesting aspect is that MQTT uses TCP as the underlying transport. All these factors are designed to better accommodate IoT.

As mentioned above, MQTT implements a publisher-subscriber model. The publisher-subscriber model decouples the client that publishes messages (the publisher) from the client that receives messages (the subscriber). Furthermore, compared to HTTP, which is primarily a synchronous protocol, MQTT is an asynchronous protocol that does not block clients while waiting for messages. Additionally, the MQTT protocol does not require that subscribers and publishers are directly connected.

As mentioned, MQTT is a message-oriented protocol that implements a publisher-subscriber model. A key component of MQTT is the MQTT broker. The main task of the MQTT broker is to dispatch messages to subscribers. That is, it receives messages from publishers and forwards them to subscribers. When forwarding messages, the MQTT broker uses topics to filter clients. A topic is a string that can be combined by creating topic levels.

Topics act like virtual channels that connect publishers with their subscribers. Topics are managed by the MQTT broker. Through this virtual channel, publishers and subscribers are decoupled, and clients (publishers or subscribers) do not need to know each other. Because there is no direct dependency between message producers (publishers) and message consumers (subscribers), this protocol is highly scalable.

The diagram below illustrates the MQTT architecture:

Now that we have a general understanding, let’s apply it through practical examples. MQTT has many implementations, and we will use Mosquitto developed by Eclipse. The first step is to install the MQTT broker. For our purposes, we will install it on a Raspberry Pi. To install Mosquitto, we need to add the repository where it is located so that we can download it. Before adding the repository, it is necessary to add the key to verify that the downloaded package is legitimate. We can connect to the Raspberry Pi via SSH or remote desktop. Now, in the terminal, enter the following command:

Then, import the key:

Finally, add the .list file:

With this, the MQTT server has been installed on the Raspberry Pi. This server is our MQTT broker. Now we need to install the clients (publishers and subscribers). In this example, we will install both the client and the server on the same Raspberry Pi, but you can also install them on different PCs/servers or IoT boards.

Assuming we have successfully installed and configured the client and server, we can now register a user to a specific topic and wait for messages from the publisher. We can use the command below to register a user:

As shown, the user is waiting for messages. In this example, we are using the topic called swa_news. Now, we will use the MQTT publisher to send a message with the topic swa_news.

In this example, the MQTT publisher sent the message “Hello Protocol”, and on the user side, we received the following message:

Finally, we will use an Android MQTT client to receive the messages we sent. The video below shows us how to configure an Android MQTT client.

https://www.youtube.com/embed/plnbafCBVtU

We hope you learned what the MQTT protocol is and how to use MQTT to send messages to clients through this article.