Efficient routing of messages is critical in IoT environments, where timely and accurate communication between devices can have a significant impact on system performance. This is where an MQTT broker can help. Here we show you how to implement one using EMQX.

What Is an MQTT Broker? 

MQTT, which stands for Message Queuing Telemetry Transport, is a protocol used in the Internet of Things (IoT) to provide a lightweight publish/subscribe messaging model. The MQTT broker is the server component in this model, and it's responsible for receiving all messages, filtering these messages, determining who is subscribed to each message, and sending the message to these subscribed clients.

It's important to note that the MQTT broker isn't necessarily a physical server. It could be a program running on a physical server, or it could be a cloud-based service. The broker's role is to maintain a list of topics – which can be thought of as message channels – and to distribute messages published to these topics to any clients that have subscribed to them.

The MQTT protocol was designed to be lightweight and efficient, with a minimal network footprint. This makes it ideal for use in IoT environments where network bandwidth can be limited, and where the devices involved may have limited power and processing capabilities. In these environments, the MQTT broker serves a crucial role, enabling efficient communication between devices.

Why Are MQTT Brokers Important? 

Message Routing

The MQTT broker's primary role is to route messages between publishers and subscribers. When a publisher sends a message to a particular topic, the broker is responsible for ensuring that this message reaches all clients that have subscribed to that topic. This efficient routing of messages is critical in IoT environments, where timely and accurate communication between devices can have a significant impact on system performance.

For instance, imagine a smart home system where the temperature sensor publishes a message to the 'home/temperature' topic whenever the temperature changes. The HVAC system and the smart thermostat are both subscribed to this topic and rely on these messages to maintain a comfortable temperature in the home. In this scenario, the MQTT broker ensures that these crucial messages reach their intended recipients promptly.

Scalability

Scalability is another key area where MQTT brokers prove their worth. As the number of devices in an IoT environment grows, so does the volume of messages that need to be routed. An effective MQTT broker can handle this increased load, ensuring that messages continue to be delivered reliably and efficiently, regardless of the number of clients.

Furthermore, MQTT brokers can be set up in a distributed network to handle even larger volumes of messages. These distributed brokers work together, sharing the load and providing a highly scalable solution. This level of scalability makes MQTT brokers an ideal choice for large-scale IoT deployments.

Security

Security is a critical concern in any IoT environment. MQTT brokers contribute to the overall security of the system in several ways. First, they can enforce access controls, ensuring that only authorized clients can publish or subscribe to specific topics. This prevents unauthorized devices from tampering with the system's communication.

Additionally, MQTT brokers can support secure communication protocols, such as SSL/TLS, to encrypt messages and protect them from being intercepted and read by unauthorized parties. This combination of access control and secure communication makes MQTT brokers a crucial component in maintaining the security of an IoT system.

Session Management

Finally, MQTT brokers are responsible for managing sessions between clients. When a client connects to the broker, a session is established. This session persists even if the connection is briefly lost, ensuring that the client does not miss any messages.

The broker also keeps track of each client's subscriptions, ensuring that when a client reconnects after a disconnection, it continues to receive messages from the topics it's subscribed to. This session management capability is crucial for ensuring reliable communication in an IoT environment, where connections can be unstable and devices may frequently disconnect and reconnect.

MQTT Broker Architecture 

MQTT Broker Server

At the core of MQTT broker architecture is the MQTT broker server. The MQTT broker server is the heart of the MQTT protocol. It's the component that receives all messages from the clients, processes them, and dispatches them to the appropriate clients based on their subscriptions.

The MQTT broker server is designed to handle multiple connections simultaneously, making it ideal for large-scale IoT applications. It ensures that all messages are delivered in the order they were sent, ensuring the integrity of the data transmitted.

Additionally, the MQTT broker server also manages the session of the clients. It keeps track of the clients' subscriptions and ensures that they receive all the messages they are subscribed to, even if they disconnect temporarily. This feature is very beneficial in IoT applications where devices may go offline due to various reasons.

MQTT Clients

The MQTT clients are the IoT devices that communicate with each other using the MQTT protocol. They could be anything, from a simple sensor to a complex machine. The MQTT clients send messages to the broker and receive messages from it.

The clients can be classified into two types: publishers and subscribers. The publishers are the ones that send messages to the broker, and the subscribers are the ones that receive messages from it. However, it's important to note that a client can be both a publisher and a subscriber at the same time.

The MQTT clients connect to the broker using a TCP/IP connection. They use this connection to publish messages to topics and subscribe to topics to receive messages. The clients can specify the quality of service for each message they publish or subscribe to, giving them control over the reliability of the message transmission.

Topics

Topics are a crucial part of the MQTT broker architecture. They act as the address for the messages. When a client publishes a message, it specifies a topic for the message. The broker then delivers this message to all the clients that are subscribed to this topic.

Topics in MQTT are hierarchical and are defined using a forward slash (/) as a separator. For example, a topic could be "home/livingroom/temperature". This hierarchical nature of topics allows for a high level of flexibility in defining the scope of the messages.

A client can subscribe to a specific topic, or it can use wildcards to subscribe to multiple topics at once. For example, a client can subscribe to "home/+/temperature" to receive temperature readings from all rooms in the home.