Introduction to MQTT Protocol: Essential for IoT Enthusiasts

Term

The MQTT Broker is a message middleware that is responsible for receiving messages sent by publishers and forwarding them to subscribers based on the topics subscribed by the subscribers. It manages client connections, handles subscriptions and unsubscriptions, and ensures messages are sent according to the specified Quality of Service (QoS) levels. MQTT is an open, lightweight machine-to-machine protocol designed for IoT interactions. In an MQTT network, the MQTT Broker is the server coordinating interactions between all MQTT agents. MQTT clients can be publishers, subscribers, or both. The MQTT architecture can be centralized or distributed to suit different application scenarios.

• It can achieve message interoperability between two MQTT brokers, ensuring message reliability and stability.
• It can configure a virtual IP or domain name for access, improving system availability and maintainability.
• It can achieve load balancing and failover, improving system availability and stability.
• It can specify the topic for message synchronization, avoiding traffic amplification issues.
• It is more recommended to use a vertically segmented approach for traffic diversion to further enhance system availability and stability.

MQTT Client

The MQTT Client refers to devices that run the MQTT library and connect to the MQTT broker over the network. These devices can be microcontrollers or mature servers, implementing the MQTT protocol’s client. In the MQTT protocol, a typical client will perform the following main operations: publishing messages to a topic, subscribing to messages from a topic, handling received messages, and acknowledging received QoS 1 or QoS 2 messages. The implementation of the MQTT protocol’s client is very simple and straightforward.

The Internet of Things (IoT) refers to the real-time collection of any monitored, connected, or interactive object or process through various information sensors, RFID technology, GPS, infrared sensors, laser scanners, and other devices and technologies, gathering information such as sound, light, heat, electricity, mechanics, chemistry, biology, and location, to achieve ubiquitous connectivity between objects and people, enabling intelligent perception, identification, and management of objects and processes. The IoT is an information carrier based on the Internet and traditional telecom networks, allowing all independently addressable physical objects to form an interconnected network.

• QoS0, i.e., at most once (At most once). Under this level, the sender will attempt to send the message to the receiver, but the message delivery process includes only one attempt. If the sending fails, it will not retry. This QoS level is suitable for scenarios where the reliability of message delivery is not high, but the response speed is critical.
• QoS1, i.e., at least once (At least once). Under this level, the sender ensures that the message is delivered at least once to the receiver. In some cases, the message may be delivered multiple times. The receiver can determine whether a message has been received using message IDs or other identifiers, thus handling duplicate reception. This QoS level is suitable for scenarios where it is necessary to ensure that messages are delivered at least once but allows for duplicate delivery.
• QoS2, i.e., exactly once (Exactly once). Under this level, the sender ensures that each message is delivered only once to the receiver. In some cases, message delivery may be delayed. The receiver can rely on some mechanism (such as database transactions) to ensure that the processing of a message does not occur multiple times. This QoS level is suitable for scenarios where it is necessary to ensure that each message is delivered only once, and there is a certain tolerance for message delivery delays.