MQTT for Lightweight IoT

A temperature sensor may need to send a reading every few seconds over an unreliable mobile connection. A dashboard wants the latest reading, an alerting service wants only dangerous readings, and a storage service wants every reading it can receive. Requiring the sensor to discover and call each consumer would make the device tightly coupled to the backend and difficult to operate.

MQTT solves this problem with a lightweight, brokered publish/subscribe model.

The sensor publishes a message to a named topic. It does not address any consumer directly. A central broker receives the message, matches the topic against subscriptions, and forwards the message to interested clients.

This architecture is useful far beyond small sensors. MQTT is used for vehicle telemetry, industrial systems, smart buildings, mobile applications, home automation, and backend event distribution.

“Lightweight” does not mean “free.” MQTT uses compact headers and can keep one connection open for many messages, but reliable delivery, persistent sessions, queued messages, TLS, and millions of subscriptions still consume real memory, bandwidth, storage, and CPU. A sound design understands where that cost moves.

This chapter uses MQTT 5. The core publish/subscribe and QoS model also exists in MQTT 3.1.1, but properties, Session Expiry, richer reason codes, and standardized shared subscriptions described here are MQTT 5 features. Client and broker must agree on a protocol version.