MQTT

MQTT is a lightweight publish/subscribe messaging protocol, standardized by OASIS, that was designed for constrained devices and unreliable, low-bandwidth, high-latency networks. A client publishes messages to a topic on a broker, and the broker fans them out to every client subscribed to that topic. Its tiny footprint is why it became the default nervous system for the Internet of Things.

• Publish/subscribe over a broker - Producers and consumers are decoupled through topics, so neither needs to know the other exists.
• Configurable quality of service - Three QoS levels trade off delivery guarantees against overhead, from fire-and-forget up to exactly-once.
• Minimal overhead - A compact binary header keeps message size small enough for battery-powered and metered-connection devices.
• Retained messages and last will - Brokers can hold a topic’s last value and announce ungraceful client disconnects, which matters for spotty device links.

In API operations, MQTT is one of the core event-driven protocols that sit alongside HTTP request/response, and it is a first-class binding in AsyncAPI so its topics and messages can be described, documented, and governed like any other interface. I see it most in IoT telemetry and command-and-control, and treating it as part of the API surface, rather than something outside governance, is what keeps a multi-protocol estate coherent.