MQTT
also: MQTT broker · publish/subscribe messaging
A lightweight publish/subscribe messaging protocol widely used to move telemetry and commands between connected devices and the cloud through a broker.
Devices publish messages to named topics (for example site/unit42/temperature) on a server called a broker; other devices and applications subscribe to the topics they care about. A device needs only one outgoing connection, which works well through firewalls and on intermittent networks. MQTT is an OASIS open standard; version 3.1.1 is also published as ISO/IEC 20922, and version 5 adds features.
It offers three delivery levels (QoS 0: at most once; QoS 1: at least once; QoS 2: exactly once), retained messages so new subscribers immediately get the latest value, and a "last will" message published if a device disconnects unexpectedly.
Where it bites: MQTT does not define payload formats, so topic naming and message schemas are your design responsibility. Security depends on TLS and per-device credentials, which need to be provisioned in the factory.
Founder takeaway
Agree on a topic structure, payload schema and per-device credentials before the first device ships; they are hard to change across a fleet.
engineer review pending