The dominant IoT messaging protocol.
WHAT IT IS
A lightweight publish-and-subscribe protocol designed for constrained devices and unreliable networks.
WHAT PUBLISH AND SUBSCRIBE MEANS
Devices publish to topics; interested parties subscribe to them, without knowing each other.
WHAT A BROKER DOES
Receives published messages and distributes them to subscribers.
WHY THAT DECOUPLING MATTERS
Devices and applications can be added or removed without either knowing.
WHAT A TOPIC IS
A hierarchical string identifying a channel.
WHAT GOOD TOPIC DESIGN LOOKS LIKE
A consistent hierarchy: type, identifier, then measurement.
WHY STRUCTURE MATTERS
Subscribers use wildcards, and an inconsistent hierarchy makes that impossible.
WHAT TO NEVER PUT IN A TOPIC
Values that change, such as readings.
WHY
Each distinct topic is a separate channel, and the broker retains them.
WHAT QUALITY OF SERVICE LEVELS PROVIDE
At most once, with no guarantee At least once, with possible duplicates Exactly once, with more overhead
WHAT TO USE
At least once, with idempotent handling, for most cases.
WHAT A RETAINED MESSAGE IS
The last message on a topic, delivered immediately to new subscribers.
WHAT IT SUITS
Current state, so a subscriber need not wait for the next update.
