Topic
In MQTT, topic allows filtering of messages. A hierarchical structure separated by a slash (/) is used.
Examples:
- home/living_room/temperature → Temperature data for the living room.
- vehicles/car/speed → Vehicle speed data.
Quality of Service (QoS)
The QoS level in MQTT determines the reliability of message delivery.
- QoS 0 (At Most Once) → The message is sent without waiting for confirmation. Available if data loss can be tolerated.
Only available if the transmitter and receiver are connected by cable.
- QoS 1 (At Least Once) → Message is transmitted at least once but can be repeated.
- QoS 2 (Exactly Once) → Most reliable level, the message is delivered only once.
Retain
The Retain feature allows messages to be saved and forwarded to newly connected clients.
- Example: If a temperature value was sent with retain, all newly connected clients can see this value.
- To clear a retain message: Retain true should send an empty message.
