Data from sensors will need to be stored. But how much do you really need to store?
Many sensors like state sensors (ON/OFF), temperature etc will fluctuate very little over the course of a day so does it makes sense to store the same sensor value every x seconds?
Take for example a light sensor that publishes its status every second.
After 1 hour it has published 3600 status messages.
If we assume that the light status hasn’t changed during that period then it doesn’t make much sense to store the 3600 status messages.
The only messages that need to be stored are change messages.
With this method if our light change state twice in the hour then we would store 2 messages.
Over a day assuming 20 state changes we would store 20 messages rather than 86,400(24*3600) messages.
Preferably it would be better if the sensor only sent changes as this would also reduce the network traffic.
However even if it does send data a regular intervals regardless of state then it doesn’t make send to store it.
Note: the sensor needs to publish with the retain message flag set.
Resources and related tutorials
- Simple Controllable MQTT Sensor Simulator in Python
- MQTT Sensors and Network Traffic Observations
- MQTT Topic and Payload Design Notes
- Checking Active MQTT Client Connections