Logging MQTT Sensor Data -Notes

Data from sensors will need to be stored. But how much data 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 changed messages.

With this method if our light changed 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 the sensor does send data a regular intervals regardless of state then it doesn’t make sense to store it.

Note: the sensor needs to publish with the retain message flag set.

Both the The Simple MQTT Data logger and the SQL MQTT data Logger are designed only to store changed data.

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