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.

Therefore a data logger should have the ability to only log changes.

Notes:

1. The sensor needs to publish with the retain message flag set.

2. A sensor shouldn’t publish a timestamp with the sensor data if it needs to be logged.

If it does the message will appaer to have changed and the data logger would need to take this into account.

Data Loggers

I have created several data loggers for logging IOT data and specifically MQTT data.

Simple Python MQTT Data logger Python script Logs data to text file as JSON encoded Data.

SQL MQTT data Logger  Python script which logs data to a sqlite database.

Simple MQTT Topic logger -Python script Logs data by topic to a text file as JSON encoded Data

Node-Red Data and Topic Logger- Node Red Flow with web user interface that lets you choose to log by topic or not.

Resources and related tutorials

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