How to update insert new record with updated value from staging table in Azure Data Explorer - azure-iot-hub

Question

I have requirement, where data is indigested from the Azure IoT hub. Sample incoming data
{
"message":{
"deviceId": "abc-123",
"timestamp": "2022-05-08T00:00:00+00:00",
"kWh": 234.2
}
}
I have same column mapping in the Azure Data Explorer Table, kWh is always comes as incremental value not delta between two timestamps. Now I need to have another table which can have difference between last inserted kWh value and the current kWh.
It would be great help, if anyone have a suggestion or solution here.
I'm able to calculate the difference on the fly using the prev(). But I need to update the table while inserting the data into table.

Answer 1

As far as I know, there is no way to perform data manipulation on the fly and inject Azure IoT data to Azure Data explorer through JSON Mapping. However, I found a couple of approaches you can take to get the calculations you need. Both the approaches involve creation of secondary table to store the calculated data.
Approach 1
This is the closest approach I found which has on-fly data manipulation. For this to work you would need to create a function that calculates the difference of Kwh field for the latest entry. Once you have the function created, you can then bind it to the secondary(target) table using policy update and make it trigger for every new entry on your source table.
Refer the following resource, Ingest JSON records, which explains with an example of how to create a function and bind it to the target table. Here is a snapshot of the function the resource provides.
Note that you would have to create your own custom function that calculates the difference in kwh.
Approach 2
If you do not need a real time data manipulation need and your business have the leniency of a 1-minute delay, you can create a query something similar to below which calculates the temperature difference from source table (jsondata in my scenario) and writes it to target table (jsondiffdata)
.set-or-append jsondiffdata <| jsondata | serialize
| extend temperature = temperature - prev(temperature,1), humidity, timesent
Refer the following resource to get more information on how to Ingest from query. You can use Microsoft Power Automate to schedule this query trigger for every minute.
Please be cautious if you decide to go the second approach as it is uses serialization process which might prevent query parallelism in many scenarios. Please review this resource on Windows functions and identify a suitable query approach that is better optimized for your business needs.