How select and send to BBDD only the changed values

Dear all,

I am using the node s7 to read some data from a PLC,
I am reading and getting the data correctly.
In the node configuration exist a checkbox to get data only if a value has changed, that is, if one value changed, I get an actualization of the complete list of variables I am reading.

Data I am getting from PLC:

The thing is that I would like to write on mysql only the data that has changed.... I dont have idea how to do it being efficiently...

I have made a test making a function for each variable and works ok... But is the right way??? I could do it, but what will happen if I am reading 400 variables? 400 functions? hahah...

Test I did for some variables...(for each PLC Variable...)

Let me know your opinions, please!!!

You will need to retain the previous value. Then when you get a new value, walk through each property on the payload and compare with the same property from the saved value. If different, write to a new object and then output that object.

In JavaScript, you can walk through all the properties of an object with something like:

const out = {} // create output object

Object.keys(msg.payload).forEach( keyName => {
    // ...

Many Thanks @TotallyInformation !!!

I have been making some test like this:

Being honest, I dont have idea how can I retain each previous value and then construct a query with the only the data has changed... could you give me some advice?

Thanks again!

You can use a context variable for retention. context.get and context.set - you will find the details in the official docs.

You can use a filter function if you want or can do things manually with a simple comparison. Filter is probably slightly more efficient but I doubt you would notice unless really pushing a lot of traffic.

context variables are memory based and will be lost if Node-RED restarts. You can, however, set up a file-based persistent variable store in your settings.js file should you wish to.

While certainly do-able - depending on the total number of variables you are potentially writing I can't help feeling that trying to figure out just the changed fields and constructing a suitable insert/update for just those is going to be more complex and less efficient than just re-writing the entire row - and indeed if they are timeseries type data is MySQl even the right sort of database for that sort of data?

1 Like

You don´t need any function for that. Just use the "All variables, one per message" mode of S7 node and add the RBE node right after that.

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.