Those sound like good options. I think the "last seen" is really useful on bot the node and sensor. I would only want to send notification for specific sensors or nodes and ideally the timing could be configurable, e.g. sensor x: 15 minutes, sensor b: 60 minutes.

Probably the custom script that runs every couple of minutes and checks the last seen is the most flexible approach.

When receiving a SET message from certain sensors i like to store the data in Grafana. Is there any way to configure a post request when the controller receives a SET message?

I would need to be able to configure the request variables and possibly the authentication information.

Typically my sensors send updates every 4 minutes and if nothing changes every 10 minutes. With several sensors on a node this results in usually the node communicating withe gateway/controller every minute. If the controller does not receive any messages from a node after 15 minutes I would like to send an SMS message - "Communication lost with Node 123 - possible power failure". Only 1 message should be sent no matter how long the communication failure lasts. After the node establishes communication again I would like to send an SMS - "Communication with Node 123 has been restored."

How would you suggest accomplishing this? Do you have the concept of a node being "online" or "offline"? Could the timers be used?