I dug a little into the JS (JS isn’t my area though), I can find a function called list_format_updated that seems to the last updated time of the feed and format it out for the UI.
I can’t quite trace it back far enough to work out where the data comes from.
I dumped all the Redis keys too, but nothing jumped out at me.
If I can extract that last update time from EmonCMS somehow, I think I could do a NodeRed script that would alert me if the feed is inactive.
Whilst I love “going to the horse’s mouth” for data, the benefit with using the API here is that you’ll be somewhat insulated from technical changes at the backend. As we move between MySQL, PHPFINA, Redis and other improvements you’ll probably want to avoid having to re-write your alerting
It tends to be a bit hungry for memory. If you install netdata, you can monitor the memory usage remotely. Alternatively monit will do that too if you expose the httpd.