register client on web server for push notifications

I have a web server that i want to use as a central contact point for IFTTT which will use PHP (maybe) to relay messages from IFTTT to a registered client.

What i mean by registered client is to have an arduino/esp32 etc that does something such as an HTTP request to the webserver to register its existance, eg 'hi im a light and my location is the kitchen' :)

My code on the server can then contact the registered client to give it the communication payload to, for example, turn on the light.

my question is: what is the process to achieve this ? How do i registered the client device onto my web server (i could store information in a mysql db but what information would i store) how does the server then use that information to push comms to the client device

any help is very much appreciated