You don't need a backend server to make a smart doorbell that pings your phone over Wlan. You only need one if you want to add loads of non-trivial functionalit

You don't need a backend server to make a smart doorbell that pings your phone over Wlan. You only need one if you want to add loads of non-trivial functionality for non-technical users who might not be on the same network. You're welcome to build your own device with a wifi capable microcontroller and a simple phone app directly accessing the socket interface.