It does not matter how the two communicate: http, functions, tcp, parallel wires, usb, etc…
They don’t even have to have the same communication system for registration and the call back. Its perfectly okay to register via http, and receive data back via usb, vice versa or any other combination.
What is needed is some special information.
- This special information is what the subscriber provides to the publisher on registration.
- This special information is also what is required for the publisher to transmit information back to the subscriber at some point in the future.
In the case of http, the special information is the ip, port, and whatever other configuration is required to get the right data, in the right format, sent to that ip and port.