How are applications such as Whatsapp, Viber, etc. synchronized with the server?

Question

Java

Hello!
I am making a client-server application on android: client-A in the background thread must send its coordinates to the server every N-th number of seconds, client-B must synchronize with the server in case new coordinates appear and, if there are new ones, process them.
The thought just comes to my mind: the client-B service must make a request to the server every N seconds and check for an update. BUT! In this case, the phone must constantly perform a task, which will lead to increased battery consumption or, in the worst case, the system can be beaten for irrational work in general. Then the question arises: how then do such applications as Whatsapp work, they are instantly synchronized. What method is used?
Thanks in advance

Answer 1

While running, keep the connection to the server. When closed, receive new messages via FCM .

Answer 2

I don't know how WhatsApp does it, but if I wanted to create something like WhatsApp, I would use WebSocket.
https://en.wikipedia.org/wiki/WebSocket
https://hackernoon.com/how-to-build-your-own-real-...