Android Question what is the best protocol for an app with 1m users

There isn't enough information to answer your question.

What is the role of the backend server?

jServer has excellent performance. Some numbers: https://www.b4x.com/android/forum/threads/37502/#content

Erel said:

There isn't enough information to answer your question.

What is the role of the backend server?

jServer has excellent performance. Some numbers: https://www.b4x.com/android/forum/threads/37502/#content

Click to expand...

the main role of server is to handle the messages of users to each other ( P2P )

You should use Firebase Push framework to send messages while your app is not running.

Once your app is running then you can use WebSockets + regular handlers to send messages. WebSockets are used for server events and regular handlers can be used to send larger pieces of data.

As your app grows you can add more servers to the backend.

Thanks Erel