Android Question How B4J Server (backend) can send messages to mobile phone app?

Hi,

What is the best or common way to B4J Server (backend) send communication to mobile phone app?
For example: server send a message to one specific mobile phone.
I've read about MQTT, Serversocket/socket, push notification.
Real scenario is: server should know the "status" app (more than 300 clients [mobile phone]). If status is off, server will notify an administrator by email.
I think about the clients regularly send status to server but I'm more interested on "Server send the messagem to clients".

Thank you.