How to make two-way communication applications?

Question

Java

Hello!
I have a server (Spring), and a bunch of devices (Rasbery PI - there is an application in pure Java). Devices send data to the server, there are no problems with this.
But it is necessary:
​​1. Check whether the device is online or not.
2. Periodically send commands from the server to a specific device (encrypted). How to do this (the option when the device makes a request and checks if there are any commands for it is not suitable)?
XMPP comes to mind (didn't work with it), AMPQ, but I only used it for queues - is it intended for such a task? Is there something special for such tasks?
It should be noted that commands from the server to devices are very rare: from once an hour to once a day.
Thanks in advance.

Answer 1

WebSockets.