How are realtime notifications for VKontakte and others implemented?

Question

Programming

Good day!
Tell me how real-time alerts are implemented on sites?
And how can this be implemented in PHP + JS?
I don't mean push, but when, for example, a message arrives, it is immediately visible, or when someone posted, it immediately displays: "XX new messages."
Apparently, this is not a feeble load on the database?
I will be glad to simple references on the topic.
Thanks in advance!

Answer 1

longpooling/websocket