How is receiving VK messages arranged?

Question

PHP

I noticed that VK sends a request to the server after some indefinite time. Maybe in a second, or maybe even in a minute. But when a message is sent to me, VK immediately makes a request. Maybe the server itself sends a signal about a new message? The research was done in developer tools.

Answer 1

I don't think it's worth duplicating answers.
How is the notification system a la Vkontakte organized?

Answer 2

most interesting