Hello interested question. For example, as in VK or gmail or the same icq. Just in the console, requests to the server (VC), leave at intervals of about 15 seconds, but the message, as expected, comes immediately after sending them to the interlocutor. When the message arrives, the page accesses the server even in the middle of the 15-second interval. With the help of what can be implemented like this or on what principle does the update of incoming messages work or how is it called? Thank you in advance.
PS You can of course with a second interval knock on the server, but it seems to me that it is like a collective farm or something.