The http protocol is essentially asymmetrical: a client application such as a browser issues requests and the server responds. It is not possible for the server to initiate communication with the client. Certain types of applications are therefore more difficult to design.
WebSocket is a network protocol for creating bidirectional communication channels between browsers and web servers. Most browsers support WebSocket, although the standard is relatively recent (2011). It enables the client to be notified of a change in server status, without having to make a request.
WebSocket is relatively efficient. I wrote an elementary WebSocket benchmark in JavaScript. In my benchmark...
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