(The traditional HTTP protocol is stateless. Each request must be initiated by the client (such as the browser), and the server will return the response result after processing, so it is difficult for the server to actively send data to the client; This traditional web mode, in which the client is the active party and the server is the passive party, has less trouble for web applications with infrequent information changes, but brings great inconvenience for web applications involving real-time information, such as instant messaging apps Real time data, subscription push, etc. Before the websocket specification was proposed, developers often used tradeoffs to implement these real-time functions: polling and comet technology. In fact, the latter is essentially a kind of polling, but it has been improved.)
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