Polling and Long Polling

1. Polling: The client sends Ajax requests to the server regularly, and the server returns the response information and closes the connection immediately after receiving the request.
   Advantages: It is easier to write back-end programs.
   Disadvantages: Most of the requests are useless, wasting bandwidth and server resources.
   Example: Suitable for small applications.

2. Long polling: The client sends an Ajax request to the server, the server holds the connection after receiving the request, returns the response information and closes the connection until there is a new message, and the client sends a new request to the server after processing the response information.
   Advantages: Infrequent requests in the absence of messages.
   Disadvantage: Server holding connections consumes resources.
   Examples: WebQQ, Hi web version, Facebook IM.