Polling and long polling

1. Polling : The client periodically sends an AJAX request to the server, returning the response information and closing the connection as soon as the server receives the request.
   Pros: It's easier to write back-end programs.
   Cons: Half 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 receives the request and hold the connection until a new message returns the response information and closes the connection, and the client finishes processing the response and sends a new request to the server.
   Advantage: No more frequent requests in the absence of messages.
   Disadvantage: Server hold connections consume resources.
   Example: WEBQQ, HI Web version, Facebook IM.

In addition, there is a distinction between long connections and socket connections:

1. Long connection : Embed a hidden iframe in the page, set the src attribute of this hidden iframe to a long connection request, and the server can continuously input data to the client.
   Advantage: Instant message arrives without unwanted request.
   Disadvantage: The server maintains a long connection that increases overhead.
   Example: Gmail Chat

2. Flash Socket: Embed a flash program JavaScript using the socket class within the page to communicate with the server-side socket interface by invoking the socket interface provided by this flash program. JavaScript controls the display of the page after it receives information from the server side.
   Pros: Realize real instant communication, not pseudo-instant.
   Disadvantage: The client must have a Flash plugin installed, a non-HTTP protocol, and cannot automatically traverse the firewall.
   Example: Network interactive game.

The above is the introduction of four kinds of request methods and comparison of advantages and disadvantages.

Polling and long polling