HTTP server push (also known as HTTP streaming) is a mechanism for sending data from a web server to a web browser. HTTP server push can be achieved through several mechanisms.
Generally the web server does not terminate a connection after response data has been served to a client. The web server leaves the connection open such that if an event is received, it can immediately be sent to one or multiple clients.
如：Non-Parsed Headers scripts on Apache
The server never terminates the response, effectively fooling the browser into continuing in "loading" mode after the initial page load would normally be complete
With long polling, the client requests information from the server in a similar way to a normal poll. However, if the server does not have any information available for the client, instead of sending an empty response, the server holds the request and waits for some information to be available. commet就是这么玩的。
Flash XMLSocket relays