Comet & asychronous HTTP

Comet [1] is web application model to achieve "server push" or "reverse ajax".

Servlet 3.0 provides the asynchronlus support which allows the servelet request to be suspended and resumed, i.e., non-blocking, to avoid thread starvation.

HTTP persistent connection, also called HTTP keep-alive, is the idea of using a single TCP connection to send/recieve mulitple HTTP request/response. It is also controversial as that it may tie up server side thread for too long. And there is a related idea about HTTP pipelining in which multiple HTTP requests are sent on a single TCP connection without waitting for the corresponding responses.

Proxy server can be use to deal with cross-domains issue for hidden iframe and XMLHttpRequest, making them appears to originate from the same domain.