http

http ajax

http polling ajax

http long-polling ajax

html5 server sent events

html5 websocket

comet

reference：

0. http://stackoverflow.com/questions/11077857/what-are-long-polling-websockets-server-sent-events-sse-and-comet/12855533#12855533

1. websocket 实现对比: http://einaros.github.io/ws/clients/index.html

2. 

http realtime response 基于http的实时响应方式的演进