websocket 和 http的比较
http://www.ruanyifeng.com/blog/2017/05/websocket.html
https://zhuanlan.zhihu.com/p/40381471
https://juejin.im/post/6844903955240058893
为什么用websocket,不用socket:
https://github.com/onlyliuxin/coding2017/issues/497
使用websocket的几个场景:
https://www.oschina.net/translate/9-killer-uses-for-websockets