CAP
http://ksat.me/a-plain-english-introduction-to-cap-theorem/
http://www.julianbrowne.com/article/brewers-cap-theorem
http://mwhittaker.github.io/2016/01/08/paxos/
http://mwhittaker.github.io/2014/08/16/illustrated-proof-cap-theorem/
http://rystsov.info/2016/01/05/raft-paxos.html
https://martin.kleppmann.com/2015/05/11/please-stop-calling-databases-cp-or-ap.html
Zookeeper并不保证读取的是最新数据
