http://snoopyxdy.blog.163.com/blog/static/6011744020157511536993/
http://www.csdn.net/article/2012-11-15/2811920-mongodb-quan-gong-lue
https://ruby-china.org/topics/20128
http://www.tuicool.com/articles/JRVNbin
https://docs.mongodb.org/manual/tutorial/perform-two-phase-commits/
http://www.runoob.com/mongodb/mongodb-tutorial.html
http://www.csdn.net/article/2015-01-14/2823551
http://www.tuicool.com/articles/BFfuMbr