http://www.cnblogs.com/dinglang/p/5679542.html
http://blog.csdn.net/zxl315/article/details/53433707
http://blog.csdn.net/zheng0518/article/details/51194942
幂等性的概念
http://www.cnblogs.com/weidagang2046/archive/2011/06/04/idempotence.html