When a method is idempotent, in
short, you are guaranteeing that regardless of when or how often it's
used, no "side effects" will occur.
對每一個相同的query string
不管你執行多少次
所得到的結果要一樣
doPost()
This method does not need to be either safe or idempotent. Operations
requested through POST can have side effects for which the user can be held
accountable, for example, updating stored data or buying items online.