참조 : Wikipedia-Optimistic concurrency control
[http://en.wikipedia.org/wiki/Optimistic_concurrency_control]
> optimistic concurrency control, (OCC) is a concurrency control method used in
relational databases without using locking. It is commonly referred to as
optimistic locking, a misnomer.Optimistic concurrency control
Non-lock concurrency control 방법들 중에 하나로 Locking을 사용하지 않고