It doesn't sound like a homework problem, or "personal research", where reinventing the wheel has pedagogic value. Based on that, I'm not going to try to answer either question.

Your first line of attack should be to use an RDBMS that provides row level locking. You get the benefit of the considerable effort others have put into making it work right. You also get to focus your attention on your application instead of having to become a database engine writer as well as an application developer.