Serializability schedules: can someone help me understand it?

I am struggling with the Databases courses I have, so I am trying to find answer to my question. This has been asked: > Add the appropriate locks to enforce serializability on the following > schedule: > > T1: W(Y) > > T2: R(Y) And: > Add the appropriate locks to enforce serializability on the following > schedule: > > T1: R(Y) R(Z) W(Y) > >T2: W(Y) R(Z) For me, right now, it's hocus pocus. Could someone explain to me what exactly I need to do here... as in: **what's the structure I need to follow**? I need some structure to understand it. > R(Y) is a read action on database object Y > > W(Y) is a write action on Y > > S(Y) sets a shared (read) lock on Y > > X(Y) sets a exclusive (write) lock on Y > > U(Y) releases any lock on Y PS: it's not about the answer to the questions, although I would appreciate it... yet it's about me understand WHY that's the answer. Hope someone can help me. I'd appreciate it a lot!