How two-phase locking helps avoid the non-serializable schedule?

Question

Now, I have two transactions here. The consistency requirement be A=0 v B=0, with A=B=0 the initial values

T1: read(A);
    read(B);
    if A=0 then B:=B+1;
    write(B).

T2: read(B);
    read(A);
    if B=0 then A:=A+1;
    write(A).

I know that there is no parallel execution resulting in a serializable schedule. So I want to apply two-phase locking to guarantee serializability.

How dose two-phase locking guarantee serializability? Can you give me one example of these two transactions?

Thank you.

What would two-phase locking mean in the context of this problem - lock A then B before entering either T1 or T2? — tariksbl, May 04 '14 at 00:03

score 0 · Answer 1 · answered Oct 30 '14 at 21:29

As you mentioned there is no parallel execution resulting in a serializable schedule i.e the schedule is not conflict serializabe.

If a schedule is not conflict serializable then two-phase locking protocol leads to deadlock.So you cannot use two-phase locking to guarantee serializability.

How two-phase locking helps avoid the non-serializable schedule?

1 Answers1