For the schedules below, assume that each transaction takes a lock on each datab

Question

For the schedules below, assume that each transaction takes a lock on each database element immediately before it reads or writes the element, and that each transaction releases its locks immedi­ ately after the last time it accesses an element. Tell what the locking scheduler would do with each of these schedules; i.e., what requests would get delayed, and when would they be allowed to resume?

1. r1(A); w1(B); r2(B): w2(C); r3(C); w3(A);

2.  r1(A); r2(A); w1(B); w2(B); r1(B); r2(B); w2(C); w1(D);

Explanation / Answer

1. r1(A); w1(B); r2(B): w2(C); r3(C); w3(A);

In this read operation is being performed on A so that ead lock is acquired on this, then on B write operation is being performed therefore it is holding a lock.

so that the instruction r2(B) will get delayed it must wait untill the lock on it releases , then scheduler acquires lock on C with write operation so that r3(C) is not possible untill the w2(C) completes. then w3(A) is also get's wait utill the lock releases.

2. r1(A); r2(A); w1(B); w2(B); r1(B); r2(B); w2(C); w1(D);

In this  r1(A); r2(A); doesn't get any problem as they both are read operations. then w1(B) will hold the lock, and w2(B) must wait untill lock releases then r1(B); r2(B) also must wait untill the write lock releases then w2(c) can put lock on C and then w1(D) also can put lock on D.

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