A DBMS system has transactions T1, T2, and T3 that are reading and writing data

Question

A DBMS system has transactions T1, T2, and T3 that are reading and writing data elements X, Y. read(T1, X) means T1 reads X from disk into memory write(T2, Y) means T2 writes Y to disk from memory. Consider the following schedules (actual order of operations). These schedules show the complete transactions -- there are no other changes to the data files beyond what is show here. For each schedule, circle SAFE (for no consistency problems) or UNSAFE (for consistency problems, e.g., lost update). read(T1,x), read(T2,x), write(Tl, x), write(T2,x), commit(T1), commit(T2) SAFE UNSAFE read(T1,x), read(T2,y), write(T3,x), read(T2,x), read(Tl, y), commit(T1), commit(T2), commit(T3) SAFE UNSAFE read(T1,x), read(T2,y), write(T1,x), read(t1,y), commit (T1), commit (T2) SAFE UNSAFE

Explanation / Answer

Schedule A is SAFE because any number of different Transactions can READ the same variable but can not allowed to perform the WRITE operation.But here WRITE operations are performed in the same order as they READ the variable x.

Schedule B is UNSAFE becase Tranction1 was READ the x but Tranction3 was done WRITE on same x which leads to data inconsistency problem.

Schedule C is SAFE because the READ of T3 and WRITE of T1 are in sequence and their COMMIT statements are also in order.

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