Consider a simple checkpointing protocol and the following set of operations in the log. (start, T4); (write, T4, y, 2, 3); (start, T1); (commit, T4); (write, T1, z, 5, 7); (checkpoint); (start, T2); (write, T2, x, 1, 9); (commit, T2); (start, T3), (write, T3, z, 7, 2); If a crash happens now and the system tries to recover using both undo and redo operations, what are the contents of the undo list and the redo list?

Consider a simple checkpointing protocol and the following set of operations in the log. (start, T4); (write, T4, y, 2, 3); (start, T1); (commit, T4); (write, T1, z, 5, 7); (checkpoint); (start, T2); (write, T2, x, 1, 9); (commit, T2); (start, T3), (write, T3, z, 7, 2); If a crash happens now and the system tries to recover using both undo and redo operations, what are the contents of the undo list and the redo list? Correct Answer Undo: T3, T1; Redo: T2

Concept:

Check pointing is a mechanism where all the previous logs are removed from the system and stored permanently in the storage disk. Checkpoint declares a point before which the DBMS was in consistent state, and all the transactions were committed.

It maintains two list - undo list and redo list.

• If the recovery system sees a log with <Tn, start> and <Tn, commit> or just <Tn, commit> , it puts the transaction in the redo list.

• If the recovery system sees a log with <Tn, start> but no commit or abort log found, it puts the transaction in undo list.

Explanation:

Above operation are shown in table as:

Now, as T1 and T3 are uncommitted, they must be undone. T2 is committed but it is after checkpoint so, it must be redone. As, T4 is already committed before checkpoint so, it neither comes in undo list nor in redo list.