GATE | GATE-CS-2006 | Question 85

Consider the following log sequence of two transactions on a bank account, with initial balance 12000, that transfer 2000 to a mortgage payment and then apply a 5% interest.

  1. T1 start
  2. T1 B old=12000 new=10000
  3. T1 M old=0 new=2000
  4. T1 commit
  5. T2 start
  6. T2 B old=10000 new=10500
  7. T2 commit 

Suppose the database system crashes just before log record 7 is written. When the system is restarted, which one statement is true of the recovery procedure?
(A) We must redo log record 6 to set B to 10500
(B) We must undo log record 6 to set B to 10000 and then redo log records 2 and 3.
(C) We need not redo log records 2 and 3 because transaction T1 has committed.
(D) We can apply redo and undo operations in arbitrary order because they are idempotent


Answer: (B)

Explanation: We must undo log record 6 to set B to 10000 and then redo log records 2 and 3 bcoz system fail before commit operation. So we need to undone active transactions(T2) and redo committed transactions (T1)

Note: Here we are not using checkpoints.

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

Recovery:
When a system with concurrent transactions crashes and recovers, it behaves in the following manner −

=>The recovery system reads the logs backwards from the end to the last checkpoint.

=>It maintains two lists, an undo-list and a redo-list.

=>If the recovery system sees a log with and or just , it puts the transaction in the redo-list.

=>If the recovery system sees a log with but no commit or abort log found, it puts the transaction in undo-list.

All the transactions in the undo-list are then undone and their logs are removed. All the transactions in the redo-list and their previous logs are removed and then redone before saving their logs.

So Answer is B redo log records 2 and 3 and undo log record 6

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