DATABASE SYSTEM CONCEPTS (LOOSELEAF)
DATABASE SYSTEM CONCEPTS (LOOSELEAF)
7th Edition
ISBN: 9781260515046
Author: SILBERSCHATZ
Publisher: MCG
Expert Solution & Answer
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Chapter 19, Problem 11PE

Explanation of Solution

a.

Example for using normal transaction undo mechanism:

Consider a bank account S with balance $200.

  • Let two transactions individually deposit $20 in the account. Thus, the final balance should be $240.
  • Now, when the transactions are executed in sequence T1, T2 the log records would be as follows:

    < T1, S, 200, 220 >

    < T2, S, 220, 240 >

  • Now, transaction T1 needs to be undone...

Explanation of Solution

b.

Implementing point-in-time recovery using database dumps:

To modify the recovery algorithm from section 19.4 to implement point in time recovery using database dumps;

  • Identify the latest archival dump (say D) before the start of the erroneous transaction (say Te) that is < Te, start>.
  • Restore the database using this dump D.
  • Now, perform a redo of all log records starting from the dump till the transaction committed that is < Te, commit >.
  • Let S be the set of those transactions which were active at the commit time of Te.
  • Rollback all transactions in the set S including Te...

Explanation of Solution

c.

Re-executing non-erroneous transactions logically:

To understand the reason the later non-erroneous transactions cannot be re-executed logically using log records, consider again the example from part (a);

  • Let both the transactions be undone and the balance be reverted back to $200.
  • Now, if T2 needs to be redone while using the log record < T2, ...

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