Databases System. The complete book. Exercise 17.3.3. The following is a sequenc

Question

Databases System. The complete book. Exercise 17.3.3. The following is a sequence of redo logging records written by two transactions T and U:

< START T>; <T, A, 10>; < START U>; <U, B, 20>; <T, C, 30>; <U, D, 40>; <C0MMIT U>; < T ,E , 50>; <C0MMIT T>

The question: Describe the action of the recovery manager, including changes to both disk and the log, if there is a crash and the last log record to appear on disk is:

(a) < START U> (b) <C0MMIT U> (c) < T ,E , 50> (d) <C0MMIT T>

[ Note to the Expert: Please follow the specific instructions provided, please do not answer anything if you are not sure of your complete answer. Otherwise, I will fail the class and I cannot afford it again. Please write in understandable English. Thank you so very much. ]

please do not forget part b <C0MMIT U>

Explanation / Answer

The Recovery Manager guarantees Atomicity & Durability. Redo rule: before commit, new values it wrote must be in the stable storage (DB or log) These 2 transaction are mutually exclusive and has no dependency on each other So if one transaction is committed,it doesn;t effect the other.

a) Crash and the Log has <START U>

========================================

On the disk: Nothing is written
On the log : 3 incidents are records
1. < START T>;
2. <T, A, 10>;
3. < START U>;

*********************************************************************************

b)<C0MMIT U>

==============================================

On the disk:
1. <U, B, 20>;
2. <U, D, 40>;

=========================================

On the log : Below incidents are records ( Redo rule: before commit, new values it wrote must be in the stable storage (DB or log)

1. < START T>;

2. <T, A, 10>;

3. < START U>;

4. <U, B, 20>;

5. <T, C, 30>;

6. <U, D, 40>;

7. <C0MMIT U>;

*******************************************************************************

c) < T ,E , 50>;

On the disk:

1. <U, B, 20>;
2. <U, D, 40>;

On the log : Below incidents are records ( Redo rule: before commit, new values it wrote must be in the stable storage (DB or log)

1. < START T>;

2. <T, A, 10>;

3. < START U>;

4. <U, B, 20>;

5. <T, C, 30>;

6. <U, D, 40>;

7. <C0MMIT U>;

8.< T ,E , 50>;

********************************************************************************************

d) <C0MMIT T>
On the disk:
1. <T, A, 10>;
2. <U, B, 20>;
3. <T, C, 30>;
4. <U, D, 40>;
5. < T ,E , 50>;

On the log : Below incidents are records ( Redo rule: before commit, new values it wrote must be in the stable storage (DB or log)

On the log :

1. < START T>;

2. <T, A, 10>;

3. < START U>;

4. <U, B, 20>;

5. <T, C, 30>;

6. <U, D, 40>;

7. <C0MMIT U>;

8.< T ,E , 50>;

9. <C0MMIT T>

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