| 3. |
(a) Consider the following
scenario for three users of a database system: |
|
|
| Time |
User A |
User B |
User C |
| t1 |
Lock record X |
|
Lock record Z |
| t2 |
|
Lock record Y |
|
| t3 |
Request record Y |
|
|
| t4 |
|
Request record Z |
|
| t5 |
|
|
Request record X |
|
|
|
|
(i) Will the users be able to process their
transactions? Explain what will happen. |
[4] |
|
|
(ii) Describe two ways of managing the above
situation. |
[4] |
|
|
|
|
(b) Consider the following
scenario, showing the timing of various transactions, leading up to a system failure: |
|
|
 |
|
|
tc represents the time of
the last checkpoint,and tf represents the time of the system failure. |
|
|
|
(i) When the system failure occurs, how many
transactions need to be rolled back? Explain your answer. |
[4] |
|
|
(ii) Explain what is meant by database
recovery. |
[2] |
|
|
(iii) Describe three facilities which a DBMS
might provide to aid recovery. |
[6] |