| 3. |
(a) Consider the following
scenario for three users of a database system: |
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| 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 |
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|
|
(i) Will the users be able to
process their transactions? Explain what will happen. |
[4] |
|
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No, the
users will not be able to process their transactions, because deadlock will occur. |
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|
[1 mark] |
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|
|
Explanation
of how deadlock occurs |
|
|
[3
marks] |
|
|
|
(ii) Describe two ways of managing
the above situation. |
[4] |
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|
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deadlock prevention; |
[1] |
|
|
|
explanation |
[1] |
|
[2
marks] |
|
|
|
|
deadlock resolution; |
[1] |
|
|
|
explanation |
[1] |
|
[2
marks] |
|
|
|
|
|
(b) Consider the following scenario, showing
the timing of various transactions, leading up to a system failure: |
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|
 |
|
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tc represents the time of
the last checkpoint,and tf represents the time of the system failure. |
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|
|
(i) When the system failure
occurs, how many transactions need to be rolled back? Explain your answer. |
[4] |
|
|
only
transactions T3 and T5 need to be rolled back |
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[1 mark] |
|
|
|
Explanation
of why |
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|
[3
marks] |
|
|
|
(ii) Explain what is meant by
database recovery. |
[2] |
|
|
restoring
a database quickly and accurately after loss or damage. |
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|
[2
marks] |
|
|
|
(iii) Describe three facilities
which a DBMS might provide to aid recovery. |
[6] |
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|
|
backup facility |
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|
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journaling facility |
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|
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checkpoint facility |
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For each
facility: one mark for identification, and one for description. |
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[6
marks] |
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