COMP9315 Final Exam
COMP9315 Final Exam
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The University of Wales
COMP9315 DBMS Implementation
Final Exam
DBMS Implementation
[Instructions] [Notes] [PostgreSQL] [C]
[Q1] [Q2] [Q3] [Q4] [Q5] [Q6] [Q7] [Q8]
Question 8 (6 marks)
Consider the followng three transactions:
T1: T2: T3:
read(X) read(Y) read(X)
X = X + 1 Y = Y * 2 read(Y)
write(X) read(X) X = X – Y
read(Y) X = X + Y Y = Y – 3
Y = Y + 1 write(Y) write(X)
write(Y) write(X) write(Y)
commit commit commit
Consider also the following concurrent schedule on these three transactions:
T1: R(X) W(X) R(Y) W(Y) C |
T2: R(Y) R(X) W(Y) W(X) | system
T3: R(X) R(Y) | failure
Note that we have omitted the computation on X
and Y from the schedule.
You should assume that it occurs somwhere between the
read and the write of the relevant variable.
The line of | symbols indicates where
a system failure occurred.
Making the following assumptions:
R = read, W = write, C = commit, A = abort
initial value of X was 5, initial value of Y was 2
the system uses write-ahead undo/redo logging
log entries are written to disk just before the relevant write operation starts
the log is empty before T1 starts; scans of the log can go all the way to its start
the transaction descriptions show all of the intended operations
any operations not completed before system failure do not actually happen
answer these questions:
Show all entries written to the log as the transactions progress in the given schedule
(include
as well as entries for updates)
Show how recovery takes place when the system restarts after the system failure
which transactions are placed on the undo and redo lists
which undo operations occur; which redo operations occur
Show all working.
Instructions:
Type your answer to this question into the file called q8.txt
Submit via: give cs9315 exam_q8 q8.txt
or via: Webcms3 > exams > Final Exam > Submit Q8 > Make Submission
End of Question
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