Question

Question 1: Assume that initially A-5 and B:10. The following are 3 transactions on A and B (→ denotes assignmen): TI : A+B → A: A+B → B T2: A+B → B: A+B → A * For each transaction, a) add necessary read- and write-actions to the computation; b) show the undo/redo-log records for each of the transactions. Question 2: The following is a sequence of undo/redo-log records generated by two transactions, T and U: COMMIT U>:<T,E,50.51: <COMMIT T> Suppose there is a crash and, upon recovery, the last log record appeared on disk is one of the following four possibilities: (a) <START U (b) COMMIT U> (d) <COMMIT T> For each possibility, to recover from the crash, describe what values written by T and U must appear on disk, and which values might appear on disk. Describe the actions of the recovery manager, including changes to both disk and the log.

Answer 1

Hi,
Its against HomeworkLib policy to ask multiple questions in one, please post others as separate questions
Question 1:
Given transactions are
T1:A+B->A; A+B->B
T2:A+B->B;A+B->A
T3:B+1->A;A+1->B
a) read and write transactions.
To perform any computation on any variable, we need to read it first
so in T1, the read writes would be as follows
read A
read B
write A+B to B
read A
read B
write A+B to A
for T2:
read A
read B
write A+B to A
read A
read B
write A+B to B
for T3:
read B
write B+1 to A
read A
write A+1 to B

b) undo/redo log are written when a write happens so that the system can be recovered when a crash happens
therefore
in T1: we have the following reads and writes as shown above
read A
read B
write A+B to B - this is added as <T1,B, 10,15> in the format <transactionId, element, before value, after value>
read A
read B
write A+B to A - this is added as <T1,A, 5,20> in the format <transactionId, element, before value, after value>

for T2:
read A
read B
write A+B to A- this is added as <T1,A, 5,15> in the format <transactionId, element, before value, after value>
read A
read B
write A+B to B- this is added as <T1,B, 10,25> in the format <transactionId, element, before value, after value>

for T3:
read B
write B+1 to A - this is added as <T1,A, 5,11> in the format <transactionId, element, before value, after value>
read A
write A+1 to B - this is added as <T1,B, 10,12> in the format <transactionId, element, before value, after value>
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