Concurrency Problems

Question

T1	T2
R(A)
	R(A)
	R(B)
R(B)
	W(A)
	COMMIT
R(A)
W(A)
COMMIT

in the above problem identify the concurrency in the following Schedule involving 2 Transaction T1 AND T2 (r: read and w : write )

a) Dirty Read problem 

b) Unrepeatable Read problem 

c) Lost update Problem 

d) Both a and b 

Comments

According to me my answer is Lost update Problem , because whatever T2 update its final value of A it is replaced By T1 . Hence Lost update problem was one of my option .

And Second would be Unrepeatable Problem because when T1 read value of A first time it would differ with second Read of same data item (A) because  in between T2 has update it . So this doesnt  preserve Isolation Property . Hence i opted for this answer .

Am i right ?

Answer 1

• Dirty read ( uncommitted W-R ) : does not exist here.
• Unrepeatable read ( R-W ) : exists. As R(A) is read in T1 and W(A) is done in T2.
• Lost update ( W-W ) : does not exist here'

 

So , Unrepeatable read exists here.

Answer 2

every write commit before next operation..

no Write Write conflict so no Lost update problem.
no Dirty read also..

Comments

Okay should i say "WRITE-WRITE CONFLICT " happen between any 2 consecutive 2 write operation if there is no operation in between them .

Surely the above question is NOT DIRTY READ BUT IT  is "UNREPEATABLE READ PROBLEM " !