Test by Bikram | Databases | Test 2 | Question: 3

Question

Consider the following schedule:

$\begin{array}{|c|c|} \hline T1&  T2 \\ \hline R(A) &{} \\ \hline R(B) & {} \\ \hline R(C) & {} \\ \hline {} & R(A) \\ \hline {} & R(B) \\ \hline {} & F: \text{_____} \\ H: \text{______} & {} \\ \hline {} & R(C) \\ \hline \end{array}$

What are the possible values of F & H for which the above schedule is allowed under Thomas Write Rule but NOT under Basic Timestamp Ordering Protocol if $\text{timestamp(T2)} > \text{timestamp(T1)}$?

• A. $W(B), W(C)$
• B. $W(A), W(A)$
• C. $W(A) ,W(B)$
• D. $W(C), W(A)$

Comments

@Bikram sir,please explain the answer

---

Sir Please Explain

---

i think sir option b is also not allow because thoms rules can help only for w-w conflict

if we put b option in question than t1R(x)------>t2w(X)

                                                 t1W(x)<------t2R(X)

---

In this none of the options are correct.

Answer 1

The answer looks bigger(I noticed after writing) it is because I copied the above table twice, otherwise there are just a few lines. 

T1	T2
R(A)
R(B)
R(C)
	R(A)
	R(B)
	F:___W(B)______
H:____W(C)____
	R(C)

Option A allows both in basic timestamp ordering and Thomas write rule,
W(C) of T1 checks the Read_TS(C)>TS(T1) || Write_TS(C)>TS(T1)
which will be false
i.e. no new transaction has read C before T1,  so T1 can write successfully.
 After that T2 will check for Write_TS(C)>TS(T2) which will be false i.e now newer transaction has written before T2 want to read it, so allowed to read C.

T1	T2
R(A)
R(B)
R(C)
	R(A)
	R(B)
	F:__W(A)_______
H:__W(A)______
	R(C)

    Option B:

Let T1, T2 have timestamps as 100,200 i.e T1 is older and T2 is newer.

Then when T2 want to write(A) it will check
Read_TS(A)>TS(T2) || Write_TS(A)>TS(T2) which will be false as lastly T2 itself read the value and now newer transaction has written or read A. So T2 will be allowed to Write A and update Write_TS(A)=200. 

when T1 want to write(A) it will check
Read_TS(A)>TS(T1) || Write_TS(A)>TS(T1) which will be true, hence it will be not allowed in to do this operation and will be rolled back.
But in timestamp ordering this will allowed as just this Write(A) of T1 will be just ignored, T2 has already written the new value of A.
 

To get the last line, it means if T1 has to write the value of A as 10 and T2 as 20 when they executed in timestamp order(T1->T2 as TS(T1)<TS(T2)) last value of A will be written by T2 i.e 20. But Thomas is a smart guy, he thought when T2 has already updated A's value as 20 why to bother about T1 which is updating it to an obsolete value, so he just ignored the T1's Write(A).

Comments

No, Option B doesn't fail Thomas Write Rule. TWR ignores obsolete (blind) writes. Read the last line of your image @Sambhrant Maurya

---

@JashanArora In either Basic Timestamp Ordering or Thomas Write rule, first the Timestamp will be checked against RTS, and then WTS. Here Option B fails against RTS itself. The last line of my image says that timestamp should be greater than RTS and here it's not !

---

 Sambhrant Maurya I missed that. You're right. :)