Test by Bikram | Databases | Test 2 | Question: 10

Question

We are using timestamp-based concurrency control, and there are two transactions T1 and T2 with timestamps 100 and 200, respectively.

The following schedule is executed in the order shown below using time stamping protocols:

$r1(A); \ w1(A); \ r2(A); \ r2(B); \ w1(B); \ c1; \ w2(A); \ c2;$

Which of the following is TRUE about the above schedule?

• A. It is allowed under Basic Timestamp Ordering Protocol.
• B. It is not allowed under Basic Timestamp Ordering Protocol since T1 is rolled back.
• C. It is not allowed under Basic Timestamp Ordering Protocol, but is allowed under Thomas Write Rule.
• D. It is not allowed under Basic Timestamp Ordering Protocol and Thomas Write Rule, but is allowed under Multi-version Timestamp Ordering Protocol.

Answer 1

Given - Two transactions T1 and T2 with time-stamps 100 and 200, respectively. 

Timestamp ordering rule says :

if r_i (x) and w_j (x) are conflicting operations then r_i (x) is processed before w_j (x) iff ts(Ti ) < ts(Tj )

This is the table for given schedule :

T1: timestamp value	T2 : timestamp value
r1(A);  101
w1(A); 102
	r2(A); 201
	r2(B); 202
w1(B); 103
c1;
	w2(A); 203
	c2;

T1 is rolled back as it is committed before complete  Write() operation of T2 ,  .. 

1. Whenever a transaction T issues a write_item(X) operation, the following is checked:

a. If read_TS(X) > TS(T) or if write_TS(X) > TS(T), then abort and roll back T and reject the operation.

This should be done because some younger transaction with a timestamp greater than TS(T)— and hence after T in the timestamp ordering—has already read or written the value of item X before T had a chance to write X, thus violating the timestamp ordering. 

2. Whenever a transaction T issues a read_item(X) operation, the following is checked:

a. If write_TS(X) > TS(T), then abort and roll back T and reject the operation. This should be done because some younger transaction with timestamp greater than TS(T)—and hence after T in the timestamp ordering—has already written the value of item X before T had a chance to read X.

Here Timestamp of T2 is 203 > Timestamp of T1 which is 103 so T is roll back .

 Transaction T not satisfying any one of the Basic timestamp ordering protocol , Thomas Write Rule and  Multi version Timestamp ordering protocol . 

Hence only option B is true .

Comments

Sir why C is not true?

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For a specific transaction, timestamps should be same for all the read and write operations.

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What about Thomas Write Rule, according to that rule also the Transaction T1 will be restarted with a newer timestamp....