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Consider the schedule $S = \{R1(A),R2(B),W2(A),W1(A)\}$

Assume that Transaction T1 has started execution before Transaction T2.

Which of the following is TRUE in the above case?

  1. Allowed under Basic Timestamp Protocol, but not under Thomas Write Rule.
  2. Not allowed under Basic Timestamp Protocol, but allowed under Thomas Write Rule.
  3. Allowed under both Basic Timestamp Protocol and Thomas Write Rule.
  4. Not allowed under both Basic Timestamp Protocol and Thomas Write Rule.
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The schedule written in the tabular form:

T1 T2
R(A)  
  R(B)
  W(A)
W(A)  

In Thomas' write rule, we "ignore the outdated updates".

Since T1 started execution first, so, the W1(A) (in the last row), is an outdated updated. This will be ignored in Thomas' write rule. But in the basic timestamp protocol, this will cause T1 to roll back.

So, correct option is (B)

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