Validation based Protocol
Consider two schedules S1 and S2, where the same set of transactions participates in both schedules.
These two schedules S1 and S2 are said to be ‘View Equivalent’, if the following three conditions are met.
a) For each data item Q,
if transaction Ti reads the initial value of Q in schedule S1,
then transaction Ti must, in schedule S2, also read the initial value of Q.
b) For each data item Q,
if transaction Ti executes read(Q) in schedule S1,
and if that value was produced by write(Q) operation executed by transaction Tj,
then the read(Q) operation of transaction Ti must, in schedule S2, also read the value of Q that was produced by the same write(Q) operation of transaction Tj.
c) For each data item Q,
The transaction that performs the final write(Q) operation in Schedule S1 must perform the final write(Q) operation in schedule S2.
The concept of ‘view equivalent’ leads to the concept of ‘view serializability’.
A schedule S1 is said to be ‘view serializable’, if it is view equi-valent to serial schedule.
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