Thomas write Rule
Thomas Write Rule provides the guarantee of serializability order for the protocol. It improves the Basic Timestamp Ordering Algorithm.
The basic Thomas write rules are as follows:
If we use the Thomas write rule then some serializable schedule can be permitted that does not conflict serializable as illustrate by the schedule in a given figure:
Figure: A Serializable Schedule that is not Conflict Serializable
In the above figure, T1's read and precedes T1's write of the same data item. This schedule does not conflict serializable.
Thomas write rule checks that T2's write is never seen by any transaction. If we delete the write operation in transaction T2, then conflict serializable schedule can be obtained which is shown in below figure.
Figure: A Conflict Serializable Schedule
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