Define Thomas write rule

Thomas' write rule: The Thomas' write rule is a modification of timestamp-ordering protocol for concurrency control. Assume that transaction T_i issues write (Q) and T_S (T_i) is its timestamp after that the subsequent actions are taken depending on T_S(T ) and the read-write timestamps of the data item Q:

• If T_S (T_i) i is generating was previously needed, and it had been assumed that the value would never be produced. Therefore, the system rejects the write operation and rolls Ti back.
• If T_S (T_i) < W-timestamp (Q), after that Ti is attempting to write an obsolete value of Q. Therefore, this write operation can be ignored otherwise, the system carries out the write operation and sets W-timestamp (Q) to T_s(T_i).