Thomas write rule

In computer science, particularly the field of databases, the Thomas write rule is a rule in timestamp-based concurrency control. It can be summarized as ignore outdated writes. It states that, if a more recent transaction has already written the value of an object, then a less recent transaction does not need to perform its write since the more recent one will eventually overwrite it. The Thomas write rule is applied in situations where a predefined logical order is assigned to transactions when they start. For example, a transaction might be assigned a monotonically increasing timestamp when it is created. The rule prevents changes in the order in which the transactions are executed from creating different outputs: The outputs will always be consistent with the predefined logical order. For example, consider a database with 3 variables (A, B, C), and two atomic operations C := A (T1), and C := B (T2). Each transaction involves a read (A or B), and a write (C). The only conflict between these transactions is the write on C. The following is one possible schedule for the operations of these transactions: