In computer science, particularly the field of databases, the Thomas write rule is a rule in timestamp-based concurrency control.
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.
Enforcing the Thomas write rule only requires checking to see if the write timestamp of the object is greater than the time stamp of the transaction performing a write.