Thomas Write Rule, also known as the Strict Two-Phase Locking (Strict 2PL) protocol, is a rule used in database management systems to ensure data consistency and correctness in the face of multiple concurrent transactions.
The Thomas Write Rule states that a transaction can only write to a database item if it holds a lock on that item that was obtained during the current transaction, and that lock has not been released yet. In other words, a transaction must hold all locks on the items it modifies until the transaction completes.
The Thomas Write Rule differs from the Timestamp-Based Concurrency Control protocol, which uses timestamps to order transactions and determine which transactions are allowed to access a given item. In Timestamp-Based Concurrency Control, each transaction is assigned a unique timestamp when it begins, and transactions are allowed to read or write items only if their timestamps are greater than the timestamp of the last transaction that accessed that item. This allows for more concurrency than the Thomas Write Rule, as multiple transactions can access the same item as long as they don't overlap in time.
However, Timestamp-Based Concurrency Control can lead to the occurrence of conflicts and rollbacks, as transactions may need to be rolled back if they try to access an item that has already been modified by a newer transaction. The Thomas Write Rule, on the other hand, ensures that transactions do not interfere with each other and that all changes to the database are made in a consistent and correct manner. However, it can also lead to a lower degree of concurrency, as transactions may need to wait for locks to be released before they can access items they need to read or modify.
In summary, the Thomas Write Rule and Timestamp-Based Concurrency Control are two different approaches to concurrency control in database management systems. The Thomas Write Rule ensures strict locking and consistency, while Timestamp-Based Concurrency Control allows for more concurrency but may lead to conflicts and rollbacks. The choice of which protocol to use depends on the specific requirements of the application and the trade-off between concurrency and consistency that is acceptable.
