Let transaction T1 has obtained a shared-mode lock S on data item Q and transaction T2 has obtained an exclusive-mode lock X on data item R. Consider the following statements: I:
T1 can read Q but cannot write Q. II:
T2 can read R but cannot write R.
Which of the above statement(s) is/are valid ?

Question

Let transaction T1 has obtained a shared-mode lock S on data item Q and transaction T2 has obtained an exclusive-mode lock X on data item R. Consider the following statements: I: 
T1 can read Q but cannot write Q. II: 
T2 can read R but cannot write R. 
Which of the above statement(s) is/are valid ?

Accepted Answer

The correct option is (1):
In database concurrency control, the locking protocol dictates the operations a transaction can perform on a locked data item. A **Shared-mode lock (S)** is primarily used for read-only operations. When Transaction T1 obtains an S lock on data item Q, it signifies T1's intent to read Q, and it is permitted to do so. Crucially, an S lock is established specifically to prevent modification, ensuring that T1 is reading a consistent state; therefore, T1 is restricted from performing a write operation on Q. Consequently, Statement I, which says "T1 can read Q but cannot write Q," is an accurate definition of the capabilities afforded by a Shared lock, making it valid.

The incorrect option is (2):
This option states that only Statement II is valid. Statement II, "T2 can read R but cannot write R," is factually incorrect regarding an **Exclusive-mode lock (X)**. The fundamental purpose of an X lock is to grant the locking transaction, T2, complete and exclusive control over data item R. This control explicitly includes the permission to perform *both* read and write (modification) operations on the data item. The X lock is the most restrictive lock and is required for writing, so the claim that T2 *cannot write* R is false. Since Statement I is valid and Statement II is invalid, this option is incorrect.

The incorrect option is (3):
This option claims that both Statement I and Statement II are valid. As established, Statement I accurately reflects the properties of a Shared (S) lock (read allowed, write denied), as its primary purpose is read access. However, Statement II incorrectly limits the capabilities of an Exclusive (X) lock. The entire point of an X lock is to allow the transaction to make exclusive modifications, meaning it must permit writing. Since the second statement is false—T2 *can* write R—this option is incorrect.

The incorrect option is (4):
This option suggests that neither Statement I nor Statement II is valid. This is incorrect because Statement I perfectly describes the standard operation of a Shared lock. A Shared lock is a read lock; it allows reading and blocks writing, which is exactly what Statement I describes for T1 on Q. Therefore, since Statement I is valid, an option that invalidates both statements cannot be the correct answer.