In context of 2-phase locking protocol in DBMS, a transaction issues lock requests in _______ phase and unlock requests in _______ phase.

Question

Accepted Answer

The correct option is (1):
The Two-Phase Locking (2PL) protocol is a fundamental concurrency control technique in Database Management Systems (DBMS) designed to ensure conflict serializability of transactions. The protocol is strictly divided into two consecutive stages. The first stage is correctly termed the **Growing Phase** (or Expanding Phase). During this phase, a transaction is permitted to obtain (issue requests for) any new locks it requires, such as Shared or Exclusive locks, but it is absolutely forbidden from releasing any locks it already holds. Once a transaction releases its very first lock, it officially enters the second stage, known as the **Shrinking Phase** (or Contracting Phase). In this phase, the transaction is only allowed to release its existing locks (issue unlock requests) and is strictly prohibited from acquiring any further new locks. This rigid separation of lock acquisition and lock release into two distinct, sequential phases is the mechanism that guarantees data consistency.

The incorrect option is (2): Shared, Exclusive.
Shared (S) and Exclusive (X) refer to the *types* or *modes* of locks, not the phases of the 2PL protocol itself. A transaction issues a request for a Shared lock for reading data or an Exclusive lock for writing data. Both Shared and Exclusive lock requests are issued during the **Growing** phase, and both types of locks are released during the **Shrinking** phase. Therefore, these terms classify the lock's function and compatibility, not the stages of the transaction's lifecycle as defined by 2PL.

The incorrect option is (3): Wait, Wound.
Wait-Die and Wound-Wait are specific, preventative strategies employed for **deadlock handling** in a DBMS, often used in conjunction with 2PL. They are not the names of the lock acquisition and release phases. The goal of these algorithms is to break a potential circular waiting condition (deadlock) between transactions by making an older transaction either wait for a younger one (Wait-Die) or force a younger transaction to roll back (Wound-Wait). They represent a policy decision for concurrency control, not the two fundamental phases of lock management.

The incorrect option is (4): Explicit, Implicit.
Explicit and Implicit describe *how* a lock is requested, not the phases of the 2PL protocol. An Explicit lock is one a programmer directly requests using a command, while an Implicit lock is one automatically acquired by the DBMS when a transaction accesses a data item. Regardless of whether a lock is explicit or implicit, the timing must still conform to the two-phase rule: all lock acquisitions (explicit and implicit) must occur in the **Growing** phase, and all lock releases must occur in the **Shrinking** phase.