Two Phase Locking
- Computer Science Department
Two-Phase Locking (2PL) is a concurrency control method used in database management systems. It ensures serializability, which means it guarantees that the execution of concurrent transactions will yield a database state as if transactions were executed one after the other in some order.
The two-phase locking protocol divides a transaction's execution into two phases - the growing phase (the expansion phase) and the shrinking phase (the contraction phase).
Growing Phase: During this phase, a transaction may obtain locks but cannot release any locks. As the transaction executes, it continues to request locks as needed for data items. By obtaining a lock, the transaction is ensuring that no other transaction can change the data item it is about to use. Locks can be either shared (for read operations) or exclusive (for write operations).
Shrinking Phase: Once the transaction releases its first lock, it enters the shrinking phase. In this phase, the transaction may release locks but cannot obtain any new ones. This phase continues until all locks have been released.
The point in time where the transaction switches from the growing phase to the shrinking phase is called the "lock point".
The main benefit of the two-phase locking protocol is that it prevents conflicts during the execution of transactions and thus ensures consistency in the database. By managing the order in which transactions are allowed to claim and release locks, 2PL prevents situations where one transaction interferes with another, leading to an inconsistent state.
However, two-phase locking does not guarantee freedom from deadlocks. A deadlock situation may occur in a system if each transaction in a set of two or more transactions is waiting for the other to release a lock, creating a cycle of dependencies.
Also, note that while 2PL can prevent non-serializable schedules, it might restrict concurrency in the system. To handle this, variations of the basic 2PL, like Conservative 2PL, Strict 2PL, and Rigorous 2PL, have been proposed to increase concurrency while maintaining serializability.