2PL

Two-phase locking (2PL) is a concurrency control method used in database systems to ensure serializability among competing transactions. In 2PL, a transaction's operations are divided into two phases: a growing phase, in which the transaction acquires locks but does not release any, and a shrinking phase, in which the transaction releases locks but does not acquire new ones. The requirement that no lock is released before the end of the growing phase guarantees a structured ordering of access to data items, which supports serializable execution.

Lock types and behavior: 2PL typically uses shared locks (S) for reading and exclusive locks (X) for writing. A transaction may acquire an S lock on a read item and later upgrade to an X lock for writing. Because of the two-phase rule, once a transaction begins releasing locks in the shrinking phase, it cannot acquire new locks. Variants differ on whether lock upgrades are allowed and how strictly the growing and shrinking phases are enforced.

Variants: Strict 2PL is a common variant where all locks are held until the transaction commits or aborts. This simplifies recovery and prevents cascading aborts, at the cost of reduced concurrency. Conservative 2PL, another variant, may acquire all needed locks before a transaction proceeds, further reducing the risk of deadlocks. Some systems implement additional mechanisms to manage upgrades, lock compatibility, and deadlock handling.

Deadlocks and recovery: Because multiple transactions may wait for locks held by others, deadlocks can occur. Databases employ detection and resolution strategies or prevention schemes such as timeouts, wait-die, or wound-wait. Overall, 2PL provides a robust path to serializable schedules but can incur locking overhead and potential contention.

In practice: Many traditional relational databases implement 2PL or strict 2PL as part of their transaction management, though some modern systems favor alternative approaches such as multiversion concurrency control (MVCC) for different performance characteristics.