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What is 2PL Two-Phase Locking?
2PL, or Two-Phase Locking, is a concurrency control mechanism in database management systems that ensures the consistency and integrity of data when multiple transactions are accessing and modifying the same data concurrently. It guarantees serializability, meaning that the transactions are executed in a way that is equivalent to a serial execution, even though they may be executed concurrently.
Basic Concepts of Database Management
To understand 2PL two-phase locking better, it is essential to grasp some basic concepts of database management.
Database: A collection of organized data that is stored and accessed electronically. The database management system (DBMS) controls and manages the storage and retrieval of data.
Transaction: A unit of work that represents a series of actions performed on a database. It is an atomic operation, meaning that it is either completely executed or not executed at all. Transactions ensure data consistency and integrity.
Concurrency Control: The management of simultaneous access to shared data by multiple transactions in a multi-user system. It prevents conflicts and ensures data integrity.
Explaining 2PL Two-Phase Locking
2PL uses locks to coordinate and control the concurrent execution of transactions. It consists of two phases: the growing phase and the shrinking phase.
1. Growing Phase:
During this phase, a transaction acquires the necessary locks before accessing or modifying data items. Locks can be either shared (read locks) or exclusive (write locks).
– Shared Lock (S Lock): Allows multiple transactions to read a shared data item but prohibits write access.
– Exclusive Lock (X Lock): Grants exclusive access to a transaction for both read and write operations. No other transaction can access the data item until the lock is released.
When a transaction requests a lock, it must wait until the lock is available if it is held by another transaction. This ensures that conflicts, such as data inconsistency or access violations, are avoided.
2. Shrinking Phase:
Once a transaction has completed its operations, it releases the locks it acquired during the growing phase. Releasing locks allows other transactions to acquire and process the data items, promoting concurrency and efficient resource utilization.
2PL ensures that conflicting operations are properly serialized, preventing any simultaneous direct updates to the same data item. This serialization guarantees data consistency and eliminates potential anomalies like lost updates or dirty reads.
In summary, 2PL two-phase locking is a concurrency control mechanism that ensures transaction serializability and data integrity in a database management system. Its two-phase structure of acquiring locks before accessing data and releasing them after completing operations prevents conflicts and promotes efficient multi-user access to shared data.
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