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What is Two-Phase Locking?
Two-Phase Locking (2PL) is a concurrency control protocol used in database management systems to ensure the consistency and integrity of data transactions. It guarantees serializability and avoids conflicts that could result in data inconsistencies.
In this protocol, a transaction is divided into two main phases: the growing phase and the shrinking phase. The growing phase, also known as the acquisition phase, is when a transaction acquires locks on requested data items before accessing them. The shrinking phase, also known as the release phase, is when a transaction releases the locks it has acquired.
Basic Concepts of Database Control
To better understand two-phase locking, let’s briefly overview some basic concepts of database control:
1. Concurrency Control: Concurrency control ensures that multiple transactions can execute concurrently in a multi-user database system without causing conflicts or data inconsistencies.
2. Transactions: A transaction is a logical unit of work that accesses and modifies data. It consists of a sequence of database operations, such as reading, writing, and updating records.
3. Locking: Locking is a mechanism used by the database system to control access to data items. It allows transactions to acquire exclusive locks (write locks) or shared locks (read locks) on the data.
4. Serializability: Serializability is a property that guarantees the execution order of transactions to be equivalent to some sequential order. It ensures that the final outcome of concurrent transactions will be the same as if they were executed serially, one after another.
Two-Phase Locking (2PL) Protocol
Now, let’s delve into the details of the two-phase locking protocol:
1. Growing Phase: During the growing phase, a transaction acquires locks on the data items it needs to access. If a transaction requires a write operation, it acquires an exclusive lock, and if it requires a read operation, it acquires a shared lock.
– Strict Two-Phase Locking (S2PL): In S2PL, a transaction holds all its locks until it reaches the shrinking phase, ensuring that no other transaction can access the locked data.
– Rigorous Two-Phase Locking (R2PL): R2PL is an extension of S2PL where a transaction releases all its locks only after it has completed.
2. Shrinking Phase: In the shrinking phase, a transaction releases the locks it has acquired. The locks are released in the reverse order of their acquisition, ensuring that no conflicts arise.
By implementing the two-phase locking protocol, the database system guarantees the serializability of transactions, eliminates conflicts, and maintains the consistency of the database. It facilitates concurrent access to data while ensuring data integrity, making it a fundamental technique in database control.
Remember, understanding two-phase locking is crucial to efficiently manage concurrent transactions in database systems and ensure the reliability and consistency of data operations.
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