Understanding Database Locking and Concurrency Control Mechanisms
In any database system, it is essential to ensure data integrity and maintain consistency when multiple users access and manipulate data simultaneously. Database locking and concurrency control mechanisms are employed to manage these situations effectively. Let’s dive into understanding what locking and concurrency control mean and how they operate.
Database Locking
Database locking is a technique used to control access to shared resources in a database system to prevent conflicting operations and ensure data integrity. When a user wants to perform an operation on a particular data item, the database management system (DBMS) first checks whether the requested resource is locked. If it is locked, the user must wait until the lock is released.
Types of Locks
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Shared Locks (Read Locks): It allows multiple users to simultaneously read data but prevents any write or modification operation on the locked resource. Multiple shared locks can be acquired concurrently, as long as no exclusive lock is held by another user.
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Exclusive Locks (Write Locks): It gives exclusive access to the user, preventing all other users from both reading and writing to the locked resource. Only one exclusive lock can be acquired at a time.
Lock Granularity
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Table-level Locks: The entire table is locked when a user acquires a lock on it. It ensures that no other user can access the table until the lock is released. Table-level locks can cause severe contention and are not suitable for systems with high levels of concurrency.
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Row-level Locks: Each individual row in a table is independently locked. It allows multiple users to access different rows simultaneously, improving concurrency. However, row-level locks may require more system resources and slow down performance in heavily loaded systems.
Concurrency Control Mechanisms
Concurrency control mechanisms are responsible for managing conflicting operations and maintaining the consistency of data in the presence of concurrent transactions.
1. Two-Phase Locking (2PL)
Two-phase locking is a widely used concurrency control protocol. It ensures conflict serializability, meaning that the execution of transactions is equivalent to executing them one after another in some order.
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Growing Phase: In this phase, a transaction can acquire locks but cannot release any locks.
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Shrinking Phase: A transaction can release locks but cannot acquire any new locks.
2PL guarantees that no transaction can read or write a data item that another transaction has locked for exclusive access.
2. Optimistic Concurrency Control (OCC)
OCC assumes that conflicts between transactions are rare, and most transactions can run concurrently without interfering with each other. It allows multiple transactions to proceed without acquiring locks explicitly while delaying the validation of data modifications until the end of the transaction.
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Read Phase: Transactions read data without acquiring any locks.
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Validation Phase: At the end of the transaction, the DBMS verifies if any conflicting writes occurred during the transaction. If no conflicts are detected, the transaction’s writes are committed, and if conflicts are found, the transaction is rolled back.
OCC is beneficial when the likelihood of conflicts is low as it reduces the overhead of acquiring and releasing locks.
3. Multi-Version Concurrency Control (MVCC)
MVCC is often used in database systems that support snapshot isolation. It allows multiple versions of a data item to exist concurrently, enabling users to read consistently and prevent read and write conflicts.
In MVCC, a new version of a data item is created when a transaction wants to modify it, leaving the old version intact until all currently running transactions are completed.
4. Timestamp Ordering
In timestamp ordering, each transaction is assigned a unique timestamp indicating its order of execution. To ensure serializability, the DBMS checks the order of operations based on their timestamps and schedules conflicting operations accordingly.
Conclusion
Understanding database locking and concurrency control mechanisms is crucial for efficient and secure database management. The choice of the appropriate locking and concurrency control technique depends on the specific requirements of the database system, the level of concurrency, and the expected workload. By utilizing these mechanisms effectively, database systems can ensure data integrity, consistency, and efficient concurrent access to shared resources. 参考文献：