Could you explain Pessimistic locking?

Pessimistic locking is a concurrency control mechanism used in multi-user environments to prevent conflicts when multiple users or processes attempt to access and modify the same data simultaneously. Unlike optimistic locking, which assumes conflicts are rare, pessimistic locking takes a more cautious approach and assumes that conflicts are likely to occur. Therefore, it restricts concurrent access to data by blocking other users from modifying the same records until the first user completes their update.

In the context of databases, pessimistic locking is typically achieved through transaction isolation levels that support locking. When a transaction wants to access a particular record, it explicitly requests a lock on that record, preventing other transactions from modifying the same record until the lock is released. There are different types of locks used in pessimistic locking, such as shared locks (read locks) and exclusive locks (write locks):

  1. Shared Lock (Read Lock):
    • A shared lock allows multiple transactions to read the data simultaneously.
    • When a transaction holds a shared lock on a record, other transactions can also acquire shared locks on the same record to read it, but no transaction can acquire an exclusive lock to modify it until all shared locks are released.
  2. Exclusive Lock (Write Lock):
    • An exclusive lock allows only one transaction to modify the data.
    • When a transaction holds an exclusive lock on a record, no other transaction can acquire any type of lock (shared or exclusive) on the same record until the exclusive lock is released.

The primary purpose of pessimistic locking is to ensure data consistency and prevent conflicts that may arise when multiple users try to update the same data simultaneously. By blocking concurrent modifications, pessimistic locking avoids issues like dirty reads, non-repeatable reads, and phantom reads, which can lead to data inconsistencies.

However, pessimistic locking can also lead to potential performance issues, such as blocking and deadlocks, where transactions wait for each other indefinitely. Therefore, it is essential to use pessimistic locking judiciously and set appropriate isolation levels based on the specific requirements of the application.

In summary, pessimistic locking is a concurrency control mechanism that restricts concurrent access to data by blocking other users from modifying the same records until the current transaction is completed. It ensures data consistency but requires careful consideration of potential performance implications.

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