What is the difference between optimistic and pessimistic locking in databases?

Concurrent data access occurs when multiple users or processes try to modify the same information simultaneously. To prevent conflicts and ensure data integrity, two main locking strategies are used: pessimistic and optimistic. The choice between them depends on the nature of the application load and the probability of simultaneous record modifications.

Pessimistic locking proceeds from the assumption that conflicts occur frequently. When it is used, a row or table is locked in the database from the moment of reading until the transaction is completed. Other processes trying to access the same data are forced to wait for the resource to be released. This approach guarantees data safety, but can significantly reduce performance due to thread idling.

Optimistic locking is based on the assumption that simultaneous modifications of the same record happen extremely rarely. Data is read without setting locks, and conflict checking is performed only at the moment of writing. To implement this approach, tables are often supplemented with a special row version or a timestamp.

The pessimistic strategy is ideal for systems with a high degree of competition for the same resources, such as booking tickets for popular events. Optimistic locking performs best in web applications with a predominance of read operations and rare updates, where DBMS-level locks would create unnecessary delays.