Definition

Pessimistic locking is the strategy of locking data resources before accessing them to prevent conflicts. A transaction acquires locks on rows it will read or write.

Core concept

Locks ensure that no other transaction can modify (or sometimes read) the data until the lock is released. This prevents anomalies but can cause contention.

Use cases

High-conflict environments (finance) where you avoid retries. Row-level locks (SELECT ... FOR UPDATE) in SQL are an example.

Trade-offs

Can lead to reduced concurrency and possible deadlocks.

Example

T1 does SELECT ... FOR UPDATE locking the row; T2 tries same row and must wait until T1 commits.

References

Common DB theory (no direct cite).