Optimistic vs Pessimistic Locking in Spring Boot: Preventing Race Conditions

Concurrent database access can cause race conditions where multiple users simultaneously book the same resource, such as a cinema seat, leading to duplicate transactions and data inconsistency. Spring Boot addresses this through two locking strategies: Optimistic Locking uses a @Version field so the JPA layer detects conflicting updates and throws an OptimisticLockException, making it suitable for low-concurrency scenarios. Pessimistic Locking uses @Lock with PESSIMISTIC_WRITE to issue a SELECT FOR UPDATE, physically blocking other transactions until the current one completes, which is better suited for high-contention situations like flash sales or auctions. For distributed systems running multiple service instances across different databases, neither database-level lock alone suffices, and a distributed locking mechanism such as Redis RedLock becomes necessary. Choosing the right strategy depends on the expected concurrency level, system architecture, and whether the application is single-instance or distributed.