Definition

Optimistic locking is a concurrency control strategy where a transaction proceeds without locking resources, checking for conflicts only at commit. Typically implemented via version checks.

Core concept

When reading, the transaction notes a version (timestamp or version number) of the data. On write/commit, it verifies the version is unchanged. If it changed, the transaction aborts and retries. No locks are held during the transaction.

Use cases

Web apps and ORMs often use optimistic locking for performance when conflicts are rare (e.g., many users read the same data but few update it).

Trade-offs

Risk of transaction abort: more work on retry if conflicts occur. Best for low-contention scenarios.

Example

A row has a version column. T1 reads version=1. T2 updates row and sets version=2. T1, on update, sees version changed and rolls back.

References

Common knowledge in DB concurrency (no direct cite).