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Explain the difference between optimistic and pessimistic locking in a database, in two short paragraphs, and say when to use each.

**Pessimistic locking** assumes conflicts are likely, so it locks a record as soon as a transaction reads it for update (e.g., `SELECT ... FOR UPDATE`). Other transactions that try to modify it must wait until the lock is released. This guarantees no one else can change the data mid-transaction, but it reduces concurrency and can cause blocking or deadlocks. Use it when contention is high, conflicts are costly or hard to retry, or transactions are short and critical, as with bank balance transfers or inventory decrements on hot items. **Optimistic locking** assumes conflicts are rare, so it takes no lock while reading. Instead, it checks at write time whether the data changed since it was read, typically with a version number or timestamp (e.g., `UPDATE ... SET version = version + 1 WHERE id = ? AND version = ?`). If another transaction got there first, the update affects zero rows and the application retries or reports the conflict. This scales well and avoids holding locks during long operations, such as a user editing a form. Use it when reads far outnumber writes, conflicts are infrequent, or work spans long periods where holding a lock would be impractical.

Generated 2026-09-27. Answers vary from run to run.