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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.

Optimistic locking assumes conflicts are rare. Transactions read data without locking it, and when they try to write, the system checks whether the data changed since it was read (typically via a version number or timestamp). If it did change, the update is rejected and the application must retry. Pessimistic locking assumes conflicts are likely: a transaction locks the data as soon as it reads it, blocking other transactions from modifying (or sometimes even reading) it until the lock is released. Use optimistic locking when contention is low—such as web applications where many users read data but rarely update the same records—since it avoids the overhead and blocking of locks and scales better. Use pessimistic locking when contention is high or the cost of a failed retry is expensive—such as financial transactions or inventory systems where multiple processes frequently update the same rows—since holding a lock guarantees the update will succeed without conflicts.

2026-09-27 生成。実行ごとに異なる応答が返されます。