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What's difference between œOptimistic and œPessimistic locking?

In pessimistic locking, when a user opens a data to update it, a lock is granted. Other users can only view the data until the whole transaction of the data update is completed.

In optimistic locking, a data is opened for updating by multiple users. A lock is granted only during the update transaction and not for the entire session. Due to this concurrency is increased and is a practical approach of updating the data.