Transactional Isolation

Transactional isolation is usually implemented by locking whatever resource is being accessed (db table for example) during a transaction thus isolating the resource from other transactions.  There are two different types transactional locking: Pessimistic locking and optimistic locking:
Pessimistic Locking : With pessimistic locking, a resource being accessed is essentially locked from the time it is first accessed in a transaction until the transaction completes, making the resource unusable by any other transactions during that time. If competing transactions simply need to read the resourceonly (a SELECT of a data row for example) then an exclusive lock may be overkill. The lock exists until the transaction has either been committed or rolled back at which point, the resource is made available again for other transactions.
Optimistic Locking : Optimistic locking works a little differently.A resource being accessed isn’t locked when first used, but the state of the resource is noted.  This allows other transactions to concurrently access to the resource and the possibility of conflicting changes is possible. At commit time, when the resource is about to be updated to persistent storage, the state of the resource is read from storage again and compared to the state previously noted when the resource was first accessed. If the two states differ, a conflicting update was made, and the transaction will roll back.

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