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Pessimistic Approach: This approach causes transaction to be delayed in case they conflict with each other at the some time in the future.

Pessimistic Execution: The validate operation is performed first, if there is a validation according to compatibility of lock then only read, compute and write operations are performed

Optimistic Approach: The optimistic method of concurrency control is based on the assumption that conflicts of database operations are rare and that it is better to let transactions run to completion and only check for conflicts before they commit. An optimistic concurrency control method is also known as validation or certification methods. No checking is done while the transaction is executing. The optimistic method does not require locking or time stamping techniques. Instead, a transaction is executed without restrictions until it is committed.

It allows transactions to proceed unsynchronized and only check conflicts at the end. This approach is based on the premise that conflicts are rare

 

Optimistic Execution: It perform read and compute operation without validation and perform validation just before write operation.

Read Composite Validate Write

The link below gives a good descp of your ques. Some part of it I've taken from there. Kindly visit d link mentioned below. :)

http://ecomputernotes.com/database-system/rdbms/concurrent-control-algorithms

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