Optimistic Concurrency Control

Optimistic Concurrency Control (OCC) is a protocol based on the assumption that conflicts between concurrent transactions are rare. Instead of using locks to prevent conflicts (a 'pessimistic' approach that assumes conflicts will happen), OCC allows transactions to proceed without acquiring any locks. Transactions read and modify data in a private workspace or local copy. The protocol is divided into three phases. First is the Read Phase: The transaction reads values from the database and stores them in its local variables. All write operations are performed on a temporary, local copy of the data. The database itself is not modified. Second is the Validation Phase: Just before the transaction is about to commit, the system validates whether its modifications will violate serializability. It checks if any other committed transaction has modified the data that this transaction has read. If the validation check fails, it means there is a conflict. The third phase is the Write Phase. If the validation is successful, the transaction's changes are made permanent in the database. If the validation fails, the transaction is aborted and rolled back, and it can be restarted. OCC works well in environments with low data contention (few conflicts), as it avoids the overhead of locking. However, in high-contention environments, the cost of frequent transaction rollbacks can make it less efficient than pessimistic locking.