Question

Beside the locking protocol, what is the additional requirement to ensure the transactional isolation property

Answer 1

Also transaction isolation level is used to define upto what extent a data must be protected from the other transaction. It contains Serializability also which is managed by Multiversion concurrency control. It is also called MVCC. It maintains copies of the data to perform any transaction and gets updated instantly once a process write data in the original data. The MVCC do not update the original snapshots but create a new one each time that's why it is called multi version.