Instead, they are logged in the transaction log together with the old data. Regardless of any shortcomings, this mode has one advantage - speed. Data is never blocked. Every task works based on the optimistic assumption that no other task is accessing the DB and no one is waiting for anything. If a collision occurs, the transaction may be repeated.
Speed is a significant asset in mass service systems.
This is a classic transaction mode. All the modifications are sent directly to the DB as well as to the system log. However, all modifications are transparent to other transactions until the transaction has been committed. Until committed, a blocking mechanism prohibits modifying read data but modified data can be read. Deadlock situations are usually resolved in favor of the transaction with a higher priority.