This is called roll forward recovery.
Some database systems use a write-ahead log, but also make changes to the database before the transaction is
formally committed. In such a system, recovery from failure can be accomplished by restarting the DBMS and
???undoing??? transactions in the log that were not committed. This approach is called rollback recovery.
TRANSACTION ISOLATION LEVELS
When multiple users access a database simultaneously, there is a chance that one person??™s changes to the database
will interfere with another person??™s work. For instance, suppose two people using an on-line flight reservation
system both see that the window seat in row 18 is available, and both reserve it, nearly simultaneously. Without
proper controls, both may believe they have successfully reserved the seat but, in fact, one will be disappointed.
This is an example of one sort of concurrency problem, and it is called the lost update problem.
The database literature describes desirable characteristics of transactions using the acronym ACID??”
transactions should be atomic (all or nothing), consistent (all rows affected by the transaction are protected from
other changes while the transaction is occurring), isolated (free from the effects of other activity in the database
at the same time), and durable (permanent).
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