All of the modifications made to data within a transaction
are uniformly applied to a database with a COMMIT statement, or the data
affected by the changes is uniformly returned to its initial state with a ROLLBACK
statement. Once a transaction is committed, the changes made by that transaction
become permanent and are made visible to other transactions and other users.
Transactions always occur over time, although most transactions occur over a very
short period of time. Since the changes made by a transaction aren??™t official until the
transaction is committed, each individual transaction must be isolated from the
effects of other transactions. The mechanism used to enforce transaction isolation is
the lock.
188 | Chapter 8: Oracle Multiuser Concurrency
Locks
A database uses a system of locks to prevent transactions from interfering with each
other. A lock prevents users from modifying data. Database systems use locks to
keep one transaction from overwriting changes added by another transaction.
Figure 8-1 illustrates the potential problems that could occur if a system did not use
locks.
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