<P> It is also possible to keep a separate journal of all modifications to a database management system . (sometimes called after images). This is not required for rollback of failed transactions but it is useful for updating the database management system in the event of a database failure, so some transaction - processing systems provide it . If the database management system fails entirely, it must be restored from the most recent back - up . The back - up will not reflect transactions committed since the back - up was made . However, once the database management system is restored, the journal of after images can be applied to the database (rollforward) to bring the database management system up to date . Any transactions in progress at the time of the failure can then be rolled back . The result is a database in a consistent, known state that includes the results of all transactions committed up to the moment of failure . </P> <P> In some cases, two transactions may, in the course of their processing, attempt to access the same portion of a database at the same time, in a way that prevents them from proceeding . For example, transaction A may access portion X of the database, and transaction B may access portion Y of the database . If, at that point, transaction A then tries to access portion Y of the database while transaction B tries to access portion X, a deadlock occurs, and neither transaction can move forward . Transaction - processing systems are designed to detect these deadlocks when they occur . Typically both transactions will be cancelled and rolled back, and then they will be started again in a different order, automatically, so that the deadlock doesn't occur again . Or sometimes, just one of the deadlocked transactions will be cancelled, rolled back, and automatically restarted after a short delay . </P> <P> Deadlocks can also occur among three or more transactions . The more transactions involved, the more difficult they are to detect, to the point that transaction processing systems find there is a practical limit to the deadlocks they can detect . </P> <P> In systems where commit and rollback mechanisms are not available or undesirable, a compensating transaction is often used to undo failed transactions and restore the system to a previous state . </P>

Which of the following is an example of transaction data