Transaction control also provides the basis for protection of the database against hardware failure.
Changes made to a database may be logged, to provide back-up protection in the event of hardware failure, provided that the changes occur within a transaction and that the databanks involved have the LOG option. Transaction handling is, therefore, important even in standalone environments where concurrency control issues do not arise.
The system logging databank, LOGDB is where transaction changes are recorded. It contains a record of all transactions executed since the latest back-up copy of a databank was taken and the log cleared. The latest back-up copy of the databank, together with the contents of LOGDB, may be used to restore the databank in the event of a databank crash.
Transaction control and logging is determined at the databank level by options set when the databank is defined.
The options are:
LOG All operations on the databank are performed under transaction control. All transactions are logged. TRANS All operations on the databank are performed under transaction control. No transactions are logged. NULL All operations on the databank are performed without transaction control (even if they are requested within a transaction), and are not logged.Sets of operations (DELETE, UPDATE or INSERT on several rows) which are interrupted will not be rolled back.
All important databanks should be defined with LOG option, so that valuable data is not lost by any system failure.
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