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
LOGoption. Transaction handling is, therefore, important even in standalone environments where concurrency control issues do not arise.
The system logging databank,
LOGDBis 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:
All operations on the databank are performed under transaction control. All transactions are logged.
All operations on the databank are performed under transaction control. No transactions are logged.
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 (
INSERTon several rows) which are interrupted will not be rolled back.
READ ONLY Only read only operations are allowed, i.e.
INSERTcan not be performed on tables in a databank with this option.
All important databanks should be defined with
LOGoption, so that valuable data is not lost by any system failure.
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