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Database Consistency


Database consistency is handled on two levels: physical and logical.

Physical consistency means that the tables are readable by Mimer SQL. This is ensured as long as the databank file is not physically damaged.

Logical consistency means that the tables contain valid data. This is ensured by Mimer SQL's transaction handling. All transactions are saved in the TRANSDB databank during build-up and are applied to the databanks when they are committed. To use transaction handling, the databank must be created with the TRANS or LOG option.

Transaction handling makes it possible to ensure that a user cannot commit a non-read only transaction which has read data that is being concurrently updated by another user. If a transaction is successfully committed then all operations in the transaction are performed. If the transaction is aborted due to a conflict, none of the operations in the transaction are performed.

The tables may be logically inconsistent if Mimer SQL is stopped before all operations in a committed transaction have been performed. At some time after the system is restarted, all uncompleted transactions will be read from TRANSDB for automatic completion. This happens in the background on a per-databank basis, after a databank is first accessed following the restart. Transactions that were not committed before the stop are aborted.

The DBOPEN facility, see DBOPEN Functionality, can be used to open all databanks in one operation and thus achieve transaction consistency quickly.



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