Backup and Recovery
Mimer SQL's backup and recovery procedures are designed to guarantee that a consistent and up-to-date database can be recreated following all types of system failure.
Mimer SQL handles database consistency is handled on two levels: physical and logical.
Physical consistency means that the tables are readable by the Mimer SQL database manager. The Mimer SQL system guarantees physical consistency as long as the databank file is not physically damaged. If a databank file was not closed properly last time it was accessed, an internal consistency check is performed when the databank is opened the next time. If physical errors are detected the databank can be restored from a backup copy.
Logical consistency means that the tables contain correct data and no transactions are incomplete. Mimer SQL's transaction handling ensures logical consistency. Details of all database accesses are saved in the TRANSDB databank during build-up and no changes are made to the database. The Commit command initiates the application of the changes that are carried out as if they were a single 'atomic' change. If a transaction is successfully committed then all operations in the transaction are applied. If the transaction is aborted due to a conflict, or a user request, none of the operations in the transaction are applied.
The databanks may be temporary logically inconsistent if Mimer SQL is stopped (either deliberately or by a system failure) before all operations in a successfully committed transaction have been performed. When the system is restarted all uncompleted transactions are read from TRANSDB and are automatically applied to get the system into a consistent state again.
During the Commit Phase of a transaction the changes to a databank are written to the transaction log (the databank LOGDB). Mimer SQL's logging system allows a backup copy of a databank to be rolled forward to recreate an up-to-date version of the databank following a file being physically damaged or other hardware faults making it inaccessible or corrupt.
The Mimer SQL system allows an online backup (i.e. a 'full backup') of a databank to be taken, whilst maintaining full access to all the data. The backup is a complete copy of the databank file and may be used as the basis for a databank recovery operation.
As a complement to backup copies it is possible to archive the transaction log changes, forming the equivalent of an incremental backup of all logged databanks. This allows several backup copies of a databank of varying ages to be kept, along with a sequence of incremental LOGDB backup files. By using the appropriate sequence of LOGDB backups and the current LOGDB, it is possible to roll forward any of the backup copies of the databank to an up to date state.
As Mimer SQL databanks are ordinary Operating System files, the normal OS facilities for backing up files can be used for them. Also incremental backups can be combined with host system backup copies. This means that the backup process can be incorporated into the normal site procedures for backup. It also allows the most efficient utilities to be used for this purpose.
The LOGDB file can also be used as an audit trail. The audit trail is accessed by a utility, where the following information about performed transactions in the system can be retrieved:
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