Updates Recorded in LOGDB

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Updates Recorded in LOGDB

The LOGDB system databank contains logged update information for each databank with LOG option.

It is recommended that all databanks, including LOGDB, are backed up at the same time and that LOGDB is cleared after backup by resetting the log. Thus, the backed up LOGDB will contain the information required to make databanks from the preceding backup consistent with the current backup.

This will provide double backup protection by allowing a lost databank to be recovered in one of the two ways listed below:

restore the databank from the most recent backup and apply the updates currently held in LOGDB, or
restore the databank from an earlier backup, then sequentially use the LOGDB files from each subsequent backup to make the databank consistent with the most recent backup, and finally apply the updates currently held in LOGDB.

The records in LOGDB should be cleared after a complete backup, in order to maintain consistency between the backup and LOGDB. This ensures that LOGDB only contains information about changes made to a databank since the last backup of it was taken. (It is possible to backup databanks without clearing LOGDB records, although care must be taken as this leaves the backup and LOGDB in an inconsistent state).

The ability to restore databank updates from a backup copy of LOGDB replaces the databank incremental backups which were supported in previous versions of Mimer SQL. These are still supported for backward compatibility but it is now recommended that LOGDB backups always be taken to offer the same double protection.

Caution: If, for any reason, the LOGDB databank is lost, no problems will be encountered immediately. All changes will have been properly recorded in the application databanks. A new, empty, LOGDB can simply replace the log that was lost.

However, a backup of the entire database must be taken immediately. The new LOGDB will be empty, and therefore in a state consistent with a backup of all databanks having just been taken and all LOGDB records cleared.

If a backup is not taken immediately, a later attempt to restore a databank is likely to fail because the restore operation will expect to find information in LOGDB that was lost when LOGDB was destroyed.

Caution: If, for any reason, the LOGDB databank is lost, no problems will be encountered immediately. All changes will have been properly recorded in the application databanks. A new, empty, LOGDB can simply replace the log that was lost. However, a backup of the entire database must be taken immediately. The new LOGDB will be empty, and therefore in a state consistent with a backup of all databanks having just been taken and all LOGDB records cleared. If a backup is not taken immediately, a later attempt to restore a databank is likely to fail because the restore operation will expect to find information in LOGDB that was lost when LOGDB was destroyed.

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