Databank shadowing means updating one or more copies of a databank simultaneously.
The master is the "normal" databank file which is accessed for data storage and retrieval. The copies are called shadows.
Any changes to the master data is automatically made to the shadows, thus protecting data from a disk crash or other event that might cause a databank to be lost.
A databank must support transaction control, see Transaction Control, to be shadowed.
All changes made to a shadowed databank are automatically made to all the shadows for the databank.
If a master databank is lost, a shadow will automatically take over from the master and operations can be resumed immediately (assuming the shadow is not also damaged in the disk crash). A shadow can be transformed to a master databank to permanently replace it and this process is much faster than restoring a databank from a backup copy.
It is recommended that conventional backups be taken as a supplement to databank shadows, to protect data in the event of a crash that destroys both the master and the shadow.
Because operations are not interrupted when backups are taken, and because Mimer SQL databanks are automatically reorganized, true 24 hour-a-day operation is possible.
Databank shadowing is entirely invisible to the application. This means that shadows can be added to existing applications. No special handling is needed to access tables in a shadowed databank.
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