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Resilience


Database Shadowing means that the database works with two, or more, copies of the database information at the same time. One copy - the master - is the 'normal' file from which data is retrieved. The other copy or copies are shadows. Alterations made to the master data are also made to the shadows, so that a shadow is always an up-to-date copy of the master. Applying the alteration separately to each shadow so that any physical corruption of the master are not transferred to the shadow.

In Mimer SQL, shadowing is controlled at the databank level. Any databank where transaction control is in force can be shadowed to as many copies as required. Important data can be protected against a disk crash without having to rely on frequent conventional backups.

Should there be a disk failure on the disk where master databanks reside, operations automatically continue towards the shadows. As there is no time-consuming process of restoring the database from backup files held on tape or other media the application can be kept running without any disturbance. The database manager can choose a suitable moment to transform the shadows to master databanks, and create new shadows.

During normal running the shadowing has no impact on response times. The Background threads perform changes made to the master databank and shadows, without impacting the users.



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