Integrity constraints express important properties of data, but the task of preserving data consistency is becoming increasingly problematic with new database applications. For example, in the case of integration of several data sources, even if the sources are separately consistent, the integrated data can violate the integrity constraints. The traditional approach, removing the conflicting data, is not a good option because the sources can be autonomous. Another scenario is a long-running activity where consistency can be violated only temporarily and future updates will restore it. Finally, data consistency may be neglected because of efficiency or other reasons.
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