Soft Cardinality Constraints on XML Data - How Exceptions Prove the Business Rule

We introduce soft cardinality constraints which need to be satisfied on average only, and thus permit violations in a controlled manner. Starting from a highly expressive but intractable class, we establish a fragment that is maximal with respect to both expressivity and efficiency. More precisely, we characterise the associated implication problem axiomatically and develop a low-degree polynomial time decision algorithm. Any increase in expressivity of our fragment results in coNP-hardness of the implication problem. Finally, we extensively test the performance of our algorithm. The performance evaluation provides first-hand evidence that reasoning about expressive notions of soft cardinality constraints on XML data is practically efficient and scales well. Our results unleash soft cardinality constraints on real-world XML practice, where a little more semantics makes applications a lot more effective in contexts where exceptions to common rules may occur.