Efficiency frontiers of XML cardinality constraints

XML has gained widespread acceptance as a premier format for publishing, sharing and manipulating data through the web. While the semi-structured nature of XML provides a high degree of syntactic flexibility there are significant shortcomings when it comes to specifying the semantics of XML data. For the advancement of XML applications it is therefore a major challenge to discover natural classes of constraints that can be utilized effectively by XML data engineers. This endeavor is ambitious given the multitude of intractability results that have been established. We investigate a class of XML cardinality constraints that is precious in the sense that it keeps the right balance between expressiveness and efficiency of maintenance. In particular, we characterize the associated implication problem axiomatically and develop a low-degree polynomial time algorithm that can be readily applied for deciding implication. Our class of constraints is chosen near-optimal as already minor extensions of its expressiveness cause potential intractability. Finally, we transfer our findings to establish a precious class of soft cardinality constraints on XML data. Soft cardinality constraints need to be satisfied on average only, and thus permit violations in a controlled manner. Soft constraints are therefore able to tolerate exceptions that frequently occur in practice, yet can be reasoned about efficiently. (C) 2012 Elsevier B.V. All rights reserved.