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XML-Native Constraint Evaluation

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Technical rept.

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This paper discusses approaches to validating XML documents for compliance to constraints. Our particular focus is on structural and content constraints that go beyond what is readily expressible in XML Schema technologies. We provide examples and solutions drawn from our specific experience building an XML-native constraint validator based on a mathematical language called Structural Notation SN. SN is used to express operational constraints as machine-processible Rules against a particular category of hierarchically structured, text-oriented military messages, called Message Text Formats MTFs, which have been migrated to a corresponding XML-based representation. We discuss the challenges we faced in implementing this XML-native constraint evaluator. For example, we discuss how, to build a Rule validator, we found it necessary to extend the underpinnings of logical evaluation in XPath 2.0 to use three-valued logic 3VL rather than two-valued logic. We detail some general principles for expressing and enforcing constraints against regularly structured text, when rendered as an XML document. We enumerate minimal capabilities needed by a constraint language and evaluator for XML documents and suggest some ways our approaches can be generalized for use in other domains. Because the need to apply constraints to incomplete or flawed documents is not unique to the military messaging world, a constraint evaluation model such as we propose, grounded in 3VL, is relevant to the XML user community at large.

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  • Operations Research
  • Computer Programming and Software

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