Schemapath, a minimal extension to xml schema for conditional constraints

Abstract

In the past few years, a number of constraint languages for XML documents has been proposed. They are cumulatively called schema languages or validation languages and they comprise, among others, DTD, XML Schema, RELAX NG, Schematron, DSD, xlinkit.One major point of discrimination among schema languages is the support of co-constraints, or co-occurrence constraints, e.g., requiring that attribute A is present if and only if attribute B is (or is not) present in the same element. Although there is no way in XML Schema to express these requirements, they are in fact frequently used in many XML document types, usually only expressed in plain human-readable text, and validated by means of special code modules by the relevant applications.In this paper we propose SchemaPath, a light extension of XML Schema to handle conditional constraints on XML documents. Two new constructs have been added to XML Schema: conditions -based on XPath patterns -on type assignments for elements and attributes; and a new simple type, xsd:error, for the direct expression of negative constraints (e.g. it is prohibited for attribute A to be present if attribute B is also present).A proof-of-concept implementation is provided. A Web interface is publicly accessible for experiments and assessments of the real expressiveness of the proposed extension.

Expressiveness and complexity of XML Schema

“…For example, in Fiorello et al [2004] an approach called DTD++ 2.0 is introduced that allows one to define schemas which are then translated into SchemaPath [Coen et al 2004], which is strictly more expressive than XML Schema and requires a transformation of the XML documents prior to validation. In this article, we present in Section 6 an instantiation of P -schemas whose expressive power is at most that of XML schema.…”

“…A syntactic approach using conditional alternatives similar to SchemaPath [Coen et al 2004] is suggested. However, rather than full XPath expressions, conditions would be limited to linear XPath (or general regular expressions) so that the expressive power of XML Schema is not exceeded.…”

“…The advantage of the pattern-based approach is that it makes explicit the way in which context-dependencies can be expressed rather than hiding it through the general use of recursive types restricted by the EDC constraint. The patternbased approach can be migrated into a full fledged schema language in two ways: (1) as an extension of DTDs with contextual patterns; or, (2) as an extension of XML Schema itself (e.g., like SchemaPath [Coen et al 2004], with the crucial difference that our extension is conservative).…”

Use of standard vocabulary services in validation of water resources data described in XML

“…Schema languages Schema languages are classified as either grammar-based or rule-based (Coen et al (2004)). W3C XML schema (W3C (2001)) and ISO Relax NG (Clark and Murata (2001)) are both grammar-based languages for XML validation.…”

“…W3C XML schema (W3C (2001)) and ISO Relax NG (Clark and Murata (2001)) are both grammar-based languages for XML validation. ISO Schematron (Jecliffe (2002)), is a rule-based schema language utilising Xpath based context matching for activating context-sensitive rules, and Schemapath (Coen et al (2004)) extends XML Schema with conditional constraints. The rules-based XML languages have the advantage of transparency with respect to the XML encoding.…”