Streaming validation of schemata: The lazy typing discipline

Assertions, identity constraints, and conditional type assignments are (planned) features of XML Schema which rely on XPath evaluation. The XPath subset exploitable in those features is limited, for several reasons, including (apparently) to avoid buffering in evaluation of an expression. We divide XPath into subsets with varying streamability characteristics. We also identify the larger XPath subset which is compatible with the typing discipline we believe underlies some of the choices currently present in the XML Schema specification. Such a discipline requires that the type of an element be decided when its start tag is encountered and its validity when its end tag is encountered. An alternative “lazy typing” discipline is proposed in which both type assignment and validity assessment are fired as soon as they are available. Our approach is more flexible, giving schema authors control over the trade-off between using larger XPath subsets (and thus increasing buffering requirements) and expeditiousness.