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 to various ends. The allowed XPath subset exploitable in those features is trimmed down for streamability concerns partly understandable (the apparent wish to avoid buffering to determine the evaluation of an expression) and partly artificial. In this paper we dissect the XPath language in subsets with varying streamability characteristics. We also identify the larger subset which is compatible with the typing discipline we believe underlies some of the choices currently present in the XML Schema specifications. We describe such a discipline as imposing that the type of an element has to be decided when its start tag is encountered and its validity has to be when its end tag is. We also propose an alternative lazy typing discipline where both type assignment and validity assessment are fired as soon as they are available in a best effort manner. We believe our discipline is more flexible and delegate to schema authors the choice of where to place in the trade-off between using larger XPath subsets and increasing buffering requirements or expeditiousness of typing information availability.