Re: [xml-dev] ID/IDREF is evil

Michael Kay wrote: "Ideally, the choice of document boundaries shouldn't make much difference; queries should work the same way regardless of where the document boundaries are. In practice, that's very hard to achieve, and one of the reasons is that intra-document linking is so very different from cross-document linking. There are actually three basic ways of modelling relationship in XML: use of the XML containment hierarchy, use of intra-document links, and use of cross-document links; and the way you write queries is totally dependent on which representation has been chosen."

I propose a slightly different perspective, which distinguishes two kinds of navigation: structural navigation, and semantic navigation. The first is offered by conventional XPath expressions, using navigation axes, node tests and predicates. Performing "semantic navigation", on the other hand, the pilot does not need to know structural relationships - the destination may be located in the same document, or in a different document whose URI need not be known, neither. Navigation is driven by specifying "what" one wants, not where one goes. This specifying of "what" is accomplished by specifying properties of the destination - a property-based query which the navigation engine resolves to a set of nodes. I think this is an extremely interesting perspective for the future evolution of XML: having achieved world class navigation within documents, starting to add serious support for inter-document (or more precisely: document-agnostic) navigation. Work in this direction plays an important role in the concept of an "info space" which binds the sum total of accessible XML resources into a unified substrate of navigation.

(Note. RDF may certainly be put to great uses in this context.)

Regards,
Hans-Juergen Rennau

Michael Kay <mike@saxonica.com> schrieb am 11:31 Donnerstag, 20.Februar 2014:

On 19 Feb 2014, at 22:45, Kurt Cagle <kurt.cagle@gmail.com> wrote:

> Actually, this brings up something I've been thinking about for a while. It is typical to think of an XML document as being a self-contained entity

I think this is one of the big problems with the use of XML as a database model.

Sometimes the concept of a "document" makes sense, it relates to something in the real world, like an insurance claim. Sometimes it makes no sense at all, e.g. when you're modelling the human genome. Ideally, the choice of document boundaries shouldn't make much difference; queries should work the same way regardless of where the document boundaries are. In practice, that's very hard to achieve, and one of the reasons is that intra-document linking is so very different from cross-document linking. There are actually three basic ways of modelling relationship in XML: use of the XML containment hierarchy, use of intra-document links, and use of cross-document links; and the way you write queries is totally dependent on which representation has been chosen. That violates the basic principles of data independence (which was the topic of my PhD thesis in 1975...)

I wouldn't normally recommend using an XML database unless it is natural to think about the data as being a set of documents.

Michael Kay
Saxonica
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