Wednesday, May 11, 2005


As Drummond Reed, Paul Trevithik and I continue to delve into the XDI Universal Schema and it's graph representation my understanding of it, and my belief in it, continue to grow.

I understand Kim’s assertion of his 7 laws. He’s not legislating; “You must do this”. He’s not playing god, he’s proposing that he has uncovered some basic truths about the state of things. I am starting to feel the same way about the power of the XDI Universal Schema. I believe that Drummond (it really is his brain child) has lead us into a VERY powerful, new way of representing data.

The Universal Schema juxtaposes, or superimposes, 4 hierarchical graphs on top of each other, one at right angles to the others. Every resource in the dataweb can be accessed through the constrained 3 level syntax (Auth, Type and Instance) axis or through it’s ‘natural’ schema (or meta-schema) representation on the second axis (once you have put it in the context of the authority that can permission that ‘document’).

Let’s Take a look…

In this image I see 4 distinct hierarchies; the first runs across the Authority level. This is a registry and the vertical graph would have been equally valid rooted in any of these Authorities.

The second hierarchy runs across the Type level. This graph describes a VERY simple schema; it shows the component parts of a US mailing address.
The third graph runs across the Instance level of the graph this MUST describe graphs that are valid instances of the schemas represented at the Type level.

Finally, the fourth graph is the one that runs from top to bottom. This is the graph that associates authorities with the schemas and the instances of those schemas. It provides XRI addressing directly to any point in that schema or schema instance. This enables very efficient traversal of the data as you can navigate the abstract schema until you NEED to drop down to the instance level to get the specific instances that you need.

Notably the first 3 graphs are totally un-constrained. At the Authority level this means that our registry is totally extensible. At the Type level this means that ANYTHING that can be represented in XML (which is a hierarchical metaphor) can be represented at the Type level (if you can describe it in RDF or XML Schema or RelaxNG you can put it here). At the Instance level it simply means that you can represent instances of the schemas from the type level.

Now, the final mind blower for the night…

I was talking to Jamie Lewis and I said “any object can be described as a list of name value pairs”… eyes, blue, hair, brown, street address, …, etc… in a simple world that might be true but even as I said it I knew that it was overly simplistic. But now looking at the XDI Universal Schema in the terms I just described I see what the vertical graph ‘also’ represents; it’s the association of, not ‘name,value’ pairs with an object, (an Authority) but ‘Schema, Instance’ pairs with an object. And That, in my mind, seems to be a mighty powerful way to describe an object.

1 comment:

Steve Austin said...

Interesting blog. I have a oracle xml blog.