1. An Analysis of the Structure of the Proposed XDI Metaschema
How XDI Builds on RDF
An Analysis of the Structure of the Proposed XDI Metaschema
v1, July 30, 2004

2. The relationship of XDI and RDF
It has long been clear that both are data description models
What has not been clear is the relationship between the two models
This analysis, based on the proposed v4 XDI metaschema, answers that question
It also reveals the fundamental pattern of the v4 metaschema design

3. Association objects
Association objects are used to describe the relationship between two other objects
They can themselves be first class objects
See: A C B
Object C describes the relationship between object A and object B
Object A has a relationship with object B

4. RDF (Resource Description Framework)
RDF uses association objects to describe the properties of resources
A Subject is identified with a uriref or bnode (blank node)
A Predicate is a uriref
An Object is a uriref, bnode, or literal
Subject 1
Predicate 1 1
Property
Object
Property value

5. XDI builds on RDF by creating a metamodel of the RDF model
This metamodel makes the 3-node RDF tuple repeatable in a recursive or fractal pattern
For a good visualization of a 3-node fractal model (triangles), see:

6. Step 1 in creating the XDI metamodel
Subject
The first step is to create the XDI tuple
Subject = Resource
Predicate = XRI
Object = Data
This tuple allows multiple predicates (XRIs) to describe the same property
It also makes the property value optional 1
Predicate 1 1
Property
Object
RDF
Property value
Resource *
XRI
1..*
0..1
Property
Data
XDI
Property value

7. Step 2: create a metamodel
The second step is to use the XDI tuple to make a metamodel of the same association object pattern on which the tuple itself is based
In other words, “blow up” the association object model so that each of the 3 objects is itself modeled with the XDI tuple C B

8. A C B Step 2 (continued) Each Subject (A) is modeled as a Resource*
Each Subject (A) is modeled as a Resource
Each Object (B) can be modeled as a nested Resource
Each Predicate (C) can also be modeled as a nested Resource
The model is now fractal – it can be infinitely recursed
XRI
1..*
0..1
Data *
Resource 1 1 *
XRI
1..*
0..1 C
Data 1
Resource *
XRI
1..*
0..1 B
Data

9. Step 3: addressing A
Resource *
Each Resource node is individually addressable using one or more XRIs
Traversing this graph requires XRI syntax that can differentiate between XRIs representing:
Attribute objects (B)
Association objects (C)
XRI
1..*
0..1
Data *
Resource 1 1 *
XRI
1..*
0..1 C
Data 1
Resource *
XRI
1..*
0..1 B
Data

10. * / A C B Step 3 (continued)
Proposed XRI 1.1 syn-tax can support this via:
Using slash to address attribute objects (B)
Using star to address association objects (C)
In essence, slash would mean “has attribute of”
Star would mean “has association with” * * C / 1 1 1 B

11. XDI addressing
Proposed XDI addressing rules for the v4 metaschema are summarized in a separate document
They provide the ability to reference and cross-reference any XRI-addressable node from any other XRI-addressable node
This meets the requirements of the Dataweb

12. Conclusion
By building directly on the RDF model, the XDI metaschema can now be boiled down to just three XML elements (and no attributes)
It is fully fractal (turtles all the way down), and can be used to describe both attributes and associations to any degree of complexity
The rules for addressing of XDI documents using XRIs can be stated clearly and compactly