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

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

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 http://ootips.org/association-objects.html http://www.riehle.org/computer-science/programming/patterns/association-objects/

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

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: http://math.rice.edu/~lanius/fractals/ http://math.rice.edu/~lanius/fractals/sierjava.html

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

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

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

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

* / 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

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

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