Open GUID Specification

Working Draft 21 October 2008


This document formally defines Open GUID, a system for establishing identity across the Semantic Web. Open GUID consists of:

Status of This Document

This document is a Working Draft published by The contents of this specification may be revised over time. Revisions will appear here, with major changes summarized in the Appendix.

Open GUID solicits feedback on this Working Draft. Please forward any comments to the public working group. Other Contributions are welcome.

Table of Contents

1. Introduction

1.1. Background and Motivation

Many ontologies exist for publishing machine processable data on the Semantic Web. Defined with languages such as OWL, these ontologies are typically targeted to a specific domain.

OWL guidelines recommend that ontologies be Designed for Use. This means that the same real world concept can be represented quite differently across domains. Given this flexibility, it can be lost that entities defined in separate ontologies share a common referent. This prevents complete analysis and searchability of the Semantic Web.

Open GUID is designed to link RDFS Classes, OWL Individuals, SKOS Concepts, and any other resource to the same real world idea.

1.2. How to Read this Document

Section 2 defines the nature of an Open GUID. The complete vocabulary is given in sections 3 and 4. Sections 5 and 6 describe how Open GUID relates to existing Semantic Web technologies.

1.2.1. URI Prefixes

URIs cited in this document are abbreviated with the following prefixes.

URI Prefixes

Thus, for example, owl:equivalentClass is short for

1.2.2. Examples

RDF examples use the Turtle language. Assume the examples are preceded by the following directives:

@prefix rdf: <> .
@prefix rdfs: <> .
@prefix owl: <> .
@prefix skos: <> .
@base <> .

2. Definition

2.1. What is an Open GUID?

An Open GUID is a permanent global identifier for any person, place, thing, abstract concept, or even a group of these. As long as something is determined unique by the web community, it can have an Open GUID. Identifiers are established using the Open GUID Services.

2.2. Keywords

An Open GUID is associated with multiple keywords or keyphrases in every language. These are used to help resolve an Open GUID.

2.3. Description

An Open GUID is associated with a short description in every language. The description is meant to help disambiguate the concept from similar ones. It is not meant to provide any detailed information about the concept.

2.4. Tags

An Open GUID can be tagged with other Open GUIDs. These tags convey no semantic meaning; rather they are used to help navigate related concepts. The link text of a tag is one of the keywords associated with the Open GUID.

2.5. Dereferenceable URI

An Open GUID is assigned a Dereferenceable URI. The URI is simply the GUID appended to See the sample below.

Sample URI

2.6. Permanency

Open GUIDs are permanent. This means, that once an Open GUID is established, it will always mean the same thing. Only grammatical and disambiguatory changes are allowed to the description.

2.7. Duplication

It is possible for duplicate Open GUIDs to emerge. Duplication is determined democratically by the web community using the Open GUID Services. If two GUIDs are deemed duplicates, one GUID will be arbitrarily assigned as the primary identifier, and an identical relationship will be established between them. The dereferenceable URI for the duplicate will respond with a 301 redirect to the primary GUID.

3. Open GUID Namespace & Vocabulary

The Open GUID namespace URI is:

The Open GUID vocabulary consists of one property, as seen below.

Open GUID Vocabulary
oguid:identical4. The oguid:identical Property

All URIs in the vocabulary are constructed by appending the listed URI to the Open GUID namespace URI.

3.1. RDF+XML

The formal definition of the Open GUID vocabulary can be found as an RDF+XML document here.

4. The oguid:identical Property


4.1. Semantics

The property oguid:identical indicates two resources are co-referent. In other words, they refer to the same real world thing.

Open GUID identity is defined pragmatically as referring to the same concept beyond a reasonable doubt. The major determinant is if the average human perceives the resources as referring to the same thing.

The linking implication can be described as being between OWL sameAs and RDFS seeAlso. The sameAs property declares equivalence of semantic representations; thus it should be used carefully to avoid confusing logical reasoners. The seeAlso property provides additional information; nothing useful can be inferred from the link. The identical property declares equivalence of referents; thus it allows reasoners to use local representations while still maintaining global context.

It is suggested that the Open GUID Services be used to establish a permanent identifier for the concept being declared identical. The uniqueness can then be evaluated by the web community, and a link established to an existing GUID if necessary. However, registration with Open GUID is not required for either subject or object in an RDF statement.

oguid:identical is an instance of rdf:Property.

4.2. Usage

The oguid:identical property can link any two resources. For example, an RDFS Class can be declared as identical to an instance and vice versa.

The rdfs:domain of oguid:identical is rdfs:Resource.
The rdfs:range of oguid:identical is rdfs:Resource.

4.3. Example

The RDF statement below indicates that <SampleOne> is equivalent to <SampleTwo>.

Example 1
<SampleOne> oguid:identical <SampleTwo> .

4.4. Symmetry

The oguid:identical property is symmetric. Thus, Example 1 implies that Example 2 is also true.

Example 2 (Implied by Example 1)
<SampleTwo> oguid:identical <SampleOne> .

4.5. Transitivity

The oguid:identical property is transitive. Thus, in Example 3, the third statement is implied by the first two.

Example 3
<SampleOne> oguid:identical <SampleTwo> .
<SampleTwo> oguid:identical <SampleThree> .

<SampleOne> oguid:identical <SampleThree> . (Implied)

4.6. Open World Assumption

Open GUID is designed for distributed systems (i.e. the Web). Thus, it makes an Open World Assumption. That is, any given document cannot be considered complete, as there might exist additional documents that define identical resources. This is desired flexibility because new ontologies can be linked to existing ones in a manner beneficial to the machines interpreting the Semantic Web.

5. Linking Ontologies

Classes and instances in web ontologies may be linked using the oguid:identical property. This implies that the resources refer to the same concept, as perceived by humans. Linking ontologies in this manner adds depth to the semantic web.

It is suggested that ontological resources be linked to a permanent GUID generated by the Open GUID Services. This assures that relationships between subjects will never be lost, even if a specific ontology or declaration disappears.

Special semantics may arise when linking ontologies that contain the concept of equivalence. By default, the symmetric and transitive properties typically associated with equality apply. See below for specific interpretations of this behavior within existing ontological frameworks.

5.1. OWL

The OWL properties owl:equivalentClass and owl:sameAs also deal with identity. It can be determined that OWL resources associated by these properties are also associated by oguid:identical. The symmetry and transitivity properties are upheld; thus, Example 4 implies Example 5.

Example 4
<SampleOne> owl:equivalentClass <SampleTwo> .
<SampleOne> oguid:identical <SampleThree> .

Example 5 (Implied by Example 4)
<SampleTwo> oguid:identical <SampleOne> .
<SampleTwo> oguid:identical <SampleThree> .

OWL semantics cannot be inferred from resources declared oguid:identical. Thus, Example 6 is not implied by Example 4.

Example 6 (Not implied by Example 4)
<SampleOne> owl:equivalentClass <SampleThree> .

A similar correlation can be made using owl:sameAs with OWL Individuals.

5.2. SKOS

The SKOS property skos:exactMatch indicates that two concepts are exactly same. It can be determined that SKOS Concepts associated by this property are also associated by oguid:identical. The symmetry and transitivity properties are upheld; thus, Example 7 implies Example 8.

Example 7
<SampleOne> skos:exactMatch <SampleTwo> .
<SampleOne> oguid:identical <SampleThree> .

Example 8 (Implied by Example 7)
<SampleTwo> oguid:identical <SampleOne> .
<SampleTwo> oguid:identical <SampleThree> .

SKOS semantics cannot be inferred from resources declared oguid:identical. Thus, Example 9 is not implied by Example 7.

Example 9 (Not implied by Example 7)
<SampleTwo> skos:exactMatch <SampleThree> .

6. Linking Content

Specifying the subject of content on the web is a cornerstone of the Semantic Web. Existing specifications call for using URIs as RDF subjects. Using URIs as subjects is problematic for two reasons.

1. Domains are subject to market forces and thus unreliable as fixed identifiers. When URIs change or disappear, semantic data associated with them becomes orphaned.

2. There are often multiple URIs that refer to the same thing. Individual may establish their own profile on multiple websites, each offering an identifying URI. Well known entities might be entered into many common knowledge databases. It is extremely difficult for Semantic Web interpreters to make associations between these URIs, thus sacrificing valuable linked data.

Open GUID solves these problems by establishing a permanent, open RDF subject. See below for specific details about linking content to an Open GUID using existing technologies.

6.1. RDFa

RDFa defines rules for embedding semantic information into HTML. Open GUIDs make natural subjects using RDFa about attributes and objects using resource attributes. Open GUIDs can be embedded as URIs (Example 10) or URNs (Example 11).

Example 10
<div about="">
Some text relating to the subject GUID.

Example 11
<div resource="urn:uuid:0e21d666-dbcd-102b-9532-00301b462e0a">
Some text relating to the object GUID.

RDFa typeof and property attributes can be interspersed to add structured ontological content related to the subject or object.

In addition, the oguid:identical property fits perfectly into a rel attribute as seen in Example 12. This declares the subject (given in the most recent about statement) as identical to the target.

Example 12
<a rel="oguid:identical" href=""></a>

6.2. Microformats

Open GUIDs can be embedded in content using various Microformats. For example, the microformat rel-tag can be used to tag content with an Open GUID; a natural, permanent tag.

Example 13
<a href="" rel="tag">Technology</a>

A. Appendix

A.1. Change Log

Changes from the specification dated 27 September 2008:

Changes from the specification dated 24 September 2008: