Before we proceed with an overview of the Semantic Web Architecture, let us first define the goals of the Semantic Web:
- Consistent Knowledge – Knowledge about a specific thing is consistent across distributed knowledge sources. Any knowledge change should be reflected across all mutually dependent knowledge sources.
- Accessing Knowledge – Providing the most accurate knowledge aggregated from all available sources.
- Reusing Knowledge – Knowledge can be fully internally and/or externally reused by having it formally structured and open.
Architectural Principles
An architecture is a model of a system that is defined within a certain context. The model is an abstraction of a real world representation. The context defines a view of appraisal of the system. It also determines the components necessary to implement the system, the properties of these components, the relationships between the components, and relationships with external entities.
The layered architecture is an architectural pattern widely used today to present conceptual Semantic Web Architecture. Generally in a layered architecture, the components of the system are arranged in a layered structure where each layer represents a group of elements providing related services. There are two types of this architecture: open and closed. In the open architecture, higher layers can use all or subset of services from all lower layers. In the closed architecture a higher layer can use only service from the immediate lower layer. In both type of the layered architecture lower layers cannot use services from higher layers.
A typical widely used layered architecture is ISO/OSI (International Standards / Open Systems Interconnect) architecture [1]. This architecture specifies functional foundation needed to define protocols for network interoperability between applications. The Semantic Web architecture has the same purpose as ISO/OSI architecture in that it specifies the languages required for data interoperability between applications.
The context of the Semantic Web Architecture are languages required for meta-data specifications and reasoning about information (knowledge) specified via the meta-data specifications.
Towards Technology Agnostic Conceptual Semantic Web Architecture
A precise definition of architectural layers’ functionalities and their interfaces has to be provided for the Semantic Web applications to interoperate.
Gerber at al. [2] defined Comprehensive, Functional, Layered (CFL) architecture for the Semantic Web. This architecture is based on the previous versions of the Semantic Web Architecture defined by Tim Berners-Lee [3,4,5,6,7,8,9]. It defines related Semantic Web functionalities rather than the W3C technologies as the architectural layers. The CFL architecture enables use of different technologies to implement the functionality of a specific layer. However the specification of the functionality interfaces needed to clearly define interfaces between the layers has not been defined yet.
The CFL architecture proposes two orthogonal architecture stacks, the language stack and the security stack (Fig. 1).
Figure 1: CFL Architecture
The Layers of the CFL Semantic Web Architecture
The layers of the CFL architecture include:
Unicode Identification Mechanism
Used to uniquely identify resources. It also provides a mechanism for uniquely identifying all the characters in all written languages. Examples of this layer technologies are: Unicode [10]and Uniform Resource Identifier (URI) [11,12,13].
Syntax Description Language
Provides a language for specifying syntax of various data formats. The most used technologies include XML [14], XML Schema [15], and Namespaces [16].
Meta-data Data Model
Provides a mechanism to model the meta-date required to implement the Semantci Web. The most used technologies for this layer include: RDF and RDF Schema (RDFS) [17].
Ontology
This layer provides language support for creation of ontologies. It is instantiated with either RDFS or RDFS and OWL [18].
Rules
This layer belongs to rule-based languages and their processing. Rule Interchange Format (RIF) is the W3C rule-based language and it is compatible with RDF and OWL [19]. There is also an important group of rule-based languages [20] that cannot be layered on top of OWL. They should be also included in the Semantic Web Architecture together with other RDF and OWL related rule-based languages. I will review these languages in one of the future posts. In addition to allow query and filtering the rules layer also supports inference.
Logic Framework
This layer provides an answer about the reasoning of why the information is taken or appear to the user. Currently the technology specification for this layer does not exist.
Proof
Thids layer provides an answer for the question of why agents should believe the provided information. Currently the technology specification for this layer does not exist. The Knowledge Systems Laboratory at Stanford has been developing a proof language called PML [21]
Trust
This layer ensures that he information provided is valid and there is a degree of confidence in the resource that provides the information . Currently the technology specification for this layer does not exist.
Identity Verification and Encryption
This layer belongs to security (especially identification and encryption at least). It is not part of the language stack. It should be developed as a separate Security Architecture that will interface with the language stack. The W3C technology specs that belong to this layer are XML Signature [22] and XML Encryption [23].
References
1. OSI Reference Model – The ISO Model of Architecture for Open Systems Interconnection
Hubbert Zimmerman
http://dret.net/biblio/reference/zim80
2. A Functional Semantic Web Architecture
Aurona Gerber, Alta van der Merwe, and Andries Barnard, 2008
http://ksg.meraka.org.za/~agerber/Paper152.pdf
3. Semantic Web – XML2000. W3C Web site 2000
Tim Berners-Lee
http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide10-0.html
4. WWW2005 Keynote. W3C Web site 2005
Tim Berners-Lee,
http://www.w3.org/2005/Talks/0511-keynote-tbl/
5. Artificial Intelligence and the Semantic Web: AAAI2006 Keynote. W3C Web site 2006
Tim Berners-Lee
http://www.w3.org/2006/Talks/0718-aaai-tbl/Overview.html
6. The Semantic Web and Challenges. W3C Website Slideshow 2003
Tim Berners-Lee
http://www.w3.org/2003/Talks/01-sweb-tbl/Overview.html
7. Standards, Semantics and Survival. SIIA Upgrade 2003, pp. 6-10.
Tim Berners-Lee
8. WWW Past and Future. W3C Web site 2003
Tim Berners-Lee
http://www.w3.org/2003/Talks/0922-rsoc-tbl/slide30-0.html
9. SIIA. Website
http://www.siia.net/
10. Unicode
http://www.unicode.org/
11. Designing URI Sets for the UK Public Sector, V1.0, October, 20009
Paul Davidson, CIO, Sedgemoor District Council
http://www.cabinetoffice.gov.uk/media/301253/puiblic_sector_uri.pdf
12. Cool URIs for the Semantic Web
W3C Working Draft, 17 December, 2007
http://www.w3.org/TR/2007/WD-cooluris-20071217/
13. Cool URIs dont’ change
Tim Berners-Lee
http://www.w3.org/Provider/Style/URI.html
14. XML, W3C
http://www.w3.org/XML/
15. XML Schema, W3C
http://www.w3.org/XML/Schema
16. Namespaces in XML 1.1, W3C
http://www.w3.org/TR/2006/REC-xml-names11-20060816/
17. RDF Standards, W3C
http://www.w3.org/TR/#tr_RDF
18. OWL, W3C
http://www.w3.org/standards/techs/owl#w3c_all
19. RIF RDF and OWL Compatibility, W3C
http://www.w3.org/TR/2010/REC-rif-rdf-owl-20100622/
20. A Realistic Architecture for the Semantic Web
Michael Kifer, Jos de Bruijn, Harold Boley, and Dieter Fensel
http://www.kr.tuwien.ac.at/staff/bruijn/priv/publications/msa-ruleml05.pdf
21. A Proof Markup Language for Semantic Web Services
Paulo Pinheiro da Silva, Deborah L. McGuinness, Richard Fikes
http://ftp.ksl.stanford.edu/pub/KSL_Reports/KSL-04-01.pdf
22. XML Signature, W3C
http://www.w3.org/TR/#tr_XML_Signature
23. XML Encryption, W3C
http://www.w3.org/TR/#tr_XML_Encryption
