Semantic Web Languages: RDF vs. SOAP Serialisation Stefan Haustein University of Dortmund, Computer Science VIII, D-44221 Dortmund, Germany [email protected] AP just as a syntax enco ding of the ABSTRACT gestion is not to use SO RDF data mo del but to build a part of the Semantic Web Although RDF is considered the Semantic Web language, on SOAP serialisati on in the rst place. it may not b e the only one. SOAP serialisati on provides several advantages, esp ecially if the Semantic Web is not viding meta data for existing web pages, but just ab out pro 2. RDF EVOLUTION also ab out exchange of content that is machine-readabl e in The Resource Description Framework RDF [15] was the rst place. This pap er discusses some problems with originall y designed by the World Wide Web Consortium the RDF syntax and data mo del. RDF is compared to W3C as a meta-language for annotating existing web pages SOAP, and some SOAP advantages like b etter integration with additional machine-readable information. Atypical with existing standards and systems, improved readability, RDF applicatio n is the Platform for Internet Content Se- and industry supp ort are p ointed out. lection PICS [6], that is intended to annotate existing web resources with meta-data ab out e.g. the suitabili ty of the Keywords: SOAP Serialisati on, RDF, RDFS, Ob ject- content for children. Orientation Although RDF was originally designed for annotation of existing web pages only, it is currently widely considered hine-readable format for the Semantic Web and the 1. INTRODUCTION as mac DARPA Agent Meta Language DAML. What are the consequences if the term \Semantic Web" However, when using RDF as the primary information do es not just mean HTML with some meta-data, but also format, and not just for meta data annotation, some issues content that is machine-readable in the rst place, thus b e- arise. These problems are describ ed in detail in the following ing suitable for application s like software agent communica- sections. For an overview of additional general RDF issues tion? Although RDF is suitable for that purp ose, its syntax and inconsistencies , the reader is referred to [4] and [10]. and data mo del are clearly optimised for annotating exist- ing do cuments with meta data, describing existing web re- hine readable format. sources using a mac 3. RDF SYNTAX ISSUES In contrast to HTML, the exibili ty of XML allows stor- The requirement that RDF should b e usable for annotat- ing all relevant meta data in a machine-readable format ing existing pages with meta-data without breaking browser in the rst place. With the increasing separation of content compatibili ty for the actual content resulted in a syntax that and layout into XML and XSLT les, and the dynamic gen- is more complicated than necessary for plain RDF do cu- eration of XHTML, the need for a separate meta mo del ments. Also, several alternative syntax forms exist. All may decrease. Naturally, RDF is well suited for annotat- alternatives are semantically equivalent, but have di erent ing the generated HTML with information like PICS, but is e ects on the rendering pro cess in the browser that is used content annotation really all the Semantic Web is ab out? to view the do cument. This article discusses some serious issues concerning the The general RDF syntax consists of simple resource de- RDF syntax and data mo del when used as a primary ma- scriptions \prop erties" emb edded in a \description" ele- chine readable content format, instead of just adding meta- ment. In order to illustrate RDF and SOAP syntax alterna- data to existing HTML or XML pages. It presents SOAP tives, a FIPA 2000 Agent Platform description [14] is used as serialisati on as an alternative. In contrast to [16], the sug- a common example here, where the serialised instances are 1 taken from the Paris Agentcities no de ApDescription . Al- though the Paris no de Agent Platform Description consists , the corresp onding RDF serialisa- Permission to make digital or hard copies of all or part of this work for of four small ob jects only erb ose. personal or classroom use is granted without fee provided that copies are tion gure 1 b ecomes rather v not made or distributed for profit or commercial advantage and that copies rdf:Description elements are bear this notice and the full citation on the first page. To copy otherwise, to In the RDF example, the y their abbreviated form for improved republish, to post on servers or to redistribute to lists, requires prior specific already replaced b permission by the authors. 1 city.html, the ttp://www.agentcities.org/Cities/ pari s Semantic Web Workshop 2001 Hongkong, China see h y \Camel" syntax for compatibility Copyright by the authors. dashes are replaced b <ApDescription id="1"> <name>paris.agentcities.org</name> <dynamic>true</dynamic> <mobility>true</mobility> <transportProfile> <ApTransportDescription id="2"> <availableMtps> <rdf:Bag> <rdf:li> <MtpDescription id="3"> <mtpName>fipa.mts.mtp.iiop.std</mtpName> <addresses> <rdf:Bag> <rdf:li>iiop://leap.crm-paris.com:9000/paris.agentcities.org/acc</rdf:li> <rdf:li>iiopname://leap.crm-paris.com:9000/paris.agentcities.org/acc</rdf:li> </rdf:Bag> </addresses> </MtpDescription> </rdf:li> <rdf:li> <MtpDescription id="4"> <mtpName>fipa.mts.mtp.http.std</mtpName> <addresses> <rdf:Bag> <rdf:li>http://leap.crm-paris.com:8080/acc</rdf:li> </rdf:Bag> </addresses> </MtpDescription> </rdf:li> </rdf:Bag> </availableMtps> </ApTransportDescription> </transportProfile> </ApDescription> Figure 1: RDF Syntax Example readability. The example enco ding is not the only RDF en- co ding option, though. RDF allows several syntax variants: <ApDescription id="1"> <name>paris.agentcities.org</name> <dynamic>true</dynamic> <mobility>true</mobility> Resource description and typ e abbreviation: An <transportProfile> rdf:Description elementmay b e replaced byan <ApTransportDescription id="2"> element named like the typ e of the resource describ ed, ... also obsoleting a corresp onding rdf:type element. </ApTransportDescription> </transportProfile> In the example, the abbreviated form was already </ApDescription> used. All ob ject descriptions in the example could b e replaced by the corresp onding standard form. For example, is equivalentto <ApDescription id="1"> ... <ApDescription id="1"> </ApDescription> <name>paris.agentcities.org</name> <dynamic>true</dynamic> <mobility>true</mobility> <transportProfile resource="2" /> is equivalentto </ApDescription> <ApTransportDescription id="2"> ... <rdf:Description> </ApTransportDescription> <type resource="&fipaNS;ApDescription" /> ... </rdf:Description> The various RDF syntax options lead to two main prob- lems: XSLT and XML Schema compatibility problems and Obviously, the second variant adds ve extra elements problems with human readability. to the example co de. 3.1 XSLT Compatibility The Extensible Stylesheet Language Transformations Using attributes instead of elements: RDF elements XSLT were designed with the main goal of separating the may b e replaced by attributes if they o ccur only once content and layout of Web pages. The basic idea is to de- in their parent element, and contain only literal text sign the original page using an XML language. The XML without further substructures. For example, some of content is then converted to a \regular" XHTML page by the ApDescription sub-elements could b e replaced by an XSLT template. attributes: The various RDF enco ding options describ ed ab ove make development of XSLT templates for RDF dicult: In or- der to b e fully applicabl e to RDF, XSLT templates would <ApDescription id="1"> need to de ne a mapping covering all p ossible syntax alter- <name>paris.agentcities.org</name> natives. It would certainly b e p ossible to design relatively <dynamic>true</dynamic> simple XSLT templates for one concrete serialised form of <mobility>true</mobility> RDF. But then the XSLT transformation would b ecome ei- ... ther very fragile, or another pro cessing step converting any </ApDescription> RDF le to the form exp ected by the template would b e necessary. alentto is equiv 3.2 Human Readability Another problem with RDF is human-readabili ty. While one of the original ideas of XML is to provide some kind of <ApDescription id="1" compromise b etween machine and human-readabili ty, RDF name="paris.agentcities.org" is actually dicult to read for humans. Again, the main dynamic="true" mobility="true"> ... reason are the meta language ro ots of RDF. With the various </ApDescription> syntax options, it is even quite dicult to just see if two RDF do cuments are semantically equivalent. In order to read RDF do cuments, a human must b e familiar with all syntax variants of RDF. When RDF is used to annotate an existing HTML page, the situation b ecomes even worse since Nesting instead of linking Instead of referring to an ob- it is often dicult to di erentiate b etween RDF annotation ject using the rdf:resource attribute, the corresp ond- and actual content. In addition, the verb osity of RDF makes ing ob ject can b e emb edded into the predicate element. it dicult to read when compared to other XML languages In the original example, all ob jects are emb edded for or SOAP. b etter readability. tuitive mapping. 4. RDF DATA MODEL ISSUES The main problem is that there is no in All mappings have their own advantages and disadvantages, While it seems relatively simple to x the problems con- without one b eing clearly preferable to the others. More- cerning the RDF syntax, this is far more dicult for the over, except from the facets solution, which has the dis- RDF data mo del. advantage of extending RDFS, it is not p ossible to apply The RDF data mo del is very simple.