Describing Namespaces with GRDDL
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Describing Namespaces with GRDDL Erik Wilde, ETH Zurich¨ (Swiss Federal Institute of Technology) ABSTRACT information automatically. In many cases, it would be ben- Describing XML Namespaces is an open issue for many users eficial to have machine-readable namespace descriptions, be- of XML technologies, and even though namespaces are one cause these could be collected and compiled into a database of the foundations of XML, there is no generally accepted of namespaces and related resources. and widely used format for namespace descriptions. We In an effort to create a namespace description language to present a framework for describing namespaces based on combine the human-readability of HTML documents which GRDDL using a controlled vocabulary. Using this frame- machine-readable semantics, the Resource Directory Descrip- work, namespace descriptions can be easily generated, har- tion Language (RDDL) was invented. RDDL used the un- vested and published in human- or machine-readable form. popular XLink standard, and thus the Gleaning Resource Descriptions from Dialects of Languages (GRDDL) [2] lan- Categories and Subject Descriptors guage was developed, which defines a way how to expose the machine-readable information as RDF [4]. H.3.2 [Information Storage and Retrieval]: Information The GRDDL model assumes that the machine readable Storage—File Organization information of an XHTML page is extracted by using an General Terms XSLT program that transforms the GRDDL into RDF state- ments. Even though this adds little to the expressiveness Management, Languages of RDDL, it is a little bit easier to handle (because the 1. DESCRIBING SCHEMAS machine-readable information can be encoded at the users discretion, as long as it can be transformed into RDF), and While an XML Schema describes constraints for a class may gain more popularity because it uses RDF rather than of XML documents, but it does not convey any information XLink. about the semantics. For most application scenarios, the A namespace description is a set of machine-readable in- semantics of a schema are described in some informal way, formation about how other resources are related to the name- in most cases using simple prose. It would be useful to have space, such as schemas defining the namespace’s vocabulary, some standard mechanism how to associate this human- or documentation for the namespace application. In order to readable description with the XML Schema in a standard make GRDDL work, this vocabulary of how linked resources way. There is no established way for doing this, and there related to the namespace must be well-known. are two main approaches to solve this problem: (1) It is pos- sible to embed additional information within the schema. 2.1 Description Roles (2) Since the targetNamespace of a schema defines a name- space name according to the XML Namespaces [1] recom- GRDDL namespace descriptions serve as a supplement to mendation, it is possible to associate additional information the documentation, providing machine-readable documen- for a schema through the namespace name. tation that can be used to compile a directory of schemas. While both approaches solve the problem, we chose to To make this directory as rich as possible, a number of roles adapt the second approach, because it better separates the that must or can be used to describe schemas has to be de- schema implementation from the schema description. Schema fined. These roles describe how a particular resource relates descriptions as described here are generic enough to not only to the namespace being described. serve as XML Schema descriptions, but as descriptions for The description roles thus constitute that fraction of name- any vocabulary associated with a namespace name. There- space description that should be available in a machine- fore, we subsequently refer to them as namespace descrip- readable way, so that it can be collected and processed. tions. The Web architecture [3] does not define a data Technically, the resource roles are defined in a simple XML format for namespace descriptions, but recommends that document, which serves as configuration for defining the namespace names should point to some kind of description. GRDDL namespace description format described in the fol- lowing section, and for the RDF Schema used for the infor- 2. NAMESPACE DESCRIPTIONS mation extraction process described in Section 3. Despite the fact that the Namespaces specification itself does not require any resource to be available at a name- 2.2 Creating Descriptions space’s URI, it is convenient if this is the case. The approach Schema authors must create GRDDL descriptions for their of having HTML pages serving as namespace descriptions is schemas, and they must follow the guidelines requiring cer- useful for humans, but makes it very hard to process this tain kinds of roles to be present. However, schema authors are free how they create the GRDDL. While many choose to write their GRDDL by hand, others don’t want to “learn” Copyright is held by the author/owner. WWW 2005, May 10–14, 2005, Chiba, Japan. GRDDL, even though it is very easy to learn. ACM 1-59593-051-5/05/0005. For these users, we provide a simple XML Schema that 1002 can be used to capture all the information required for a Validation (as well as RDF generation) in the current im- GRDDL document, and an XSL Transformations (XSLT) plementation is done using an XSLT 2.0 program. The rea- program to generate the GRDDL that then serves as name- son for this is that XML Schema does not provide a reason- space description. This schema for namespace descriptions able way of validating a schema that is tightly integrated is generated from the list of resource roles described in the with a host language, and that XSLT 1.0 does not provide previous section. any support for checking datatypes. The result of the XSLT- A second schema — also generated from the list of re- based validation is a report containing a list of warnings or source roles described in the previous section — is required errors raised during the validation process. for describing the attributes (this schema does not define any elements) that convey the machine-readable information 4. PUBLISHING DESCRIPTIONS within the GRDDL document. Some of the attributes are After the harvesting and validation process, GRDDL doc- defined to appear on XHTML a elements, augmenting the uments are processed using XSLT, which after joining the link with well-defined semantics. Other attributes appear on individual RDF graphs results in a single RDF graph de- XHTML div elements and are used to enclose textual de- scribing all harvested namespace descriptions. This aggre- scriptions. Using these attributes, namespace descriptions gated set of descriptions is published as XML and XHTML. contain all the information that we want to make accessible XHTML is published as heavily crosslinked pages, en- in machine-readable form. abling users to retrieve all the information present in the RDF using a regular browser. Using search features, it is possible to search for specific text in all literal informa- xmlns:... tion, so that access through the XHTML pages is provided xsi:schemaLocation through search-based retrieval as well. xsd:target .html .xml Namespace For users interested in a machine-readable description of .xsd the collected data, the data is published as XML. This XML uses more traditional structures using standard ID/IDREF .html references rather than being based on RDF. Even though GRDDL uses an RDF-based data model, it was decided that an application-specific XML Schema is better suited for representing the namespace descriptions. The reason for this is that RDF is not well-suited for processing it with XPath/XSLT, whereas a suitably designed XML can be pro- This figure shows the complete model of how namespace cessed very simply by users with relatively little XPath or descriptions are used. We assume that the namespace de- XSLT experience. scription is generated, so that the actual GRDDL document located at the namespace URI of the schema is an XHTML 5. CONCLUSIONS document generated by XSLT. The generated XHTML con- Our namespace description approach implements the idea tains information about associated resources (the schema of a light-weight Semantic Web, searching for the middle described, documentation, and transformations), and some ground between the considerable effort necessary to create of these associated resources may be namespace descriptions machine-readable descriptions for many details of an IT en- themselves (versioning and other dependencies). vironment, and the absence of any machine-readable de- 3. HARVESTING DESCRIPTIONS scription in the plain namespace handling defined by the recommendation. As pointed out in Section 2, namespace descriptions are In an effort to implement the light-weight Semantic Web GRDDL documents, which means they are XHTML with as easily, standards-compliant and future-proof as possible, embedded semantic information. This makes automated we employed a variety of Web technologies such as XML, processing of namespace descriptions easy, because only the XML Schema, XSLT, GRDDL, and RDF. Using these tech- semantic information has to be extracted. This task is best nologies and combining them in the most efficient way en- done by XSLT, which means that harvesting namespace de- abled us to implement what we consider to be a gap in the scriptions means collecting GRDDL documents, and then XML landscape of today without too much effort. using XSLT to generate RDF from these documents. As pointed out in Section 2.2, we do not require schema 6. REFERENCES authors to generate their namespace description. They can [1] Tim Bray, Dave Hollander, and Andrew Layman. generate it, in which case the schema and the XSLT for the Namespaces in XML. World Wide Web Consortium, generation will guarantee an error-free namespace descrip- Recommendation REC-xml-names-19990114, January 1999.