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Xml-Based Rdf Query Language (Xrql) and Its Implementation

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Xml-Based Rdf Query Language (Xrql) and Its Implementation XML-BASED RDF QUERY LANGUAGE (XRQL) AND ITS IMPLEMENTATION Norihiro Ishikawa, Takeshi Kato, Hiromitsu Sumino NTT DoCoMo Inc, 3-5 Hikarino-oka, Yokosuka, Kanagawa, Japan Johan Hjelm Ericsson Research, Torshamnsgatan 23, Kista SE-16480, Stockholm, Sweden Kazuhiro Miyatsu Ericsson Research Japan,1-4-14 Koraku, Bunkyo-ku Tokyo, Japan Keywords: Mobile phone, RDF, XML, Query Language, Metadata, Semantic Web Abstract: Resource Description Framework (RDF) is a language which represents information about resources. In order to search RDF resource descriptions, several RDF query languages such as RQL and SquishQL have been proposed. However, these RDF query languages do not use XML syntax and they have limited functionality. xRQL is proposed to solve these issues by defining an XML-based RDF query language with enhanced manipulations of the RDF metadata. xRQL is a logic language relying on a functional approach. It consists of an operator declaration, a RDF data description and a result description. Based on RDF graph data model, xRQL defines a graphical path expression with variables, which is similar to GOQL for describing RDF data. It also adopts the object-oriented model for creation, modification and deletion operations of RDF data. Users can define their favorite XML- compliant result descriptions by themselves, which is similar to XQuery. In addition, a set of RDF operations for RDF schema is defined to manipulate the class and property hierarchies in RDF schema. xRQL has been implemented as an RDF query language over a native RDF database management system. This paper also briefly describes the implementation status. 1 INTRODUCTION RDF and logic language, there are some problems to be solved: The World Wide Web was originally built for human z Non-XML Syntax consumption. It contains such a large volume of Since the existing RDF query languages use SQL- information that it’s impossible to manage manually. like syntax to describe an RDF query, a special The use of metadata to describe the information can parser other than an XML parser is required. achieve automatic processing. RDF (Resource SQL-like syntax has limitations in expressing a Description Framework) (Ora, 1999, Dan, 2002) is a complicated RDF query, compared with XML- W3C recommendation for describing information based syntax. It is also difficult to describe the objects and processing metadata. hierarchical RDF data model with a plain text For querying RDF resource description, several syntax language. RDF query languages (G.Karvounarakis, 2002, Mike, z Limitation of RDF Metadata 2002, L.Miller,2001, Flavius,2002, Joachim,2002, Manipulations Edutella, 2002) have been developed. However, in The existing RDF query languages only support the context of processing within an XML-based RDF metadata search operations. Other functions, distributed database system, and with respect to an such as Creation, Modification and Deletion of 319 Ishikawa N., Kato T., Sumino H., Hjelm J. and Miyatsu K. (2005). XML-BASED RDF QUERY LANGUAGE (XRQL) AND ITS IMPLEMENTATION. In Proceedings of the First International Conference on Web Information Systems and Technologies, pages 319-326 DOI: 10.5220/0001229203190326 Copyright c SciTePress WEBIST 2005 - WEB INTERFACES AND APPLICATIONS RDF metadata should be considered to support z RDF Schema Query: Besides the metadata manipulation operations, as well as the manipulation functions of the distributed similar functions are supported by SQL in a heterogeneous metadata represented in RDF, relational database system. the retrieval function of schema definitions z Lack of Result Format Declaration for the vocabularies used in any given block The existing RDF query languages retrieve the of RDF data is needed. Since schema search result in an unordered and ambiguous way. definitions are defined by using the RDF It takes more time for machine to understand and Schema (RDFS) Specification, which process the retrieval results. It will be effective provide information about relationships and powerful if an expected result format is between classes and properties which might declared in the RDF query language. be used in queries, RDF query language should be able to query RDF schemas which As a solution to the above problems, we have are implicit in any RDF data model. proposed an XML-based RDF query language, z Inference: Because of the hierarchical named xRQL, to manipulate RDF resource vocabularies defined in RDFS specification, description and RDF schemas. It relies on the RDF such as subClassof and subPropertyof, an graph data model that captures the RDF modeling RDF query language should support the primitives and permits the interpretation of operations of inferring “implicit” superimposed resource description. xRQL adopts the relationships between classes and properties. functionality of an XML query language (D. z RDF Metadata Manipulation: In addition Chamberlin, 2001), several object-oriented query to basic search functions, RDF query languages (ODMG,1998, Serge, 1989, Zeki, 1996, language should support the operations of Flavius, 2002) and a graph query language creation, modification and deletion of RDF (Jeonghee, 1999, R.Fikes, 2001). So it’s expected to metadata. These operations, not yet be a standard query language for manipulation of supported by the current query languages, RDF resource description stored in the distributed will be attractive to the clients who want to RDF repositories. construct their own RDF metadata Section 2 summarizes the requirements that an repositories based on given RDF schemas. RDF query language should satisfy. In Section 3, we z Definition of Search Result Format: In depict xRQL design concepts and xRQL syntax in order to achieve the efficient processing of details, and give some typical examples. In Section search results, the capability of defining 4, a native RDF management system that supports search result format should be available in xRQL is briefly introduced. Finally, we summarize RDF query language. That will help the our contributions and show our considerations in the clients to retrieve the search results in their future research in Section 5. favorite formats. 2 REQUIREMENTS OF AN RDF 3 XML-BASED RDF QUERY QUERY LANGUAGE LANGUAGE: xRQL We summarize the general requirements of an RDF xRQL is a logic language relying on a functional query language as follows. approach. It consists of an operator declaration, a z Machine Readability: RDF query language RDF data description and a result description. Based syntax should be human readable and on RDF graph data model, xRQL defines a graphical machine understandable for processing RDF path expression with variables, in the same way as metadata easily and efficiently between the Graphical Object-Oriented Query Language (GOQL) distributed heterogeneous metadata (L.Sheng, 1999) for describing RDF data. It also repositories. adopts the object-oriented model (D.Chamberlin, z Support for RDF Data Model: RDF query 2001) for creation, modification and deletion language should support the RDF data operations of RDF data. Users can define their model (e.g. resources, properties, values), favorite XML-compliant result formats by which is similar to the object-oriented data themselves, in the same way as XQuery (ODMG, model. This means RDF query language 1998). In addition, a set of RDF schema operations should support concepts in the object- is defined to manipulate the class and property oriented data model, such as class hierarchies. hierarchies and inheritance. 320 XML-BASED RDF QUERY LANGUAGE (XRQL) AND ITS IMPLEMENTATION 3.1 Design Concept RDF Data Model We describe the design concepts of xRQL by using RDF Schema the following example of RDF resource for the web content. Figure 1 depicts the contents published on a certain web site. The lower part depicts the RDF Resource descriptions of two persons (r1 and r4) and two content of music and movie on the web (r2 and r5). Figure 2: The layered data model The middle part depicts a schema for contents Then the relations between these three levels are annotation. In this schema, property create is defined ensured by a standard type interpretation. In Figure with domain “person” and range “content”. The 1, the r1 in RDF Resource level has a directed type upper part shows the well-known RDF data model. label pointing to composer in RDF Schema level Properties (e.g. create, managed) serve to represent whose type is rdfs:class of RDF Data Model level. attributes (or characteristics) of resources as well as xRQL is designed based on this 3 layered data relationships between resources. Furthermore, both model. classes and properties include hierarchical semantics. For example, class music (or movie) is a subclass of 3.1.2 RDF Resource Manipulation content while property compose (or produce) is a subproperty of create. According to the data model in section 3.1.1, we rdfs:domain defined four basic operations to manipulate RDF rdfs:class r d rdfs:property resource under the constraint of RDF Schema level. rdfs:range RDF Data Model These operations include search, deletion, creation and modification. string address create managed name person content company The xRQL queries essentially provide the means of string tel compose manipulating RDF resource with the knowledge of string composer music produce technique the given RDF schema. For search operations, we producer movie string RDF Schema proposed a path expression based on RDF graph data washington address compose managed model. In compliance with RDF graph (i.e.
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