EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH Database Technology for the Semantic Web Vassilis Christophides Dimitris Plexousakis Computer Science Department, University of Crete Institute for Computer Science - FORTH Heraklion, Crete 1 Chri stophi des V assi l i s EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH On the Semantic Web � Main infrastructure for supporting Community Webs � groups of people sharing a domain of discourse and a set of information resources (e.g., data, documents, services) and having W orkpl ace Educati on some common interests/objectives Semantic � Higher Quality Web Information Web Services � having data and programs described in a way that facilitates Com m erce their reuse and integration by H eal t h machines across applications 2 Chri stophi des V assi l i s
EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH 4 + 1 Webs? � Computers � XHTML � Voice � Voice XML � Semantic � RDF � Wireless � WAP/WML � Television � bHTML � Semantic � RDF 3 Chri stophi des V assi l i s EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH Metadata exists for Almost Anything/Everywhere � Physical Objects, Places, People, � Devices, Networks, Infrastructure, < tag1 > < tag1 > � Digital Documents, Data, < tag2 > < tag2 > < tag3 > < tag3 > </ tag1 > Programs </ tag1 > � User Profiles, Preferences, 4 Chri stophi des V assi l i s
EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH RDF Objectives � Enables communities to define their own semantics of resource descriptions � we can disagree about semantics, but share the same infrastructure (syntax, editors, query languages, databases, etc.) � Imposes structural constraints on the expression of metadata in various application contexts � for consistent encoding, exchange and processing of metadata on the Web � Facilitates development of metadata vocabularies without central coordination � mechanisms for reusing descriptions of resources, concepts, etc. � Focus on DBMS technology for RDF metadata � Related W3C efforts on XML data management 5 Chri stophi des V assi l i s EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH Outline � Database issues for RDF metadata management � The Data Independence Issue � The Query Language Issue � The Model Issue � RDF Query Language: RQL � Querying Large RDF Schemas � Filtering/Navigating Complex RDF descriptions � Storing Voluminous RDF descriptions � Alternative DB representations � Performance Figures � The ICS-FORTH RDFSuite � Conclusions and remaining issues 6 Chri stophi des V assi l i s
EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH The Data Independence Issue � Conceptual Level: Describing resources using one or several RDF schemas � Logical Level: How RDF descriptions and schemas are physically stored � Logical-schema: Data organization using tables, objects, etc. � Physical-schema: Data organization using files, records, indices, etc. � RDF data independence is crucial for ensuring scalability of real-scale Semantic Web applications 7 Chri stophi des V assi l i s EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH The Query Language Issue F i n d r e s o u r c e s d u n d e r … c l a s s i f i e h o s e p r o p e r t y Querying the w l u e i s … . v a Semantics (RQL) Description Graphs t s F i n d s t a t e m e n u b j e c t i s Querying the w h o s e s a n d o b j e c t i s … … Structure (Squish) Triple Database n F i n d d e s c r i p t i o s w h o s e e l e m e n t Querying the t r i b u t e v a l u e a t Syntax c o n t a i n s … . (XQuery) XML Repository 8 Chri stophi des V assi l i s
EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH Why a Data Model for RDF ? � As support for physical/logical independence � RDF can be stored in files, a native repository, a relational database � RDF can be virtual, as a view of a repository, integrated sources � RDF can be in memory, using data structures in C, C++, Java, etc � RDF can be streamed between processes � To describe information content of RDF Statements � to agree and reason about information content, preservation � To define semantics of a data manipulation language: � A query language describes in a declarative fashion, the mapping between an input instance of the data model to an output instance of the data model 9 Chri stophi des V assi l i s EU-NSF Semantic Web Workshop 3-5 Oct ICS-FORTH But RDF has specifics: Serialization syntax Painter Painting r df :t ype r df :t ype created r2 : museoreinasofia.mcu.es/guernica.jpg 1937 paints fname “Pablo” & r2 r3 : w w w . ar t chi ve. com /w om an. j pg created lname & r6 r6 : pi casso132 “Picasso” paints 1904 & r3 � XML attributes vs elements for RDF properties < Pai < r df : nter r D escri df pti : I D = “pi on r df casso132"> : I D = “ pi casso132" f nam e= Pabl o l nam e= Pi casso> < f < pai nam e> Pabl nt s r df : r esour o< / f nam e> ce= "ht t p: / / m useor ei nasof i a. m cu. es/guer ni ca. gi f "/ > � fname, lname < l < pai nam e> Pi nt s r df casso< / : r esour l ce= "ht nam e> t p: / / w w w . ar t chi ve. com / w om an. j pg”/ > � XML flat vs nested structures of RDF statements < pai < r df nts> : type> Pai nt er < / r df : t ype> < / r df : < Pai D escri nti pti ng r on> df : about = "ht t p: / / w w w . ar t chi ve. com / w om an. j pg”/ > � Description vs. Painter elements < r df : D escri < created> 1904< / pt i on r df : about created> = "ht t p: / / m useor ei nasof i a. m cu. es/ guer ni ca. gi f "> < / < r pai df : type> Pai nts> nt i ng< / r df : t ype> � RDF properties are unordered, optional, and multivalued < pai < created> 1937< / nts> created> < / r df < Pai : D escri nti ng r pti on> df : about = "ht t p: / / m useor ei nasof i a. m cu. es/ guer ni ca. gi f "> � 2 paints and 0 creates < r df : D escri < created> 1937< / pt i on r df : about created> = " ht t p: / / w w w . ar t chi ve. com / w om an. j pg"> < r df : < / type> Pai Pai nti ng> nt i ng< / r df : t ype> � One more motivation for a data model : < / < created> 1904< / pai nts> created> < / < / Pai r df nter> : D escri pti on> � isolate the user from syntactic aspects of RDF/XML 10 Chri stophi des V assi l i s
Recommend
More recommend
