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D2R MAP -- A Database to RDF Mapping Language

  • June 2003
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Christian Bizer at Universität Mannheim
Christian Bizer
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Abstract

The vision of the Semantic Web is to give data on the web a welldefined meaning by representing it in RDF and linking it to commonly accepted ontologies. Most formatted data today is stored in relational databases. To be able to use this data on the Semantic Web, we need a flexible but easy to use mechanism to map relational data into RDF. The poster presents D2R MAP, a declarative language to describe mappings between relational database schemata and OWL ontologies.
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D2R MAP – A Database to RDF Mapping Language
Christian Bizer
Freie Universität Berlin
Institut für Produktion,
Wirtschaftsinformatik und OR
Garystr. 21, D-14195 Berlin, Germany
+49 30 838 54057
bizer@wiwiss.fu-berlin.de
ABSTRACT
The vision of the Semantic Web is to give data on the web a well-
defined meaning by representing it in RDF and linking it to com-
monly accepted ontologies. Most formatted data today is stored in
relational databases. To be able to use this data on the Semantic
Web, we need a flexible but easy to use mechanism to map rela-
tional data into RDF. The poster presents D2R MAP, a declarative
language to describe mappings between relational database
schemata and OWL ontologies.
Categories and Subject Descriptors
H.2.3 [Database Management]: Languages
General Terms
Standardization, Languages
Keywords
Semantic Web, relational databases, RDF data model, mapping
1. INTRODUCTION
The Semantic Web is an extension of the current Web, in which
data is given a well-defined meaning by representing it in RDF
and linking it to commonly accepted ontologies. This semantic
enrichment allows data to be shared, exchanged or integrated from
different sources and enables applications to use data in different
contexts [1].
Most formatted data today is stored in relational databases. To be
able to use this data in a semantic context, it has to be mapped
into RDF, the data format of the Semantic Web.
The data model behind RDF is a directed labelled graph, which
consists of nodes and labelled directed arcs linking pairs of nodes
[2]. To export data from an RDBMS into RDF, the relational
database model has to be mapped to the graph-based RDF data
model.
2. D2R LANGUAGE FEATURES
D2R MAP is a declarative, XML-based language to describe such
mappings. The main goal of the language design is to allow flexi-
ble mappings of complex relational structures without having to
change the existing database schema. This flexibility is achieved
by employing SQL statements directly in the mapping rules. The
resulting record sets are grouped afterwards and the data is
mapped to the created instances. This approach allows the
handling of binary and higher degree relationships, multivalued
class properties, complex conditions and highly normalized table
structures, where instance data is spread over several tables.
The mapping process performed by the D2R processor has four
logical steps as shown in Figure 1. For each class or group of
similar classes a record set is selected from the database. Second,
the record set is grouped according to the groupBy columns of the
specific ClassMap. Then the class instances are created and
assigned a URI or a blank node identifier. Finally, the instance
properties are created using datatype and object property bridges.
The division between step three and four allows references to
blank nodes within the model and to instances dynamically cre-
ated in the mapping process.
The second goal is to keep D2R MAP as simple as possible. Apart
from elements to describe the database connection and the name-
spaces used, the actual mappings are expressed with just three
elements. For each class or group of similar classes in the onto-
logy a ClassMap element is used. Each ClassMap has an sql
attribute and a groupBy attribute. To create instance URIs, pat-
terns and value substitution tables can be used. The instance prop-
erties are constructed with DataTypePropertyBridge elements for
literal properties, which can be typed using XML datatypes and
xml:lang attributes. Datatype properties can be converted simi-
larly using patterns and value substitution tables. References to
external resources or instances within the model are created with
an ObjectPropertyBridge element. To refer to the instances cre-
ated on the fly, a referredClass together with a referredGroupBy
attribute is used. Multiple values of a single property can be put in
rdf:Bag, rdf:Alt or rdf:Seq containers, using the useContainer
Copyright is held by the author/owner(s).
WWW 2003, May 20-24, 2003, Budapest, Hungary.
ACM xxx.
Figure 1. The D2R mapping process.
Table
Table
Table
Record set
Instance
Property
Property
Property
Grouped record set
Instance
Instance
Instance
Instance
attribute together with a DataTypePropertyBridge or Object-
PropertyBridge element.
3. EXAMPLE
The following example illustrates the use of a D2R MAP to ex-
port data about authors and their publications from a database into
RDF. Because authors usually have more than one publication
and publications can be written by multiple authors, the informa-
tion would typically be stored in three database tables: One for the
authors, one for their publications and a third one for the n:m
relationship between authors and publications. A D2R MAP
transformation of these tables to the classes ex:Author and
ex:Book could look as follows:
<Map>
<DBConnection odbcDSN="bookDB" />
<ProcessorMessage
outputFormat="RDF/XML-ABBREV"/>
<Namespace prefix="ex"
namespace="http://example.org#"/>
<ClassMap type="ex:Book"
sql="SELECT isbn, title FROM books;"
groupBy="isbn"
uriPattern="ex:book@@isbn@@">
<DatatypePropertyBridge
property="ex:title"
column="title" xml:lang="en"/>
</ClassMap>
<ClassMap type="ex:Author"
sql="SELECT authors.aid, name, URL,
isbn FROM authors, bookauthor
WHERE authors.aid = bookauthor.aid;"
groupBy="authors.aid">
<DatatypePropertyBridge
property="ex:fullname"
column="name" />
<ObjectPropertyBridge
property="ex:homepage"
column="URL" />
<ObjectPropertyBridge
property="ex:author_of"
referredClass="ex:Book"
referredGroupBy="isbn"
useContainer="rdf:Bag"/>
</ClassMap>
</Map>
The first three subelements define the database connection, the
desired output format and an example namespace. The first
ClassMap element describes the mapping for the ex:Book class.
The instance URIs are created using a pattern. The xml:lang
attribute is set for the title property.
The second ClassMap describes the creation of ex:Author in-
stances and links the authors to their publications using an rdf:bag
container for the ex:autor_of property. Because the ex:Author
class map contains no URI construction schema, instances are
identified as blank nodes. The following example instance is cre-
ated with the map above:
<ex:Author rdf:nodeID='A465'>
<ex:fullname>Chris Bizer</ex:fullname>
<ex:homepage rdf:resource=
'http://www.bizer.de'/>
<ex:author_of>
<rdf:Bag>
<rdf:li rdf:resource=
'http://example.org#book321230273'/>
<rdf:li rdf:resource=
'http://example.org#book884237273'/>
</rdf:Bag>
</ex:author_of>
</ex:Author>
This example shows only some features of D2R MAP. The com-
plete language specification and further examples are found at
http://www.wiwiss.fu-berlin.de/suhl/bizer/d2rmap/D2Rmap.htm.
A D2R processor prototype is publicly available under GNU
LGPL license. The processor is implemented in Java and is based
on the Jena API [3]. It exports data as RDF, N3, N-TRIPLES and
Jena models. It is compliant with all relational databases offering
JDBC or ODBC access. The processor can be used in a servlet
environment to dynamically publish XHTML pages containing
RDF, as a database connector in applications working with Jena
models or as a command line tool.
4. RELATED WORK
Other mapping approaches which have influenced the design of
D2R MAP, are developed by the AIFB Institute, University of
Karlsruhe, Germany [4] and by Boeing, Philadelphia, USA [5]. It
is planned to extend D2R MAP with conditional mappings and
more sophisticated value transformation abilities. These exten-
sions could be based on RuleML [6], RDFT [7] or further lan-
guage constructs borrowed from XSLT.
REFERENCES
[1]
James Hendler, Tim Berners-Lee, Eric Miller. Integrating
Applications on the Semantic Web. Journal of the Institute of
Electrical Engineers of Japan, Vol 122(10), October 2002,
p.676-680.
[2]
Graham Klyne, Jeremy Carroll (eds.). Resource Description
Framework (RDF): Concepts and Abstract Syntax. W3C
Working Draft (work in progress). November 2002,
http://www.w3.org/TR/2002/WD-rdf-concepts-20021108/.
[3]
Brian McBride. Jena: Implementing the RDF Model and
Syntax Specification. Technical report, Hewlett Packard
Laboratories (Bristol, 2000). http://www.hpl.hp.com/
semweb/index.html.
[4]
Nenad Stojanovic, Ljiljana Stojanovic, Raphael Volz. A
reverse engineering approach for migrating data-intensive
web sites to the Semantic Web. IIP-2002 (Montreal, 2002).
http://www.aifb.uni-karlsruhe.de/WBS/nst/docs/papers/
IIPv31finalv1.pdf.
[5]
Tom Barrett et al. RDF Representation of Metadata for
Semantic Integration of Corporate Information Resources.
WWW2002
(Hawaii, 2002).
http://www.cs.rutgers.edu/
~shklar/www11/ final_submissions/paper3.pdf.
[6]
RuleML. http://www.dfki.uni-kl.de/ruleml/.
[7]
Borys Omelayenko. RDFT: A Mapping Meta-Ontology for
Business Integration. ECAI-2002 (
Lyon, 2002)
http://www.cs.vu.nl/~borys/papers/rdft4ktsw02.pdf.
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