Got bored by the relational database?Switch to a RDF store!

Fabrizio GiudiciTidalwave s.a.s.

Who I am

● Java consultant since 1996● Senior architect● Java instructor for Sun since 1998● Member of the NetBeans Dream Team● Technical Writer, Blogger at Java.Net, DZone● http://weblogs.java.net/blog/fabriziogiudici/● http://www.tidalwave.it/people

Where I am using RDF stores

http://bluemarine.tidalwave.it

Agenda

● Why the RDBMs?● RDBMs issues● The Semantic Model● OpenSesame and Elmo● A few code samples● Conclusion

RDBMs are everywhere

What do we expect from a RDBM?

● Persistence● Reliability● Transactions● Integrability● Manageability

Lack of cohesion

● Do we really need a RDBM for those things?● No, we don't

– Persistence and transactions are good

– The specific relational schema is evil

● RDBMs sell those stuff in a single package

ER-OO impedance

● Entity-Relationship is different than OO– Primary keys

– No inheritance

– No behaviour

– Normalization rules

– Relationship through external keys

ORMs

● Tools to minimize the ER-OO impedance● Java has got a standard API: JPA

– Hibernate, TopLink, EclipseLink, OpenJPA

– Tries to abstract the database à la Java

● Good, but the RDBM has still to be designed● And maintained

Can we get rid of the relational database?

The Semantic Model

● Semantic Technology != Semantic Web● RDF: Resource Description Framework● “Triples” are the atomic information item● Subject / predicate / object

– Java / is-a / programming-language

– Fabrizio / is-member-of / NetBeans Dream Team

– Verona / is-part-of / Veneto

– Verona / has-plate / “VR”

The Semantic Model

● The subject is a resource ● The predicate is a property● The object is a value● A value is a resource or a primitive type● Resources, properties identified by URL/URN

– Just a naming scheme

– Not necessarily web-related

Formal representation

● RDF is not related to XML● XML is just one of the way to represent RDF

– XML-RDF, unfortunately referred to as RDF

● Notation 3 (N3), another popular representation– Much more human-readable

● Other formats exist● RDF representation is often referred to as

“serialization”

(XML-)RDF is near to you

● RSS/RDF● Dublin Core● XMP by Adobe

Compared to RDBMs

● There's no fixed schema– Everything is a triple

– “AAA slogan”: Anyone can say Anything about Any topic

● Adding new data types is adding triples– No need to add / alter tables

– Maintainance is just updating data

● Databases can be distributed (federations)– Can be merged by just copying triples together

What about performance?

● Not as optimized as SQL● There's no spread knowledge about tuning as

for SQL● Some missing parts

– E.g. Sesame still misses select count(*)

OpenSesame, Elmo

● Popular Java infrastructure for RDF– FLOSS

– http://www.openrdf.org

● Elmo providers JPA-like operations– Annotations

– Specific API or even subset of JPA

A simple code example

● Note the use of standard ontologies

import org.openrdf.elmo.annotations.rdf;

@rdf(GeoVocabulary.URI_GEO_LOCATION)public class GeoLocation { @rdf("http://www.w3.org/2003/01/geo/wgs84_pos#lat") private Double latitude;

@rdf("http://www.w3.org/2003/01/geo/wgs84_pos#long") private Double longitude;

@rdf("http://www.w3.org/2003/01/geo/wgs84_pos#alt") private Double altitude;

@rdf("http://www.tidalwave.it/rdf/geo/2009/02/22#code") private String code; }

A simple code example

● Declare persistent classes inMETA-INF/org.openrdf.elmo.concepts

● Choose a store– Memory

– Memory backed by file

– Database (transactional)

Repository repository = new SailRepository( new MemoryStore(new File("/tmp/RDFStore"))); repository.initialize(); ElmoModule module = new ElmoModule(); SesameManagerFactory factory = new SesameManagerFactory(module, repository); SesameManager em = factory.createElmoManager();

Use as JPA EntityManager

em.getTransaction().begin(); GeoLocation genova = new GeoLocation(); genova.setLatitude(45.0); genova.setLongitude(9.0); genova.setCode("GE");

em.persist(genova); em.getTransaction().commit();

Queries

● There are specific query languages● SPARQL is one of the most popular● Similar to SQL, but triples in place of tables

PREFIX wgs84: <http://www.w3.org/2003/01/geo/wgs84_pos#>SELECT ?location WHERE { ?location wgs84:lat ?lat }

Running a query

em.getTransaction().begin(); String queryString = "PREFIX wgs84: <http://www.w3.org/2003/01/geo/wgs84_pos#>\n" + "SELECT ?location WHERE \n" + " {\n" + " ?location a ?type.\n" + " ?location wgs84:lat ?lat\n" + " }"; final ElmoQuery query = em.createQuery(queryString). setType("type", GeoLocation.class). setParameter("lat", 45.0);

final List<GeoLocation> result = query.getResultList();

for (GeoLocation l : result) { System.err.println(l); } em.getTransaction().commit();

Scratching the surface

● Elmo is powerful● Supports advanced constructs

– Objects with “multiple personality”

– Mixins

Open issues

● OpenSesame doesn't support all databases● Lack of experience

– Programming skills

– Maintainance

– Tuning

– Managerial culture

● Not widespread● Performance?

Conclusion

● RDBMs are mainstream, but old● They lead to rigid schemata, don't fit the OO● It's possible to use something different● RDF stores can be a viable alternative

Fabrizio.Giudici@tidalwave.it