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Creating Custom Rules for Jena

In this post I will show you

  1. how to add your own custom rules to Jena,
  2. how to use rule primitives, and
  3. I will mention some things you may want to keep in mind when using rules.

Add and Activate Rules

We assume we start with the following simple triples:

:Peet :takesCourse :ComputerScience .
:Ruth :teachesCourse :ComputerScience .

for which we add the following rule to a student1.rules file:

[hasStudentRule: 
  (?student :takesCourse ?course) 
  (?lecturer :teachesCourse ?course) 
    -> (?lecturer :hasStudent ?student)] 

The hasStudentRule says that if a student takes a course and that course is presented by some lecturer, then the student is a student of that lecturer. The (?student :takesCourse ?course) (?lecturer :teachesCourse ?course) part of the rule is referred to as the premise or body of the rule and the (?lecturer :hasStudent ?student) part as the conclusion or head of the rule.

To activate the rules in Jena we have to create a model that is capable of doing inferences. In the context of Jena inferencing means that it will re-evaluate the rules against the data, which will cause additional statements to be potentially added to the model. This can be achieved by calling methods that will cause Jena to re-evaluate the model, i.e. calls like InfModel.rebind() and InfModel.validate().

Path path = Paths.get(".").toAbsolutePath().normalize();

// Load RDF data
String data = path.toFile().getAbsolutePath() + 
  "/src/main/resources/data.ttl";
Model model = ModelFactory.createDefaultModel();
model.read(data);
    
// Load rules
String rules = path.toFile().getAbsolutePath() + 
  "/src/main/resources/student.rules";
Reasoner reasoner = new GenericRuleReasoner(Rule.rulesFromURL(rules));

InfModel infModel = ModelFactory.createInfModel(reasoner, model);   
infModel.rebind();

When the hasStudentRule is activated a new statement will be added to the Jena model:

:Ruth :hasStudent :Peet .

Using Rule Primitives

Jena supports a number of builtin rule primitives that are intuitive to understand, i.e.

[pensionerRule: 
  (?person :hasAge ?age) greaterThan(?age, 60) 
    -> (?person a :Pensioner)]

which states that when ?person has an age greater than 60, ?person is considered to be a pensioner. Assuming we have the data

:Peet :hasAge 90 .

the following triple will be added to the Jena model:

:Peet a :Pensioner .

Things to Keep in Mind

There are two main things I think one needs to keep in mind with Jena rules:

  1. The purpose of rules are to manipulate the triples in the Jena model. For the most part it adds triples to the model, but it can also remove triples from the model if primitives like remove and drop are used.
  2. Adding and removing of triples can be achieved through SPARQL queries which can perform better or worse than rules. It is therefore best to check both approaches for your given use case.

Conclusion

In this post I gave a brief introduction in how to use custom rules and builtin rule primitives in Jena. This code is available at github.

Creating, Writing and Reading Jena TDB2 Datasets

Jena TDB2 can be used as an RDF datastore. Note that TDB (version 1 of Jena TDB) and TDB2 are not compatible with each other. TDB2 is per definition transactional (while TDB is not). In this post I give a simple example that

  1. create a new Jena TDB2 dataset,
  2. create a write transaction and write data to the datastore,
  3. create a read transaction and read the data from the datastore, and
  4. release resources associated with the dataset on writing and reading is done.

Create TDB2 Dataset

To create a Jena TDB2 dataset, we use the TDB2Factory. Note that the class name is TDB2Factory and not TDBFactory. We need to specify a directory where our dataset will be created. Multiple datasets cannot be written to the same directory.

Path path = Paths.get(".").toAbsolutePath().normalize();      
String dbDir = path.toFile().getAbsolutePath() + "/db/"; 
Location location = Location.create(dbDir);      
Dataset dataset = TDB2Factory.connectDataset(location); 

Create WRITE Transaction and Write

dataset.begin(ReadWrite.WRITE);
UpdateRequest updateRequest = UpdateFactory.create(
  "INSERT DATA { " 
  + " \"Grace Hopper\" .}");
UpdateProcessor updateProcessor = 
  UpdateExecutionFactory.create(updateRequest, dataset);
updateProcessor.execute();
dataset.commit(); 

Create READ Transaction and Read

dataset.begin(ReadWrite.READ);
QueryExecution qe = QueryExecutionFactory
  .create("SELECT ?s ?p ?o WHERE {?s ?p ?o .}", dataset);
for (ResultSet results = qe.execSelect(); results.hasNext();) {
  QuerySolution qs = results.next();
  String strValue = qs.get("?o").toString();
  logger.trace("value = " + strValue);
}  

Release Dataset Resources and Run Application

The dataset resources can be release calling close() on the dataset.

dataset.close();

Running the application will cause a /db directory to be created in the directory from where you run your application, which consists of the various files that represent your dataset.

Conclusion

In this post I have given a simple example creating a TDB2 dataset and writing to and reading from it. This code can be found on github.

Creating a Remote Repository for GraphDB with RDF4J Programmatically

In my previous post I have detailed how you can create a local Ontotext GraphDB repository using RDF4J. I indicated that there are some problems when creating a local repository. Therefore, in this post I will detail how to create a remote Ontotext GraphDB repository using RDF4J. As with creating a local repository, there are three steps:

  1. Create a configuration file, which is as for local repositories.
  2. Create pom.xml file, which is as for local repositories.
  3. Create the Java code.

The benefit of creating a remote repository is that it will be under the control of the Ontotext GraphDB Workbench. Hence, you will be able to monitor your repository from the Workbench.

Java Code

package org.graphdb.rdf4j.tutorial;

import java.io.FileInputStream;
import java.io.InputStream;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Iterator;

import org.eclipse.rdf4j.model.Model;
import org.eclipse.rdf4j.model.Resource;
import org.eclipse.rdf4j.model.Statement;
import org.eclipse.rdf4j.model.impl.TreeModel;
import org.eclipse.rdf4j.model.util.Models;
import org.eclipse.rdf4j.model.vocabulary.RDF;
import org.eclipse.rdf4j.repository.Repository;
import org.eclipse.rdf4j.repository.RepositoryConnection;
import org.eclipse.rdf4j.repository.config.RepositoryConfig;
import org.eclipse.rdf4j.repository.config.RepositoryConfigSchema;
import org.eclipse.rdf4j.repository.http.config.HTTPRepositoryConfig;
import org.eclipse.rdf4j.repository.manager.RemoteRepositoryManager;
import org.eclipse.rdf4j.repository.manager.RepositoryManager;
import org.eclipse.rdf4j.repository.manager.RepositoryProvider;
import org.eclipse.rdf4j.rio.RDFFormat;
import org.eclipse.rdf4j.rio.RDFParser;
import org.eclipse.rdf4j.rio.Rio;
import org.eclipse.rdf4j.rio.helpers.StatementCollector;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.Marker;
import org.slf4j.MarkerFactory;

public class CreateRemoteRepository {
  private static Logger logger = LoggerFactory.getLogger(CreateRemoteRepository.class);
  // Why This Failure marker
  private static final Marker WTF_MARKER = MarkerFactory.getMarker("WTF");
	
  public static void main(String[] args) {
    try {		
      Path path = Paths.get(".").toAbsolutePath().normalize();
      String strRepositoryConfig = path.toFile().getAbsolutePath() + "/src/main/resources/repo-defaults.ttl";
      String strServerUrl = "http://localhost:7200";
		
      // Instantiate a local repository manager and initialize it
      RepositoryManager repositoryManager  = RepositoryProvider.getRepositoryManager(strServerUrl);
      repositoryManager.initialize();
      repositoryManager.getAllRepositories();

      // Instantiate a repository graph model
      TreeModel graph = new TreeModel();

      // Read repository configuration file
      InputStream config = new FileInputStream(strRepositoryConfig);
      RDFParser rdfParser = Rio.createParser(RDFFormat.TURTLE);
      rdfParser.setRDFHandler(new StatementCollector(graph));
      rdfParser.parse(config, RepositoryConfigSchema.NAMESPACE);
      config.close();

      // Retrieve the repository node as a resource
      Resource repositoryNode =  Models.subject(graph
        .filter(null, RDF.TYPE, RepositoryConfigSchema.REPOSITORY))
        .orElseThrow(() -> new RuntimeException(
            "Oops, no <http://www.openrdf.org/config/repository#> subject found!"));

		
      // Create a repository configuration object and add it to the repositoryManager		
      RepositoryConfig repositoryConfig = RepositoryConfig.create(graph, repositoryNode);
      repositoryManager.addRepositoryConfig(repositoryConfig);

      // Get the repository from repository manager, note the repository id 
      // set in configuration .ttl file
      Repository repository = repositoryManager.getRepository("graphdb-repo");

      // Open a connection to this repository
      RepositoryConnection repositoryConnection = repository.getConnection();

      // ... use the repository

      // Shutdown connection, repository and manager
      repositoryConnection.close();
      repository.shutDown();
      repositoryManager.shutDown();					
   } catch (Throwable t) {
     logger.error(WTF_MARKER, t.getMessage(), t);
   }		
  }
}   

Conclusion

In this post I detailed how you can create remote repository for Ontotext GraphDB using RDF4J, as well as the benefit of creating a remote repository rather than a local repository. You can find the complete code of this example on github.