KRAAL

KRAAL (Knowledge and Reasoning Awesome Algorithms with Limits) is a set of Graql scripts and ontologies aiming at transforming Grakn TypeDB into a RDF triple store with integrated RDFS (and in the future OWL) reasoner.

In addition, some "glue" code to import RDF into Grakn is provided.

How does it work?

Grakn does not support RDF stack natively. It comes with its own query language (Graql); in addition, it is a strongly typed knowledge graph that needs a schema definition before data can be inserted.

The idea around KRAAL is quite simple. The provided Graql scripts define the below concepts (among others used for support):

1. rdf-node which represents a node in a RDF graph. This is an abstract string attribute that has some sub-concepts:

   1. rdf-non-literal An abstract string attribute that has some concrete sub-concepts:
      1. rdf-IRI a IRI.
      2. rdf-blank-node a blank node, notice each blank node has a unique identifier as its value.
   2. rdf-literal a literal. Literals have an optional data type and language tag attached.

2. rdf-triple is a ternary relationship representing an RDF triple.

In addition, scripts create rules to represent semantic of predicates in a way that triples are inferred and materialized accordingly to existing facts.

Lastly, some Java code is provided to import RDF triples from various file format into Grakn, using the above concepts.

In the end, once the above schema and rules are imported, Grakn can be used as an RDF triple store (with triples stored as rdf-triple relations) with an integrated reasoner (provided by the semantic rules).

Prerequisites

KRAAL has been developed and tested with Grakn Core version 2.0.2 on Windows 10 (but nothing should be specific to Windows).

Java

Grakn 2.0.2 needs Java 11 or higher to run. The Java code in this repository is also targeted to Java 11.

Please make sure you have downloaded and installed Java 11 on your machine. Both Oracle Java and OpenJDK will work.

The grakn.bat file in Grakn distribution below, calls Java to start the server. If your default JRE is not compatible with Grakn, you can edit this file to point to the correct JRE.

Grakn

1. Grakn Core (which comes with an integrated text-based console) can be downloaded and installed from the Grakn website.

2. Grakn Workbase is a graphical UI that can be useful in querying Grakn. You can download it as an executable file and simply executing it with double clicking.

Please see the quickstart on the Grakn website to familiarize a bit with Grakn and its console.

RdfImporter

This is a small Java utility to import RDF files into Grakn. It is provided as a RdfImporter.jar under the Graql folder. Notice that, in order to use the tool, you must have created the required RDF support (that is schema and rules) as explained in next section below.

java -jar RdfImporter.jar -k <arg> -f <arg> [-u <arg>] [-s <arg>] file1 [file2] ...

	-f <arg>   Format of input file.
	-k <arg>   Database to use for importing.
	-s <arg>   "Batch" size; perform this many insertions before committing a
				transaction. Higher values might speed up execution, as long
				as you have enough memory.
	-u <arg>   Base IRI to use for nodes without a base.

	Recognized file formats:
	

		RDF:	RDF/XML
		NT:	N-Triples file format (.nt)
		TTL:	Turtle file format (.ttl)
		TTLS:	Turtle* (TurtleStar) file format (.ttls)
		N3:	N3/Notation3 file format (.n3)
		TRIX:	TriX
		TRIG:	TriG file format (.trig)
		TRIGS:	TriG* (TriGStar) file format (.trigs)
		BRF:	A binary RDF format (.brf)
		NQ:	N-Quads file format (.nq)
		JSONLD:	JSON-LD file format (.jsonld)
		RJ:	RDF/JSON file format (.rj)
		RDFA:	RDFa file format (.xhtml)
		HDT:	HDT file format (.hdt)

Graql scripts

The folder Graql contains Graql script needed to implement required concepts and rules. In addition, it contains RDF files with vocabularies.

RDF and RDF Schema support

The file rdf.gql is a Graql schema that adds support for RDF. In addition, it adds some rules so to implement RDF Schema 1.1 semantics.

1. To import the schema you can use Grakn console. the below shows how to create a blank database named rdfand import the schema there. Please refer to Grakn console documentation for details.

   Welcome to Grakn Console. You are now in Grakn Wonderland!
   Copyright (C) 2021 Grakn Labs
   
   > database create rdf
   Database 'rdf' created
   > transaction rdf schema write
   rdf::schema::write> source rdf.gql
   Concepts have been defined
   rdf::schema::write> commit
   Transaction changes committed
   

2. Then you must import RDF and RDF Schema vocabularies into the rdf database using RdfImporter import utility as shown below. In this example we assume .jar and vocabularies are in the same folder from where you run the command.

   java -jar RdfImporter.jar -k rdf -f TTL 22-rdf-syntax-ns.ttl rdf-schema.ttl

Graql Editor

To create Graql files (schema and rules) we suggest using Notepad++: The file Notepad++/graql.xml is a language definition that adds syntax coloring for Graql to Notepad++.

How to release

Export RdfImporter from Eclipse as executable .jar file inside Graql folder. The main class is io.github.mzattera.semanticweb.kraal.RdfImporter.