· RSAComb ·

Combined approach for Conjunctive Query answering in RSA
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About

This is an improved re-implementation of the combined approach for CQ answering over RSA ontologies described in [1].

Please note that the prototype mentioned in [1] is not available (and the contributors to this repository have never seen it); therefore, this "re-implementation" could be completely different from that prototype (potentially using different tools and programming language).

Preliminaries

In order to use this program you need to have RDFox available in your system, along with a valid license. RDFox is proprietary software and as such we are not able to distribute it along with our code. Please refer to this link to request a free trial.

This software has been developed and tested with RDFox v5.5

Changes introduced

We tried to implement the system as close as possible to the theoretical description provided in [1]. Regardless, we had to deal with the fact that we are using different tools to carry out reasoning tasks and we are probably using a different language to implement the system. The following is a (non exhaustive) summary of fixes (🔧), changes (🔄) and improvements (⚡), we introduced along the way:

• 🔄 RDFox is used instead of DLV as the underlying LP engine.

• ⚡ The system accepts unrestricted OWL ontologies as input and takes care of normalising and approximating the ontology to RSA. At the time of writing, two approximation algorithms are provided, to compute a sound (or complete) set of answer to the input queries, respectively.

• ⚡ The different steps of the combined approach (namely, the canonical model computation and the filtering step) are executed in isolation using different named graphs. This allows us to reuse partial products of the computation and can even be used to parellalise filtering and answering steps.

• 🔧 In Def.4, the definition of built-in predicate notIn is wrong and should reflect the implicit semantics implied by the name, i.e.,

  let [...] notIn be a built-in predicate which holds when the first argument is not an element of the set given as second argument

  This has been fixed by (1) introducing a built-in predicate In (note that instances of In can be computed beforehand since they only depend on the input ontology), and (2) implement notIn as the negation of In using RDFox NaF built-in support.

• 🔄 Top (owl:Thing) axiomatisation is performed introducing rules as follows. Given p predicate (arity n) in the original ontology, the following rule is introduced:

      owl:Thing[?X1], ..., owl:Thing[?Xn] :- p(?X1, ..., ?Xn) .
  

  Note that, by definition, arity can be either 1 or 2.

• 🔄 Equality axiomatisation is performed introducing the following rules:

      rsacomb:congruent[?X, ?X] :- owl:Thing[?X] .
      rsacomb:congruent[?Y, ?X] :- rsacomb:congruent[?X, ?Y] .
      rsacomb:congruent[?X, ?Z] :- rsacomb:congruent[?X, ?Y], rsacomb:congruent[?Y, ?Z] .
  

  defining equivalence as a congruence relation over terms in the ontology. Substitution rules propagate the equivalence to all existing atoms.

• 🔧 In Def. 4, the definition of built-in predicate NI is not consistent with its use in Table 3 and related description in Sec. 4.2. We redefined NI as the set of all constants that are equal to a constant in the original ontology (according to the internal equality predicate rsa:congruent). Note that, in this scenario, there is no need to introduce NI instances as facts in the system; instead we can add a rule to populate the new predicate:

    rsa:NI[?X] :- rsa:congruent[?X, ?Y], rsa:named[?Y] .
  

  where rsa:named is an internal predicate keeping track of all constants in the original ontology.

• ⚡ In Def. 3, regarding the generation of the logic program used for the RSA check, only T5 axioms involving an unsafe role will introduce the internal predicates PE and U.

• ⚡ Both in the canonical model and the filtering program computations, rules without a body are loaded into RDFox as facts.

• ⚡ The cycle function introduced in Def.4 establishing the direction of the unraveling of loops is defined over triples (A,R,B). We are currently limiting the triple only to those appearing in a T5 axiom A ⊑ ∃R.B. Note that this greatly limits the size of cycle for a given triple, and as a consequence limits the number of rules used to compute the canonical model.

Using the software

We assume you followed these steps in order to setup RDFox on your personal machine and in particular you know the path to the JRDFox.jar library that comes with the distribution.

Alternatively, run the following commands from the root of the project to install RDFox locally. Download links for specific versions and operating systems can be found here.

mkdir -p lib && pushd lib
wget https://rdfox-distribution.s3.eu-west-2.amazonaws.com/release/v5.2.1/RDFox-linux-x86_64-5.2.1.zip
unzip RDFox-linux-x86_64-5.2.1.zip
ln -s RDFox-linux-x86_64-5.2.1.zip/lib/JRDFox.jar
popd

Provide RDFox license

The documentation, describes several ways to provide the license to RDFox.

One easy way is to put your license key in a file RDFox.lic in $HOME/.RDFox/, with adequate read permissions for the user executing the program.

Compiling and running the project

The project uses sbt to manage dependences.

To compile the project run the following from the base directory:

sbt compile

The project uses the sbt plugin sbt-assembly to produce a fat jar with all the required dependences. Run the following from the base directory of the project to produce a standalone jar file.

sbt assembly

The output of the command will print the location of the produced jar. Note that the fat jar file distributed with this repository excludes the RDFox as a dependency. Provided that you have the RDFox setup on your machine, you can run the program as follows

java -cp <path/to/JRDFox.jar>:<path/to/fat.jar> uk.ac.ox.cs.rsacomb.RSAComb [<option> ...]

Running tests

To run the suites of tests provided along with the code run

sbt test

This will run all unit tests and functional tests. If you want to limit the scope of the tests and run only a particular suite use

sbt "testOnly <test-class>"

For example, to execute only unit tests concerning the canonical model computation, run

sbt "testOnly uk.ac.ox.cs.rsacomb.CanonicalModelSpec"

or alternatively

sbt "testOnly *CanonicalModelSpec"

To run only functional tests for LUBM, excluding tests tagged as slow (that require more resources), run

sbt "testOnly *functional.LUBM -- -l org.scalatest.tags.Slow"

Debugging

You can set the logging level of RSAComb using the -l | --logger flag (see the help screen for more information). When the logger is set to verbose, RSAComb will generate a set of files that contain the intermediate products of the program execution (these include the set of rules to generate the canonical model for the input ontology and the filtering rules derived from the input query). These files are stored in the working directory, in a new folder named rsacomb-<timestamp>.

You can load these files directly into RDFox to simulate the same environment used by RSAComb, leaving you in a state just before the answer gathering process. We also provide a convenient simulate.rdfox RDFox script that can be used to load all the necessary files in RDFox for you.

Let's suppose you run the following command from the root of the project

java -cp lib/JRDFox.jar:target/scala-2.13/RSAComb-assembly-1.1.0.jar uk.ac.ox.cs.rsacomb.RSAComb -l verbose -o tests/lubm/univ-bench.owl -d tests/lubm/data/lubm1.ttl -q tests/lubm/queries.sparql

This will answers all the queries in tests/lubm/queries.sparql and generate debug information in a new folder in the current working directory (let's say, rsacomb-20211005120845/). You can run the provided RDFox script as follows

path/to/RDFox sandbox <debug-folder> "simulate <query-id>"

where

• debug-folder is the newly generated folder (rsacomb-20211005120845 in this example)
• query-id is the identifier of the query we want to simulate (if we want to simulate query 16 we will pass 16 as an argument). We can pass all to simulate all queries.

This will launch a sandboxed RDFox console, where you will be able to explore a simulation of the datastore used by RSAComb. You can also access the same datastore from the web interface at http://localhost:12110/console/.

References

[1] Feier, Cristina, David Carral, Giorgio Stefanoni, Bernardo Cuenca Grau, and Ian Horrocks. The Combined Approach to Query Answering Beyond the OWL 2 Profiles. In Proceedings of the Twenty-Fourth International Joint Conference on Artificial Intelligence, IJCAI 2015, Buenos Aires, Argentina, July 25-31, 2015, 2971–2977, 2015. http://ijcai.org/Abstract/15/420.

[2] Horridge, Matthew and Bechhofer, Sean. The OWL API: A Java API for OWL Ontologies. Semantic Web Journal 2(1), Special Issue on Semantic Web Tools and Systems, pp. 11-21, 2011.

Acknowledgements

• OWLAPI [2]
• RDFox
• Graph for Scala

Credits

• Federico Igne
• Stefano Germano
• Ian Horrocks (Scientific Supervisor)

From the Knowledge Representation and Reasoning research group in the Department of Computer Science of the University of Oxford.

License

This project is licensed under the Apache License 2.0.