PATTERNS IN ONTOLOGY ENGINEERING:CLASSIFICATION OF ONTOLOGY PATTERNS

Eva BlomqvistSchool of Engineering, Jonkoping University

P.O. Box 1026, SE-551 11 Jonkoping, Swedeneva.blomqvist@ing.hj.se

Kurt SandkuhlSchool of Engineering, Jonkoping University

P.O. Box 1026, SE-551 11 Jonkoping, Swedenkurt.sandkuhl@ing.hj.se

Keywords: Ontology Engineering, Ontology Patterns, Pattern Classification.

Abstract: In Software Engineering, patterns are an accepted way to facilitate and support reuse. This paper focuses onpatterns in the field of Ontology Engineering and proposes a classification scheme for ontology patterns. Thescheme divides ontology patterns into five levels: Application Patterns, Architecture Patterns, Design Patterns,Semantic Patterns, and Syntactic Patterns. Semantic and Syntactic Patterns are quite well-researched but thehigher levels of pattern abstraction are so far almost unexplored. To illustrate the possibilities of patterns onthese levels some examples are discussed, together with ideas of future work.

1 INTRODUCTION

Recent developments in the area of Ontology En-gineering involve semi-automatic ontology construc-tion to reduce time and effort of constructing an on-tology. In particular for small-scale application con-texts, reduction of effort and expert involvement is animportant requirement.

Ways of reducing this effort are to further facil-itate semi-automatic construction of ontologies, butalso to introduce reuse in Ontology Engineering. Pat-terns have proved to be a fruitful way to handle theproblem of reuse. In Software Engineering it is al-ready the commonly accepted way to build softwareand there are not only Software Design Patterns, butalso Data Model Patterns, generic Problem SolvingMethods, Object System Models, and many other ap-proaches. Classification of ontology patterns is a firststep towards a structured use of patterns in OntologyEngineering. The main contribution of our research,and focus of this paper, is the classification and cate-gorisation of patterns for Ontology Engineering.

In section 2 the background of the research is pre-sented. Further, in section 3, our approach for classi-fying ontology patterns is described and some exam-ples of the different levels are discussed. In the finalpart, section 4, our research is summarised and someideas for future work are presented.

2 ONTOLOGY PATTERNSTODAY

Ontology is a popular term today, used in many ar-eas and defined in many different ways. In this paperontology is defined as:

An ontology is a hierarchically structured set ofconcepts describing a specific domain of knowledgethat can be used to create a knowledge base. An on-tology contains concepts, a subsumption hierarchy,arbitrary relations between concepts, and axioms. Itmay also contain other constraints and functions.

Even using this definition, ontologies can be usedfor many different purposes, and they can be con-structed and structured in many different ways. Oneof the most common ways to describe the level ofgenerality of an ontology is by using the structure in(Guarino, 1998). The structure shows how a generalTop-level Ontology can be specialised into a DomainOntology or a Task Ontology, which in turn can becombined into an Application Ontology (describing acertain task specialised for a specific domain).

Another categorisation of ontologies can be ob-tained by classifying them by their intended use (vanHeijst et al., 1997). There are three main levels, Ter-minological Ontologies, Information Ontologies andKnowledge Modelling Ontologies, where each leveladds further complexity to the ontology structure. For

example, in a Terminological Ontology often a simpletaxonomy of terms is enough, while in a KnowledgeModelling Ontology more advanced axioms and con-straints are very often required.

The ontology community has not yet adopted thepattern idea on a broader scale. There exist a few pat-terns for ontologies, and all of them are specialised forsome specific kind of ontology (with respect to bothlevel of generality and usage area, as defined above).

For specific ontology languages, patterns are be-ing developed, e.g. the OWL-patterns of (W3C-SWBPD, 2004). Examples of meta-structures for on-tologies, which can be denoted patterns, are Seman-tic Patterns for describing implementation indepen-dent logical constructs (Stuckenschmidt and Euzenat,2001)(Staab et al., 2001). These patterns can for ex-ample describe notions like locally inverse relationsand composition of relations. Similar to Software De-sign Patterns are the Ontology Design Patterns dis-cussed in (Gangemi, 2004), and the Design Patternsdeveloped for ontologies in Molecular Biology (Re-ich, 1999). These patterns are quite general and de-scribe for example how to keep track of updates be-tween different partitions of an ontology.

3 CLASSIFICATION OFONTOLOGY PATTERNS

In order to study the use of ontology patterns in abroader perspective, there is need for a general clas-sification of patterns. In this section such a classifica-tion is presented and discussed.

3.1 Classification

Our classification of ontology patterns uses the fol-lowing levels (an illustration of the levels and theirinterdependencies can be seen in Figure 1):

• Application Patterns - Purpose, scope, usage andcontext of the implemented ontology or ontologies,including interfaces and relations to other systems.

• Architecture Patterns - A description of how tocombine or arrange implemented Design Patternsin order to fulfill the overall goal of the ontology.

• Design Patterns - A small collection of SemanticPatterns that together create a common and genericconstruct for ontology development.

• Semantic Patterns - Language independent descrip-tion of a certain concept, relation or axiom. Ameta-description of a Syntactic Pattern.

• Syntactic Patterns - Language specific ways to ar-range representation symbols, to create a certainconcept, relation or axiom.

To really benefit from ontology patterns theyshould be studied, and used, at all these levels. Asnoted in section 2, there are some ontology pat-tern approaches present today, but these are mostlyconnected to the lower levels, such as SyntacticPatterns (W3C-SWBPD, 2004), Semantic Patterns(Staab et al., 2001) and Design Patterns (Gangemi,2004) (Reich, 1999).

Ontology Application Patterns aim to describegeneric ways to use the implemented ontologies, interms of purpose, context, interfaces etc. This ideaof abstracting the best ways to apply and use an on-tology, or several ontologies, within some context orapplication is an important issue. Here many mod-els and common practices exist, but not formalised aspatterns. A simple generalisation of existing modelscould result in initial Application Patterns for ontolo-gies. The application level patterns will be very muchdependent on the ontology usage area, and may alsobe domain dependent.

Ontology Architecture Patterns aim to describe ageneric way to design the overall structure of an ontol-ogy, in order to fulfill the goal of the ontology in ques-tion. So far, no Architecture Patterns for ontologieshave been presented. Important questions are whetherto divide the ontology into components or modules, orto divide it into layers or use other construction prin-ciples. On the architecture level there could be greatpossibilities to draw from experiences in other ar-eas. Architectures tend to be similar whether it comesto Problem Solving Methods, software or ontologies.Architectures also do not tend to be application or do-main specific, but the patterns created certainly needto be adapted to either manual or automatic use.

Ontology Design Patterns aim to describe ageneric recurring construct in ontologies. When im-plemented, the construct will form a small piece ofthe complete ontology (perhaps a component or mod-ule). There exist patterns that are generic and domainindependent, for manual construction of domain on-tologies. Such patterns can be very useful but there isalso a need for more specific patterns, e.g. domaindependent patterns, both for manual and automaticuse. The main idea of patterns for automatic use isto be specific enough, allowing them to be used auto-matically or semi-automatically, and at the same timegeneric enough to be useful in several ontologies of acertain domain.

A continuous application of all levels of the patternclassification above would require defined patterns forall different kinds of ontologies (both level of general-ity and usage area as defined in section 2). Yet anotheraspect to be taken into account is whether the patternswill be applied manually or automatically. In Ontol-ogy Engineering today, the approaches tend more andmore toward semi-automatic construction, even moreso than in Software Engineering. Automatically ap-

plied patterns will probably play a very important rolein the future. Thus, when exploring and developing afull-covering system of patterns for ontologies, it isimportant to consider both the ontology categorisa-tion, the pattern categorisation and the notion of man-ual or automatic pattern usage.

Application Level

Architecture Level

Design Level

Semantic Level

Syntactic Level

Figure 1: Illustration of the classification levels as acontinuous increase of granularity.

3.2 Discussion of Selected Examples

Our research mainly has a focus on ontology patternsfor automatic use, and specifically for InformationOntologies on the domain or application level withinenterprises.

One can imagine two different approaches whenaddressing this problem. The first one is to take all ex-isting ontologies and derive patterns from them. Theother one is to develop criteria of ”good design” andconstruct patterns that reflect these principles. Unfor-tunately, the second approach is not practically feasi-ble since it is almost impossible to derive criteria ofhow to design ontologies for all situations. The firstapproach is the common way, in for example Soft-ware Engineering, to ”discover” patterns using exist-ing structures. Unfortunately, enterprise ontologiesare so far quite scarce and often sparsely documented,which makes it hard to extract patterns this way.

One ”middle course” is to draw from knowledgealready accumulated in other areas. Many patterns inComputer Science also describe some kind of knowl-edge, although they might have a different purpose.Our approach is to study those patterns in order to de-velop ontology patterns. So far the focus has beento develop an overall picture of the ontology area, bydeveloping the classification structure discussed pre-viously, and to put existing approaches into this cat-egorisation. In the following sections first ideas forpopulating the higher pattern levels are presented.

3.2.1 Ontology Application Patterns

No Application Patterns have so far been formalisedfor ontologies, as noted earlier, but there exist manymodels of ontology usage. The models typically showhow different ontologies interact and depend on eachother in a specific system. As a starting point, thesetypes of models (for example like in (Levashova et al.,2003)) could be abstracted into Ontology ApplicationPatterns.

3.2.2 Ontology Architecture Patterns

At the next level, some Software Architecture Patternsmight be quite easily adopted by the ontology com-munity. There are several approaches to modulari-sation of ontologies (Stuckenschmidt, 2003) (Rector,2003), which in combination with a Software Archi-tecture Pattern, for example similar patterns to thosedeveloped by Shaw (Shaw, 1996), could become anarchitecture for ontologies. An example could bethe Implicit Invocation Architecture Pattern (Shaw,1996), which instead of objects or processes could beused with ontology modules.

3.2.3 Ontology Design Patterns

The third level in focus is the Ontology Design Pat-terns. Here patterns similar to Software Design Pat-terns have already been suggested, so the interestingareas to explore involve more domain and applicationspecific patterns for semi-automatic use. For testingthe suitability of different patterns in Computer Sci-ence to act as templates when constructing OntologyDesign Patterns, some example ontologies were cre-ated. These were evaluated against real-world appli-cations in order to decide which patterns to focus on.Some patterns considered interesting for constructingOntology Design Patterns are Data Model Patterns(Hay, 1996) (Silverston, 2001), Object System Mod-els (Sutcliffe, 2002) and Analysis Patterns (Fowler,1997).

3.2.4 Remaining Issues

So far our focus has been on Ontology Design Pat-terns in practice, and currently an evaluation is beingconducted to see how useful the ontologies suggestedabove can be, and how they might be generalised intoontology patterns. The evaluation has shown that oneproblem is to transfer constraints in the existing pat-terns into axioms of the ontologies in a structuredway. Another problem is the differences in general-ity of the existing patterns. Also, the underlying as-sumptions that the patterns are based on might differbetween the ontology field and the field where the pat-tern originated.

4 SUMMARY AND FUTUREWORK

The main contribution of this research, so far, isthe classification of ontology patterns into five dif-ferent levels. When using these levels it is also im-portant to consider the kind of ontology to be built,its intended usage, and especially how it will be con-structed. The notion of semi-automatic constructionposes new and different requirements on possible pat-terns, they need to be more specific, appropriately for-malised and maybe even domain dependent.

Our work has so far focused on testing patternsfrom other Computer Science areas in order to eval-uate their usefulness when adapted to the ontologyarea. When considering future work, it has beenshown that many areas of the ontology pattern fieldrequire more research efforts, in order to reach a full-covering system of patterns. Our research will con-tinue to focus on the patterns intended for automaticuse since they are the natural next step in further fa-cilitating semi-automatic ontology construction.

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