DOOR

Towards a Formalization of Ontology Relations

∗

Carlo Allocca, Mathieu d’Aquin and Enrico Motta

Knowledge Media Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K.

Keywords:

Ontology Relations, Networked Ontology.

Abstract:

In this paper, we describe our ongoing effort in describing and formalizing semantic relations that link ontolo-

gies with each other on the Semantic Web in order to create an ontology, DOOR, to represent, manipulate and

reason upon these relations. DOOR is a Descriptive Ontology of Ontology Relations which intends to deﬁne

relations such as inclusion, versioning, similarity and agreement using ontological primitives as well as rules.

Here, we provide a detailed description of the methodology used to design the DOOR ontology, as well as

an overview of its content. We also describe how DOOR is used in a complete framework (called KANNEL)

for detecting and managing semantic relations between ontologies in large ontology repositories. Applied in

the context of a large collection of automatically crawled ontologies, DOOR and KANNEL provide a starting

point for analyzing the underlying structure of the network of ontologies that is the Semantic Web.

1 INTRODUCTION

Ontologies are the pillars of the Semantic Web (SW)

and, as more and more ontologies are made avail-

able online, the SW is quickly taking shape. As a

result, the research community is becoming more and

more aware that ontologies are not isolated artifacts:

they are, explicitly or implicitly, related with each

other (Kleshchev and Artemjeva, 2005). Indeed, a

number of studies have intended to tackle some of

the challenges raised by these ontology relationships,

from both theoretical and practical points of view.

At a theoretical level, studies have targeted ontol-

ogy comparison in order to identify overlaps between

ontologies (Maedche and Staab, 2002). Approaches

have been proposed to ﬁnd differences between ver-

sions of an ontologies (Noy and Musen, 2002; Konev

et al., 2008).

According to (Klein and Fensel, 2001), the

ontology versioning problem has been deﬁned as the

ability to handle changes in ontologies by creating

and managing different variants of it. In other words,

ontology versioning means that there are multiple

variants of an ontology around. The authors of (Klein

and Fensel, 2001) suggested that, ideally, developers

should maintain not only the different versions of

∗

This work was funded by the EC IST-FF6-027595

NeOn Project.

an ontology, but also some information about the

way versions differ and whether or not they are

compatible with each other. In (Gangemi et al., 1999)

ontology integration is deﬁned as the construction of

an ontology C that formally speciﬁes the union of the

vocabularies of two other ontologies A and B. The

most interesting case is when A and B commit to the

conceptualization of the same domain of interest or

of two overlapping domains. In particular, A and B

may be related by being alternative ontologies, truly

overlapping ontologies, equivalent ontologies with

vocabulary mismatches, overlapping ontologies with

disjoint domain, homonymically overlapping ontolo-

gies. Finally, in ontology matching, an alignment is

a set of correspondences between the entities of two

ontologies, therefore relating these two ontologies by

mapping there models with each other.

At a practical level, Semantic Web Applications use

the SW as a large-scale knowledge source (d’Aquin

et al., 2008): they achieve their tasks by automatically

retrieving and exploiting knowledge from the SW

as a whole, using advanced Semantic Web Search

Engines (SWSEs) such as WATSON (d’Aquin

et al., 2007). These SWSEs provide keyword based

search mechanisms to locate relevant ontologies

for particular applications. As an example, the

query “student” currently gives 1079 ontologies as

Allocca C., d’Aquin M. and Motta E. (2009).

DOOR - Towards a Formalization of Ontology Relations.

In Proceedings of the International Conference on Knowledge Engineering and Ontology Development, pages 13-20

DOI: 10.5220/0002276400130020

 SciTePress

a result in WATSON

(valid on the 22/04/2009).

However, these results are provided as a simple

list without making explicit the underlying rela-

tions that link ontologies with each other. Indeed,

on the ﬁrst page, at least 2 of the ontologies

(http://www.vistology.com/ont/tests/student1.owl

and http://www.vistology.com/ont/tests/student2.owl)

represent, apart from their URIs and the base

namespaces, exactly the same logical model,

expressed in the same ontology language. An-

other common situation is when an ontology

has been translated in different ontology lan-

guages. This is the case in the ﬁrst and second

results of the query “student, university, re-

searcher”(http://reliant.teknowledge.com/DAML/Mid-

level-ontology.owl and http://reliant.

teknowledge.com/DAML/Mid-level-ontology.daml).

These two ontologies are obviously two different

encodings of the same model. Inspecting the results

of WATSON in the same way, it is not hard to ﬁnd

ontologies connected with other, more sophisticated

semantic relations: versioning, inclusion, similarity,

etc. Leaving implicit these relations in SWSE’s

ontology repositories generates additional difﬁculties

in exploiting their results, expecting the users and the

applications to ﬁnd the “right” or “best” ontology to

achieve their goal.

Both the theoretical and practical challenges con-

cerning relations between ontologies indicate a need

for a general study of these relations, providing a for-

mal base for deﬁning, manipulating and reasoning

upon the links that relate ontologies online, explic-

itly or implicitly. Here, we chose to take an ontolog-

ical approach to this problem. We design DOOR, a

Descriptive Ontology of Ontology Relations that de-

ﬁnes ontology relations using ontological primitives

and rules. Apart from the ontology itself, the main

contributions of this work concern the realization of a

methodology to identify and deﬁne relations between

ontologies, as well as the development of a complete

system based on DOOR (KANNEL), providing ser-

vices for detecting relations, populating DOOR, and

formally querying detected and inferred relations in a

large ontology repository.

This paper is structured as follows: in Section 2

we continue discussing signiﬁcant work concerning

ontology relations; Section 3 presents the adopted

methodology for designing DOOR; Section 4 de-

scribes the DOOR ontology; In Section 5 we brieﬂy

describe KANNEL and the main role of DOOR in this

framework. Finally, Section 6 concludes the paper

and sheds the light on interesting future research on

ontology relations.

http://watson.kmi.open.ac.uk

2 RELATED WORK

J. Heﬂin (Heﬂin and Pan., 2004) was the ﬁrst to stud-

ied formally some of the different types of links be-

tween ontologies, focusing on the crucial problems of

versioning and evolution. However, currently, there is

no ontology management system that implements his

framework. The authors of (Kleshchev and Artem-

jeva, 2005) characterized, at a very abstract level,

a number of relations between ontologies such as

sameConceptualization, Resemblance, Simpliﬁcation

and Composition, without providing formal deﬁni-

tions for them, and without considering the links be-

tween these relationships. Several approaches have

been focusing on how to compare two different ver-

sions of ontologies in order to ﬁnd the differences.

In particular, PROMTDIF (Noy and Musen, 2002)

compares the structure of ontologies and OWLD-

iff (http://semanticweb.org/wiki/OWLDiff) computes

the differences by entailment, checking the two set

of axioms. SemVersion (Volkel, 2006) compares two

ontologies and computes the differences at both the

structural and the semantic levels. In addition, many

measures exist to compute particular forms of simi-

larity between ontologies (David and Euzenat, 2008).

All these studies discuss particular relations sepa-

rately and are generally based on an abstract, informal

deﬁnition of the relations they consider. A complete

model is necessary to provide a wide overview of ex-

isting ontology relations, to clearly establish what are

their deﬁnitions, formal properties, and how they are

connected with each other.

3 METHODOLOGY FOR THE

DOOR ONTOLOGY

Building an ontology of relationships between ontolo-

gies is a very ambitious task. It requires a deep analy-

sis of the ontologies available online and of the liter-

ature, at different levels. Therefore, a reasonably rig-

orous but nonetheless ﬂexible methodology is needed

to identify, describe and formalize ontology relations

and their connections, in order to build the DOOR on-

tology. Here, after deﬁning some important elements

that will be used in the rest of the paper, we present

the steps involved in the methodology we adopted and

brieﬂy detail each of them.

3.1 Deﬁnitions and Requirements

We consider the following deﬁnitions:

Deﬁnition 1 (Ontology). An ontology is a set of ax-

ioms (in the sense of the description logic) over a Vo-

KEOD 2009 - International Conference on Knowledge Engineering and Ontology Development

cabulary VOC, where VOC is the set of the primitive

terms (named entities) employed in the axioms of the

ontology;

Deﬁnition 2 (Ontology Space). An ontology space

OS, is a collection of ontologies.

Deﬁnition 3 (Ontology Relation). Given an ontology

space OS, an Ontology Relation is any binary relation

deﬁned over OS.

At the most general level, the design of the DOOR

ontology was based on three main sources to identify

relevant ontology relations:

1. We analyzed the results of SWSEs (e.g., WAT-

SON) to manually identify existing, implicit re-

lations between ontologies in these results.

2. We considered relations described in the litera-

ture, such as the ones already mentioned in the

previous sections.

3. We also included existing, explicit relations that

are primitives of the OWL ontology language.

Also, ontology relations in the DOOR ontology

should reﬂect the following important features:

• they are general enough to be applied to multiple

domains;

• they are sufﬁciently intuitive to reﬂect general

meaning;

• they are formally deﬁned to be processed auto-

matically by inference engines;

3.2 Main Steps of the Methodology

To design DOOR, we considered the methodology de-

scribed in (Gangemi et al., 2001) for selecting general

ontological categories and adapted it to the problem

of ontology relations. As a result, we divided our ap-

proach into a number of steps, as follows:

1. Identifying the top level relations between ontolo-

gies, considering our three sources (SWSEs, liter-

ature and existing OWL primitives). At this stage,

the task only consists in coming up with a list of

relations that should be relevant, giving us a gen-

eral overview of the different sections of the on-

tology. Relations such as inclusion, similarity,

incompatibility and previous version are identi-

ﬁed here.

2. Specifying the identiﬁed relations, identifying rel-

evant variants and sub-relations. Here, our three

sources of relations are also employed to derive

relations at a lower level. We also use a more sys-

tematic approach, which consists in looking at on-

tologies (and so ontology relations) from 5 differ-

ent dimensions that can characterize them:

• The Lexicographic level, which concerns the

comparison of the vocabularies of the ontolo-

gies.

• The Syntactic level, which concerns the com-

parison of the sets of axioms that form the on-

tologies.

• The Structural level, which concerns the com-

parison of the graph structure formed by the ax-

ioms of the ontologies.

• The Semantic level, which concerns the com-

parison of the formal models of the ontolo-

gies, looking in particular at their logical con-

sequences.

• The Temporal level, which concerns the anal-

ysis of the evolution of ontologies in time.

For example, considering the relation of inclusion

identiﬁed in the ﬁrst step and that led to a

property includedIn in the ontology, we can

specify this relation according to three dif-

ferent dimensions (syntactic, structural and

semantic), leading to three variants of inclusion

between ontologies (syntacticallyIncludedIn,

isHomomorphicTo and semanticallyIncludedIn)

that consider the set of axioms, the graph and

the formal models of the ontologies respectivly.

In addition, besides the systematic analysis of

this relation according to the dimensions, we

include in DOOR particular forms of inclusions

derived from existing OWL primitives (e.g.,

OWL imports) and from the literature (e.g,

isAConservativeExtensionO f (Ghilardi et al.,

2006)). More details about these relations are

given in the next section.

3. Characterizing each relation by its algebraic prop-

erties. For example, the algebraic properties for

similarity are that it is re f lexive and symmetric.

For inclusion, we can deﬁne that it is re f lexive

and transitive. Including such information in the

ontology corresponds to what (Gangemi et al.,

2001) calls deﬁning the ground axioms.

4. Establishing connections between relations. The

results obtained from the previous steps are

mainly top-level relations with a list of variants,

each of them being given algebraic properties.

Here, we want to structure these lists, in partic-

ular by giving them taxonomic relations. As an

example, it can be easily established that syn-

tacticallySimilarTo is a sub property of semanti-

callySimilarTo. In the same way, we can indicate

that a previous version of an ontology ought to be

similar to it. This corresponds to deﬁning non-

ground axioms in (Gangemi et al., 2001).

DOOR - Towards a Formalization of Ontology Relations

ontologyRelatedTo

agreesWith disagreesWith includedIn isAlignedTo isTheSchemaFor

similarTo

Figure 1: Top Level of DOOR.

5. Introducing rules to deﬁne complex relations from

atomic ones. For example, the equivalentTo

property can be deﬁned as equivalentTo(X

, X

):-

includedIn(X

, X

), includedIn(X

, X

Like in any methodology, the application of these

steps should be ﬂexible and continuous. Getting back

to a previous step is sometimes necessary and, as the

building of an ontology such as DOOR is a constantly

ongoing effort, it should be possible to re-apply the

methodology entirely to make the ontology evolve.

The intended result is an ontology made, on the

one hand, of an ontologically deﬁned and taxonomi-

cally structured set of relations, and on the other hand,

of a set of rules to deﬁne complex relations. In the

following we give a detailed overview of the ﬁrst ver-

sion of the DOOR ontology, considering only the ﬁrst

(ontological) part of it, as, due to its complexity, the

deﬁnition of rules governing complex relations is still

a work in progress and would not ﬁt in this paper.

4 FORMAL DESCRIPTION

OF DOOR

The OWL version of the DOOR

ontology can be downloaded at:

http://kannel.kmi.open.ac.uk/ontology. We start

with describing the ﬁrst level of DOOR, in Figure

2. The main relevant abstract relations are simply

represented as sub-properties of ontologyRelatedTo.

An ontology X is ontologyRelatedTo another one Y

if one of the top level relations is satisﬁed between

X and Y. The top level relations include includedIn,

similarTo, isAlignedTo, disagreesWith, agreesWith

and isTheSchemaFor. We clustered them in four

groups and each group will be explained in more

details in the next sub-sections.

4.1 includedIn and equivalentTo

includedIn and equivalentTo are two of the main on-

tology relations. The former represents the meaning

of “an ontology contains an another one”. The lat-

ter intends to convey the meaning of “two ontolo-

gies express the same knowledge”. According to our

methodology, these two relations have been analyzed

at different levels, giving origin to different kinds of

inclusion and equivalence relations. In Table 1, we

summarize the result of these analyses:

Table 1: Specialization of inclusion and equivalence rela-

tions.

includedIn equivalentTo

Semantic semanticallyIncludedIn semanticallyEquivalentTo

isAConservativeExtentionOf

Structural isHomomorphicTo isIsomorphicTo

Syntactic syntacticallyIncludedIn syntacticallyEquivalentTo

import

In particular, the sub-relations of includedIn are de-

ﬁned as follows:

syntacticallyIncludedIn(X

, X

) if the set of axioms

of X

is contained in the set of axioms of X

,which

means X

⊆ X

isHomomorphicTo(X

, X

) if a homomorphism ex-

ists between the RDF-graph of X

and the RDF-

graph of the X

semanticallyIncludedIn(X

, X

) if the set of models

of X

is contained in the set of models of X

. In

other words, if X

|= X

isAConservativeExtentionOf(X

, X

) , informally, if

syntacticallyIncludedIn(X

, X

) and all the ax-

ioms entailed by X

over the vocabulary of X

are

also entailed by X

. A more formal deﬁnition can

be found in (Ghilardi et al., 2006). The notion of

conservative extension has been used in particular

for ontology modularization (Grau et al., 2007).

import(X

, X

) if there is an explicit statement in X

indicating that it imports X

using the owl:imports

primitive. Formally, this means that all the axioms

of X

should be considered as contained in X

The sub-relations of equivalentTo are deﬁned as fol-

lows:

syntacticallyEquivalentTo(X

, X

) if and only if

SyntacticallyIncludedIn(X

, X

) and Syntac-

ticallyIncludedIn(X

, X

isIsomorphicTo(X

, X

) if an isomorphism exists be-

tween the graph of X

and the graph of X

semanticallyEquivalentTo if and only if

semanticallyIncludedIn(X

, X

) and

semanticallyIncludedIn(X

, X

Finally, following our methodology, we deﬁned the

algebraic properties of each relation

and classiﬁed

Since these are fairly obvious, we do not detail then.

KEOD 2009 - International Conference on Knowledge Engineering and Ontology Development

syntacticallyEquivalentTo

equivalentTo

isHomomorphicTosemanticallyIncludedIn

includedIn

isIsomorphicTo

semanticallyEquivalentTo

imports

isAConservativeExtensionOf

isIsomorphicTo

syntacticallyIncludedIn

Figure 2: Taxonomy for includedIn and equivalentTo.

them to create a taxonomic structure relating these re-

lations. This structure is showed in Figure 2

4.2 similarTo

Ontology similarity has been described as a measure

to assess how close two ontologies are (David and

Euzenat, 2008). Various ways to compute the sim-

ilarity between two ontologies have been described

which are relevant in different application contexts. In

our work, similarTo is used to represent the meaning

of “how an ontology overlap/cover parts of the same

area as interest of another ontology”. Following our

methodology, similarTo has been analyzed and for-

malized at the lexicographic, structural and semantic

level, giving origin to different kinds of similarity re-

lations (see Table 2).

Table 2: Specialization of the similarity relation.

SimilarTo

Semantic semanticallySimilarTo

MappingSimilarTo

Syntactic syntatticallySimilarTo

Lexicographic LexicographicSimilarTo

To deﬁne these relations, we need to introduce the

following elements: given two ontologies X

and X

we denote by LC(X

, X

) the set of axioms of X

that

are logical consequences of X

and by Voc(X

) the vo-

cabulary of X

. The following deﬁnitions depend on

a threshold T > 0.

semanticallySimilarTo(X

, X

), if

|LC(X

, X

)

LC(X

, X

max(|X

|, |X

≥ T

The arrows represent the subPropertyOf relation. For

example, syntacticallyEquivalentTo is a sub property of se-

manticallyIncludedIn.

owlIncompatibleWith

explanationEvolution

syntacticallyEquivalentTo

equivalentTo

isLatterVersionOf isPreviousVersionOf

similarTo

isIsomorphicTo

sematicallyEquivalentTo

syntacticallySimilarTopriorVersion

backwardCompatibleWith conceptualEvolutionOf explanationEvolution

lexicallySimilarTo mappingS imilarTo semanticallySimilarTo

Figure 3: Taxonomy for similarTo. Dashed elements repre-

sent elements from other sections of the ontology.

syntacticallySimilarTo(X

, X

), if

max(|X

|, |X

≥ T

lexicographicallySimilarTo(X

, X

), if

|Voc(X

)

Voc(X

max(|Voc(X

)|, |Voc(X

)|)

≥ T

Finally, in addition to the relations deﬁned above,

we also consider a similarity relation that relies on the

existence of an alignment between the two ontologies.

Indeed, mappingSimilarTo is a relation that links

two ontologies X

and X

if there exists an alignment

from X

to X

and this alignment covers a substantial

part of the vocabulary of X

(i.e., a proportion greater

than a threshold T ). Not that, since alignments can

be unidirectional, mappingSimilarTo differs from

the other similarity functions by not being symmetric.

Finally, we have classiﬁed the relations in Table 1

to create the taxonomic structure showed in Figure 3.

4.3 Versioning

Versioning is a temporal relation that concerns the

evolution of an ontology. In (Klein and Fensel, 2001),

the ontology versioning problem has been deﬁned as

the ability to handle changes in ontologies by creating

and managing different variants of it.

An ontology can evolve over time in different di-

rections, e.g. lexicographic, changing the names of

some resources, syntactic, adding or removing ax-

ioms, semantic, changing the deﬁnition of some con-

cepts or simply adding or removing axioms. There-

fore, the new ontology could be equivalent or totally

different from the previous one. When we analyze

DOOR - Towards a Formalization of Ontology Relations

different ways of linking two ontologies by the ver-

sioning relation, the two following sentences are sug-

gested immediately: “X

is the previous version of

the X

” or “X

is the latter version of the X

”. These

two typical pieces of knowledge are represented in the

DOOR ontology by the relations isPreviousVersionOf

and its inverse isLatterVersionOf respectively.

Conforming to our methodology, the isPrevi-

ousVersionOf and isLatterVersionOf relations have

been analyzed and formalized, to identify sub-

relations and variants. In Table 3 we summarize the

result of this analysis.

Table 3: Specialization of the versioning relations.

isLatterVersionOf isPreviousVersionOf

Temporal conceptualEvolutionOf priorVersion

explanationEvolutionOf

backwardCompatibleWith

owl:IncompatibleWith

Semantic conceptualEvolutionOf

Syntactic explanationEvolutionOf

According to (Klein et al., 2002; Heﬂin and Pan.,

2004; Heﬂin, 2001) the modiﬁcation of an ontology

can lead to two different types of evolutions: being

a conceptual change, meaning that the model of the

ontology changed, or being an explanation change,

meaning that the modiﬁcations happened only at a

syntactic level, without affecting the model of the

ontology. Therefore, we specialized the isLatterVer-

sionOf relation into

conceptualEvolutionOf(X

, X

) if X

is a latter ver-

sion that is not semantically equivalent to X

explanationEvolutionOf(X

, X

) if X

is a latter ver-

sion that is semantically equivalent to X

These two relations will lead to the deﬁnition of rules

to infer them from equivalence and other versioning

relations.

In addition, the OWL ontology properties

priorVersion, backwardCompatibleWith and

incompatibleWith represent explicit relations be-

tween versions of ontologies and are included in

DOOR as sub-properties of isLatterVersionO f and

isPreviousVersionO f .

To complete this section of the DOOR ontology,

we can classiﬁed the relations in Table 3 as showed in

Figure 4.

Indeed, according to (Klein et al., 2002; Heﬂin

and Pan., 2004; Heﬂin, 2001) the modiﬁcation of an

ontology can lead a new version which is completely

different from the original one. But in practice, by an-

alyzing Watson’s ontology repository, it is almost al-

ways possible establish a similarity between the two

owlIncompatibleWith

explanationEvolution

syntacticallyEquivalentTo

equivalentTo

isLatterVersionOf isPreviousVersionOf

similarTo

isIsomorphicTo

sematicallyEquivalentTo

syntacticallySimilarTopriorVersion

backwardCompatibleWith conceptualEvolutionOf explanationEvolution

lexicallySimilarTo mappingSimilarTo semanticallySimilarTo

Figure 4: Taxonomy for versioning relations.

ontologies, at least at the lexicographic level. Due to

this fact, we chose to consider the versioning relations

to be sub-properties of similarTo, to indicate that two

different versions of the same ontology should, to

some extent, be similar. Moreover, in accordance with

its deﬁnition, the explanationEvolutionOf relation is

a sub-property of semanticallyEquivalentTo.

4.4 Agree and Disagree

Based on the formal measures of the agreement

and disagreement between ontologies deﬁned in

(d’Aquin, 2009), we introduce the agreesWith and

disagreesWith relations in DOOR. Informally, the for-

mer holds the general meaning of “to have the same

opinion about something”. In other words, it connects

two ontologies, sharing the same knowledge partially

and is therefore very related to the similarTo and the

equivalentTo relations. The later indicates that the on-

tologies “contradict each other” to a certain extent,

these contradictions appearing at various levels. En-

visaged sub-relations for these two relations are listed

in Table 4.

Table 4: Specialization of agreesWith and disagreesWith.

agreeWith disagreeWith

Temporal backwardCompatibleWith owlIncompatibleWith

Semantic sematicallyEquivalentTo hasDisparateModeling

sematicallySimilarTo incompatibleWith

incoherentWith

incosistentWith

Syntactic syntacticallyEquivalentTo

syntacticallySimilarTo

explanationEvolution

In this Table, all the sub-relations of agreesWith

have already been deﬁned before. We add a few re-

lations to express speciﬁc ways for ontologies to dis-

agree, all related to the semantic dimension of the on-

KEOD 2009 - International Conference on Knowledge Engineering and Ontology Development

agreesWith

semanticallyEquivalentToBackwardCompatibleWith

explanationEvolution syntacticallyEquivalentTo

syntactiallySimilarTo

semanticallySimilarTo

Figure 5: Taxonomy for the agreement relations.

hasDisparateModelling

incompatibleWith

disagreeWith

incoherentWith inconsistentWith owlIncompatibleWith

Figure 6: Taxonomy for the disagreement relations.

tologies.

incompatibleWith(X

, X

) if incoherentWith(X

, X

)

or inconsistentWith(X

, X

incoherentWith According to (Qi and Hunter, 2007)

an ontology X

is incoherent if and only if there

is an unsatisﬁable concept name in X

. Therefore,

two ontologies are incoherent with each other if,

when they are merged, they generate an incoher-

ent result.

inconsistentWith According to (Bell et al., 2007) an

ontology X

is inconsistent if it has no model.

Therefore, two ontologies are inconsistent with

each other if, when they are merged, they generate

a inconsistent result.

hasDisparateModeling Two ontologies are consid-

ered to have disparate modeling if they represent

corresponding entities in different ways, e.g. as

an instance in one case and a class in the other.

owl:IncompatibleWith It comes from OWL lan-

guage (Patel-Schneider et al., 2004).

Finally, we have also classiﬁed the relations in Table 4

as showed in Figures 5 and 6.

4.5 Other Relations

Analyzing Watson’s ontology repository we found

out that there are many documents which only rep-

resent the TBox of an ontology and others represent-

ing just the ABox. This is captured through the is-

TheSchemaFor relation. isAlignedTo relation links

ontologies for which exists an alignment.

Figure 7: Architecture of the KANNEL framework.

5 KANNEL: AN APPLICATION

FOR THE DOOR ONTOLOGY

In the previous section, we described the DOOR on-

tology in detail. Here we provide a brief overview of

the way it is used in the KANNEL system. KAN-

NEL (Allocca., 2009) is a framework for detecting

and managing ontology relations for large ontology

repositories, such as WATSON. It is an ontology-

based system where the DOOR ontology plays an im-

portant role, providing an explicit representation of

the implicit relations between ontologies. We have

designed an architecture for this framework, as de-

picted in Figure 7. As showed in this ﬁgure, the

DOOR Ontology separates the on-line part of the

architecture–providing APIs and services that relies

on a reasoner–from the off-line part–detecting rela-

tions in the repository and populating the ontology.

The ofﬂine part is based on three components: the

Control Component (CC), the Detecting Component

(DC) and the Populating Component (PC). As a ﬁrst

step, the CC selects from the Ontology Repository on-

tologies that need to be evaluated to establish poten-

tial relations. Then, the selected sets of ontologies are

processed by the DC, which contains the main mech-

anisms to discover the possible relations between on-

tologies, relying on the deﬁnitions provided in this pa-

per. Finally, the PC populates the semantic structure

with the detected relations. What is obtained is a set

of automatically discovered relations, represented as

part of the DOOR ontology so that the reasoner used

in the system can infer new relations from the onto-

logical and rule based knowledge included in the on-

tology. As such, DOOR provides meta-information

on the ontology repository in KANNEL.

DOOR - Towards a Formalization of Ontology Relations

6 CONCLUSIONS

In this paper, general relationships between ontolo-

gies have been examined. In particular, we have cho-

sen to consider well-known relations in the literature,

as well as the ones needed to support the develop-

ment of Semantic Web Applications. To achieve that,

we adapted an ontology building methodology for the

construction of DOOR, an ontology of relations be-

tween ontologies.

This ontology describes relations both from the

point of view of their taxonomic structure and from

the point of view of their formal deﬁnitions, provid-

ing the formal properties to describe them as well as

a set of rules to derive complex relations from other

relations.

We also described KANNEL, a framework for de-

tecting and managing ontology relationships for large

ontology repositories. The DOOR ontology plays a

fundamental role in KANNEL, not only to provide an

explicit representation on ontology relations, but also

to supply meta-information that offers several advan-

tages, among which the possibility to reason upon on-

tologies and their relations. This possibility provides

a relevant support for the development of Semantic

Web Applications, which can use the semantic web as

a large-scale knowledge source (d’Aquin et al., 2008).

The ﬁrst version of the DOOR

ontology is available in OWL at

http://kannel.kmi.open.ac.uk/ontology. The KAN-

NEL framework is currently under development. The

development of DOOR is obviously a continuous

task, which requires a proper assessment of each

version. For this reason, we plan to test and validate

the ﬁrst version presented here, in particular by

populating it with automatically detected relations

between ontologies in WATSON.

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KEOD 2009 - International Conference on Knowledge Engineering and Ontology Development