=Paper=
{{Paper
|id=Vol-263/paper-3
|storemode=property
|title=Introducing Context into Semantic Web Knowledge Databases
|pdfUrl=https://ceur-ws.org/Vol-263/paper3.pdf
|volume=Vol-263
|dblpUrl=https://dblp.org/rec/conf/caise/Stoermer06
}}
==Introducing Context into Semantic Web Knowledge Databases==
https://ceur-ws.org/Vol-263/paper3.pdf
1144 CAiSE'06 Doctoral Consortium
Introducing Context into Semantic Web
Knowledge Bases
Heiko Stoermer
University of Trento,
Dept. of Information and Communication Tech.,
Via Sommarive 10, 38050 Trento, Italy
stoermer@dit.unitn.it
Abstract. Knowledge Representation in the Semantic Web is mainly
characterized by the capabilities of the languages OWL and RDF. When
attacking non-trivial problems such as the creation and maintenance of
large-scale knowledge bases in RDF, it becomes evident that RDF lacks
a central feature, namely the capability of restricting the truth value of
a statement to a context, with far-reaching consequences which we will
hint at in this paper. We propose a solution approach to this problem: the
extension of RDF knowledge bases with context features. We will present
theoretical background, envisioned experiments as well as a comparative
study based on two technically di erent approaches, and illustrate the
applications and bene ts of contextual Knowledge Representation in the
Semantic Web.
Keywords: Context, Knowledge Representation, Semantic Web, RDF
1 Introduction
The general setting of this thesis proposal is the area of the Semantic Web. Since
the rst concrete published ideas in this direction in 2001 [3], many interesting
and progressive technologies have emerged to pursue the vision of a Web that
will not only simply contain data but semantic information that is machine-
processable in a meaningful way.
The original approach was to keep things simple, as for the authors of [3]
it was obvious that complex approaches in Arti cial Intelligence (AI) and espe-
cially in the eld of Human Language Technology (HLT) had not advanced to a
level that could be called usable for the desired machine interpretation of Web
documents authored by humans.
So at the base of the layered architecture sketched in [3] there was supposed
to be the meaningful annotation of Web documents to transform them into
Semantic Web Documents. Technologies such as RDF and RDF Schema [10]
were among the rst to come into existence { RDF to annotate documents
and RDFS the de ne the regarding annotation vocabulary. Together with OWL
{ a language for conceptual modeling { these two languages and the related
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tools play a central role in the Semantic Web and have become standards for
representing what a machine can know about a document (and the world).
From the original idea of rather simple (and possibly manual) annotation,
things have developed farther, not only through the in uence of the AI commu-
nity, on who's work for example in Description Logics1 and Automated Reason-
ing the di erent variants of the OWL language are based.
More recent projects in the Semantic Web use a more self-con dent vocab-
ulary and talk about knowledge bases instead of metadata, and use inference
techniques and Automated Reasoning to provide semantic-enabled services. This
development has brought the e orts of the Semantic Web closer to research that
has been performed for many years in the area of AI and speci cally Knowledge
Representation (KR). This growing overlap opens opportunities for applying
theories from AI to the Semantic Web to ensure that the e orts taken so far re-
ceive the best theoretical base they can have and also to contribute new ndings
back to the AI eld.
This thesis proposal will deal with one aspect of KR, the notion of context,
and how to improve current KR approaches in the Semantic Web by incorpo-
rating context theories to RDF Knowledge Bases. I will illustrate motivations
of why sensible KR for the Semantic Web should be based on contexts, and
underline this opinion by outlining requirements of the large ongoing European
project Vikef 2 that I am involved in. I will furthermore give details about
work that has already been performed in this area, by myself and other people,
as well as describe the vision for the proposed research in more depth. Finally,
I will deliver an outline of which bene ts and applications a good approach in
this topic will be able to provide.
2 Problem Description and Motivation
The Vikef project deals with creating large-scale information systems that base
on Semantic Web technology. At the center of the envisioned systems there is an
RDF knowledge base (KB) that contains a large amount of information about
documents and their contents. This information is stored in an RDF triple store
that has been developed in the course of the project by other partners in the
consortium. The triple store has originally been envisioned to be a single bag of
RDF triples, i.e. all statements are stored in the same information space together.
However, from a Knowledge Representation point of view RDF statements in
general are context-free, and thus represent statements of universal truth, while
documents contain context sensitive information, i.e. information whose inter-
pretation depends on the context in which the document was written. In e ect,
the original way to proceed can easily generate contradictory statements to be
stored in the KB, such that for instance \Silvio Berlusconi is the Prime minister
1
As one of many relevant sources for information about Description Logics please
refer to [1]
2
For more information about the Vikef architecture, features and application sce-
narios please refer to the project's website: http://www.vikef.net
1146 CAiSE'06 Doctoral Consortium
of Italy" and \Romano Prodi is the Prime minister of Italy" as the result of ar-
ticles written at di erent points in time. Such phenomena are however unwanted
in a logical system because they would seriously interfere with the higher level
reasoning that is to be performed to provide the envisioned services. Addition-
ally, we would like to be able to model other aspects such as relevance, credibility
and validity of a statement, all of which require further quali cation of a single
statement or a de ned set of statements.
An innumerous amount of similar examples can be constructed for a single
information system, although in such a system it would at least be possible to
implement some heuristics to try to keep track of these issues, at least at the
time of insertion of a triple, to minimize the problem. However, if we think about
the Semantic Web as a whole, with a large number of uncoordinated information
systems, the problem becomes even more evident. If every peer builds up a KB
of unquali ed RDF statements, the set of universally true facts in the Semantic
Web becomes enormously large and impossible to handle from a semantic point
of view. In our opinion, such contradictions, contradictory beliefs and facts which
become semantically incorrect in the absence of additional pragmatic or contex-
tual information are likely to impose serious problems on the coordination and
interoperation of information systems in the Semantic Web.
3 Research Proposal: Context in RDF Knowledge Bases
We think that the mentioned issues can be attacked by introducing the notion
of context into RDF, to limit the scope of an RDF statement to the context in
which it is relevant. Especially, we want to model that a statement is true only
under a certain set of conditions, which will help us store information in the KB
that would cause contradictions or inconsistencies in a non-contextual RDF KB.
3.1 Context in KR - Multi Context Systems
The ideas presented in this paper base on the logical theory of Multi Context
Systems (MCS) and the principles of Locality and Compatibility presented e.g.
in [11], with in uences from [8, 9]. Basically, this theory states that contexts
can be seen in a peer-to-peer view, resembling more general aspects such as
human beliefs, agent knowledge and other distributed systems. The important
aspect is that reasoning within a context follows standards mechanisms, as the
non-elementary view on the axioms does not require to keep track of the con-
text they are relevant for. Relations between contexts however, i.e. to reason
across contexts, are to be expressed in so-called compatibility relations (CRs),
that formalize exactly how under certain circumstances knowledge from other
contexts becomes relevant. Regarding RDF in this case we claim that a context
can be thought of as a locally coherent set of axioms, and we envision CRs to
be modeled as a semantic attachment [16].
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3.2 Basic Work
The basic idea is to have all statements that belong to a context in a separate
named RDF graph, and extend the RDF semantics in a way to enable contexts
to appear as standard objects in RDF statements of other contexts. One way to
achieve this has been described in [6].
Then, we want to model the mentioned compatibility relations (CRs) between
contexts, to allow for reasoning across contexts. This aspect is probably the
most important one, because from an application perspective it is crucial that
sensible queries can be issued and all relevant information is taken into account -
which requires reasoning across contexts and reasoning on the relations between
contexts.
Several approaches can be thought of to model compatibility relations in
our architecture. First of all, one could think of allowing the implementer of an
information system to provide their own vocabularies (ontologies) to describe
relations between contexts. A similar option would be for us to provide such an
ontology as part of the architecture. However, in our opinion the basic problem
with these approaches is the fact that many interesting relations between ar-
chitectures semantically cannot be fully formalized with the help of a Semantic
Web ontology, which is based on Description Logics 3 .
One of the questions that might arise is how these compatibility relations are
supposed to be modeled. At the moment, we see three approaches to do this:
1. Formally de ne a xed set of compatibility relations as part of our architec-
ture and require all systems that implement the architecture to take care of
providing also an implementation of the relations.
2. Provide an ontology for context relations, so that there exists a vocabulary
to describe these relations with the help of RDF. This approach is slightly
more exible, because the ontology would be extendable.
3. De ne a CR to be implemented as a semantic attachment [16], which can be
thought of as a sort of plugin to the system, one attachment per CR. This
has the positive e ects that i) there is no restriction on how many and which
kind of CRs are part of such a system and ii) implementation of the CRs is
generally not restricted to any speci c language or system.
Very basic and preliminary results including some of the above ideas have
been presented in[6].
3.3 Open Issues and Further Work
As we are only at the very beginning of our work, there are numerous open issues
and questions to be explored. We will only list a few, to illustrate the direction
3
As an example for this claim take a relation such as . The semantics of
this relation have to be expressed algorithmically: c and c0 are taken to be compatible
in the sense that one does not contain facts that contradict with facts of the other
and that the relevant context parameters are the same; then, if no answer to a query
to c can be given, the query will be propagated to c0 .
1148 CAiSE'06 Doctoral Consortium
of the proposed work. To begin with, there are aspects about the underlying
theory that in our opinion have to be cleared. Among these are:
{ What are the semantics of overlapping contexts and how are they to be
modeled?
{ What are the semantics of adding a statement to the context, or adding a
statement to the description of a context?
{ In which way are statements in a context a ected by statements about a
context?
{ How do we discover that a statement is not coherent with a context and thus
may not be added to it?
{ Should contexts be transparent or opaque to a query language?
Secondly, as one of the major aspects of the research to be performed, we in-
tend to invest a reasonable amount of e ort into the exploration of compatibility
relations. As already mentioned before, there are several implementation options
that come to mind, but we want to make sure all possibilities are thought of.
Additionally, one of our objectives is also to see if there are limitations to the
Multi Context Systems theory, what their nature is and if they can be overcome.
Regarding the relations between contexts themselves, we would like to investi-
gate if there are general relations that can always be assumed { such as for
example the ones existing in relational database systems { and how we want to
deal with application-speci c relations. It is our goal to explore the possibilities
that the theory o ers, and to de ne the formal properties of relations we nd to
be relevant.
3.4 Comparative and Experimental Objectives
As a further step in addition to the more theoretical aspects mentioned before,
we are currently establishing close collaboration with two other research groups
to lay the base for comparative experimental work that is envisioned to be part
of the outcomes of the proposed dissertation. With the group responsible for
developing the mentioned RDF triple store we are currently working on an im-
plementation based on RDF named graphs, which we have described in [15].
Additionally, with the group behind the RDF-based P2P system DBin4 we will
try to develop a second implementation based on RDF's rei cation feature, to
explore if there are ways to overcome the limitations mentioned later in Sect. 4.
A substantial part of the proposed work will be the comparison and evalu-
ation of the two prototypes regarding soundness, limitations, performance and
development e ort.
The setting of the proposed work within a large-scale research project is very
fortunate and bene cial for this kind of comparative studies. We expect very
large datasets to be generated and available for intense evaluation, which will
hopefully make the results interesting and highly relevant for the Semantic Web
community.
4
http://www.dbin.org/
CAiSE'06 DC 1149
4 State of the Art and Related Work
We are certainly not the rst to raise the issue of underquali ed statements in
RDF. The straight forward approach to tackle this problem would be to use the
rei cation capability of RDF: for every statement inserted into an RDF graph
we also insert a number of meta-statements about this statement, containing all
relevant context parameters, e.g.:
{ <1996 IsTheTimeOf "">
{ <2005 IsTheTimeOf "">
There exist di erent opinions about the questions whether parameters de-
scribing a context can be limited or not [2]. In any case, this approach would
be implementable using standard RDF. However, with a potentially unlimited
number of context parameters, we foresee a statement explosion with the this
kind of approach, because for every statement we would have to add a signi cant
number of meta-statements describing the relevant context dimensions, so the
overhead is immense.
A completely di erent approach of representing contexts in RDF is to extend
RDF with the ability to represent a reference to a context directly in the data
model. There have been proposals in the past, by Guha [12] or Klyne [14, 13],
which do not use rei cation but implement context as a real extension of the
RDF model theory, by moving from triples to quadruples for identifying the
context to which a statement belongs. To the best of our knowledge, these ideas
have not been pursued any further. Moreover all the currently available RDF
tools would have to be extended in order to deal with such an RDF model.
One related approach that has lead to actual results is that of the W3C
Named Graph Interest Group5 . There has been published a substantial article
in 2005 [7], and implementational results are now part of the Named Graphs API
for Jena (NG4J) 6 , with Jena being the de-facto standard for the development of
software that deals with RDF. We think that this approach it could be well used
as an underlying implementation for the smaller part of what we want to do,
and to be extended for the very important aspect for Compatibility Relations
discussed above.
From a di erent point of view, we are also trying to discover the relations
between our ideas and works from the eld of Description Logics, e.g. Context-
OWL [5] and Distributed Description Logics [4]. These works are especially
important with regards to Compatibility Relations and the modelling of contexts
in general.
5 Bene ts and Applications
The bene ts of a good solution for the issues mentioned in Sect. 2 would in our
opinion be the base for advanced Knowledge Representation in the Semantic
Web. We hope to provide three major outcomes with our work:
5
http://www.w3.org/2004/03/trix/
6
http://www.wiwiss.fu-berlin.de/suhl/bizer/ng4j/
1150 CAiSE'06 Doctoral Consortium
1. A detailed solution to the problem of modeling contexts in the Semantic
Web in a coherent and general way.
2. An evaluation of the MCS theory. We would be able to put this theory to
the test, and explore its limitations.
3. Provision of comparative experimentation results, to illustrate which pos-
sibilities exist, how they behave and whether they prove appropriate for
real-world applications.
Possible applications for this kind of knowledge representation are manifold.
Aspects such as beliefs, trust, incomplete knowledge or knowledge base evolution
in our opinion can all be tackled with a sensible context system as a base. We
believe that in the long run, the vast amount of knowledge represented in the
Semantic Web can be handled much easier if represented in context.
However, we envision the outcomes of this work to go beyond local aspects
and also become relevant from a distributed point of view. As the nature of
the Semantic Web is inherently distributed, we think we can contribute to the
semantic coordination of Semantic Web agents, rstly by o ering the capabilities
to make explicit that two knowledge bases belong to their respective agents and
to enable the agents to establish semantic links to the KBs of other peers with
the help of compatibility relations.
6 Conclusion
In this proposal I have presented an envisioned framework of representing con-
text in the Semantic Web, as a way to make the currently available methods
of knowledge representation in this area become more precise, meaningful and
sensible. I have illustrated the motivations, theoretical and practical, as well as
the bene cial setting of the proposed work within a large European research
project. We have already contributed to the community with a preliminary pub-
lication that received positive response. Certainly, there are many open issues
that will have to be explored. But we are establishing the relevant cooperations
to provide for a high level of expertise in this area and to provide not only a
theoretical contribution but also practical results to illustrate how contextual
knowledge representation in the Semantic Web can help to overcome a number
of existing issues and limitations.
7 Acknowledgements
The work described in this paper has been partly funded by the European Com-
mission through grant to the project Vikef under the number IST-507173. I
would like to thank Luciano Sera ni and my advisor Prof. Paolo Bouquet for
their continuous input and support regarding this topic, and for keeping me on
track.
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