E.A. Fox et al. (Eds.): ICADL 2005, LNCS 3815, pp. 38 – 43, 2005. © Springer-Verlag Berlin Heidelberg 2005

An Ontology-Based Model of Digital Libraries

László Kovács and András Micsik

MTA SZTAKI, Computer and Automation Research Institute,

of the Hungarian Academy of Sciences, Department of Distributed Systems,

H-1111 Budapest XI. Lágymányosi u. 11. Hungary {laszlo.kovacs, micsik}@sztaki.hu

Abstract. In this paper a new unifying model is suggested for digital libraries which contains four conceptual layers, and defines the concepts of each layer as an OWL ontology. Instances of the ontology can be used to define an overall view of a digital library in terms of the four layers and the relationships be-tween them. Such a model has the advantage that the methodology is formal-ized and extensible, thus models are comparable and manageable.

Keywords: digital library concepts, ontologies, reference models, OWL.

1 Introduction

Digital libraries represent a truly interdisciplinary research domain; modeling activi-ties in this area thus have very complex requirements. Models related to digital librar-ies published so far can be grouped as follows:

• External models have some overlap with the digital library research area, but these models are widely used outside the area as well [3,4],

• Partial models are restricted to certain views or services of digital libraries [5,6,7,9]. Models for digital library evaluation form a subset of partial models, for example [2,10],

• Generalized models: the 5S model [8] is a formal model for digital libraries, while the DELOS model can be seen as a conceptual model [1].

The single general model applicable for digital libraries is the 5S model [8]. Ac-cording to this model a digital library consists of a repository, metadata catalogs, ser-vices and a society of users. The 5S refers to streams and structures for the construc-tion of digital objects, spaces for the description of digital object collections and their interrelations, scenarios for the definition of how services and activities change the state of the system, and finally societies for the interconnection of roles and activities within the user community. The 5S model is based on mathematical formalism, and has been used in various case studies, including the generation of a taxonomy of DL terms. 5SL is a declarative language based on this model for the generation of DL applications.

Another holistic view of digital library is given by the DELOS working group for digital library testbeds and evaluation [1]. Although the model focuses on evaluation,

An Ontology-Based Model of Digital Libraries 39

its view can be generalized. The model emerges from three non-orthogonal compo-nents of digital libraries: the users, the data/collection and the technology used. The interaction of these three defines the fourth component: usage. Each component con-nects the DL domain to different fields of research, with different interests and evaluation ’culture’. This model is less formal than the 5S model.

2 Suggestion of a Holistic Model for Digital Libraries

In this section we outline a model that covers all possible aspects of digital libraries on the conceptual level, and provides means for relating modeled aspects on the for-mal level. Our work is rooted in the DELOS model [1], and starts from the basic as-pects of a digital library identified as: Collection/Content, Services, Inter-faces/Infrastructure and Community.

Content

Services

Interfaces

Community

metadata user profiles archive profiles

Search Register as user Add archive

Web UI Admin UI

Discovery Connect new archive

Fig. 1. Layered approach for digital library modeling

These components can be illustrated as a layered model (Figure 1), with the con-tent as the bottom layer, and the community as the top layer. Layers provide access to lower layers, and combine individual capabilities of lower layers into more complex functionalities. On the topmost layer we reach complex work patterns such as discov-ering new relevant documents or inserting a new document collection into the digital library.

The traditional goals of reference models are to establish a common basic termi-nology and to provide a generic architecture or structural modal for the area. Digital libraries are very different in their aims, services and architecture. Still the goal of es-tablishing a common basic terminology remains necessary, and the emerging use of ontologies in connection with the Semantic Web effort offers new and valuable tools for that purpose.

An ontology describes concepts and relationships within the investigated area or phenomena [11]. OWL (the Web Ontology Language) was selected as the format of this new ontology [12]. Each layer in our model defines its key concepts and their re-lationships. Relationships exist also between layers. First, each layer is introduced in a bottom-up order, and then the overview of the whole ontology is presented. The main concepts and relationships in the content layer are:

40 L. Kovács and A. Micsik

Class(Concept owl:Thing) Class(Collection Concept) Class(PersistentCollection Collection) Class(DataCollection PersistentCollection) Class(MetadataCollection PersistentCollection) Class(DynamicCollection Collection) Class(TemporaryCollection Collection) DisjointClasses(PersistentCollection DynamicCollection TemporaryCollection) ObjectProperty(has_metadata domain(DataCollection) range(MetadataCollection)) ObjectProperty(derived_from range(Collection) domain(Collection)) ObjectProperty(refers_to range(Collection) domain(Collection))

The class Concept is used as a common root for all classes defined in our ontology. Definitions are sometimes a bit shortened for easier reading and saving space. The ba-sic concept Collection models logical groups of data, such as databases, document stores, indices or metadata. This class is divided into three disjoint subclasses: a Per-sistentCollection contains mostly static data, the ‘real content’ of the digital library, a DynamicCollection contains data which is needed for services and is often actualized (e.g. user profiles, database indices), and TemporaryCollection is a family of tempo-rary, reproducible data objects such as a search result. Relationships can be used to describe when a collection is derived from another (e.g. an index), or when a collec-tion contains the metadata for another collection. Further concepts not listed here de-scribe data formats, internal structures, etc. The characteristic elements of the next layer are:

Class(Service Concept) Class(AtomicService Service) Class(CompositeService Service) Class(ServiceGroup Concept) ObjectProperty(has_service domain(ServiceGroup) range(Service)) ObjectProperty(composed_of domain(CompositeService) range(Service)) ObjectProperty(uses domain(Service) range(Collection)) ObjectProperty(reads uses domain(Service) range(Collection)) ObjectProperty(updates uses domain(Service) range(Collection)) ObjectProperty(produces uses domain(Service) range(Collection))

In the service layer services can be atomic or composed, and services can be grouped together for easier reference. Services are usually modeled by their pre- and postcondition; the requirements for deploying the service and the effect of service de-ployment. Here, the ‘uses’ properties can be applied to connect the required input and

An Ontology-Based Model of Digital Libraries 41

the output to a service. It should be noted that services are abstract on this layer: they are accessible only through interfaces provided by the next layer:

Class(Interface Concept) Class(MachineInterface Interface) Class(HumanInterface Interface) Class(Node Concept) ObjectProperty(provides domain(Interface) range(ServiceGroup)) ObjectProperty(has_interface domain(Node) range(Interface)) ObjectProperty(communicatesWith domain(MachineInterface) range(MachineInterface))

The interface layer defines the infrastructure of the digital library system with re-spect to service deployment and communication. Services are accessible through in-terfaces, where human interfaces communicate with users (upper layer), and machine interfaces communicate with each other. Nodes may be used to represent the various separate hardware elements of the system, and their roles in the system are indicated by the interfaces they provide. Interfaces may also be characterized by their availabil-ity (PDAs, touchscreen kiosks, etc.).

Class(Role Concept) Class(WorkPattern Concept) Class(Actor Concept) Class(User Actor) Class(Professional Actor) Class(Agent Actor) ObjectProperty(has_access domain(Role) range(HumanInterface)) ObjectProperty(responsible_for domain(Role) range(Role)) ObjectProperty(participator domain(WorkPattern) range(Role)) ObjectProperty(has_role domain(Actor) range(Role))

The community layer represents the use of the system. Roles define which inter-faces are accessible for which users, and the organizational structure of the digital li-brary can also be illustrated by responsibilities between roles. Work patterns collect the roles needed to perform that work. Actors can be used to describe the user com-munity. The subclasses of actors are defined as in [9].

Figure 2 provides an overview of the main classes and their relationships in the on-tology. It can be seen that the layers are connected in a simple, hierarchical way. The ontology can be enhanced with more classes and properties in order to provide richer or finer conceptualization. When the required level of conceptualization is reached, instances are created to represent aspects of the modeled digital library, and relations are used to define connections between these instances.

42 L. Kovács and A. Micsik

Content

Services

Interfaces

Community

Collection

ServiceGroup

Interface Node

Service

Role WorkPattern

uses

provides

has_access

has_participator

has_interface

has_service

Fig. 2. The main concepts in the layers and their connections

3 Summary

A new modeling technique is suggested for digital libraries in which layers provide the separation of main aspects of digital libraries such as content and organization. The concepts and their relationships of each layer are defined as an ontology. This re-sults in a formal way of capturing the essence of a digital library which is extensible to model high-level details as well.

The presented approach is very general, and it is not limited to digital libraries, but can be applied to other complex networked information systems (CNIS) such as web portals or organizational memory applications. As the borders of the digital library field are still blurred, this generality may also help to find the “differentia specifica” of digital libraries, and to give more contour to borders.

Acknowledgement. This work was supported in part by the DELOS Network of Ex-cellence on Digital Libraries funded by the European Commission.

References

1. Norbert Fuhr, Preben Hansen, Michael Mabe, András Micsik, Ingeborg Sølvberg: Digital Libraries: A Generic Classification and Evaluation Scheme. ECDL 2001, September 4-9 2001, Darmstadt, Germany, Springer LNCS 2163, pp. 187-199

2. László Kovács and András Micsik. The Evaluation Computer: a Model for Structuring Evaluation Activities. DELOS Workshop on the Evaluation of Digital Libraries, Padova, 2004, http://dlib.ionio.gr/wp7/workshop2004_program.html

3. A Reference Model for an Open Archival Information System, Document Number: ISO 14721:2003

4. Tefko Saracevic. Relevance: a review and a framework for thinking on the notion in in-formation science. Journal of the American Society for Information Science, 26:321-343, 1975

An Ontology-Based Model of Digital Libraries 43

5. Functional Requirements for Bibliographic Records, http://www.ifla.org/VII/s13/frbr/ frbr.pdf

6. Bing Wang. A hybrid system approach for supporting digital libraries. International Jour-nal on Digital Libraries, 2(2-3):91-110, 1999.

7. Donatella Castelli, Carlo Meghini, Pasquale Pagano. Foundations of multidimensional query language for digital libraries. ECDL 2002, Springer LNCS 2458, pp. 251-265.

8. M. A. Goncalves, E. A. Fox, L. T. Watson and N. A. Kipp. Streams, Structures, Spaces, Scenarios, Societies (5S): A Formal Model for Digital Libraries, ACM Transactions on In-formation Systems, 22(2), 2004, pp. 270-312

9. Final Report of DELOS/NSF Working Group on ’Reference Models for Digital Libraries: Actors and Roles’, 23 July 2003, http://www.delos-nsf.actorswg.cdlib.org/finalreport.pdf

10. Tefko Saracevic. Digital Library Evaluation: Toward Evolution of Concepts. Library Trends 49(2): (2000)

11. O. Olsson. Two Uses of Ontologies in Digital Libraries. Second DELOS Workshop: Meta-data and Interoperability in Digital Library Related Fields, Bonn, 7-8 October 1996, ERCIM Workshop Reports No.97/W002

12. D. L. McGuinness, F. van Harmelen (eds.). OWL Web Ontology Language Overview. W3C Recommendation 10 February 2004, http://www.w3.org/TR/owl-features/