The term "Digital Libraries" corresponds to a very complex notion with several diverse aspects and cannot be captured by a simple definition. A robust model of Digital Libraries encapsulating the richness of these perspectives is required. This need has led to the drafting of The Digital Library Manifesto, the aim of which is to set the foundations and identify the cornerstone concepts within the universe of Digital Libraries, facilitating the integration of research results and proposing better ways of developing appropriate systems. The Manifesto is a result of the collaborative work of members of the European Union co-funded DELOS Network of Excellence on Digital Libraries.1 It exploits the collective understanding that has been acquired, over more than a decade, on Digital Libraries by European research groups active in the Digital Library field, both within DELOS and outside, as well as by other groups around the world.
This article presents the core parts of the Manifesto that introduce the entities of discourse of the Digital Library universe.
Digital Libraries represent the meeting point of many disciplines and fields, including data management, information retrieval, library sciences, document management, information systems, the web, image processing, artificial intelligence, human-computer interaction, and digital curation. This multidisciplinary nature has led to a variety of definitions as to what a Digital Library is, each one influenced by the perspective of the primary discipline of their proposer(s) . The concept of Digital Library has evolved substantially since the early idea of it as a system for providing access to digitized books and other text documents. The DELOS Network of Excellence on Digital Libraries , for example, now envisions a Digital Library as a tool at the center of intellectual activity having no logical, conceptual, physical, temporal, or personal borders or barriers to information . Generally accepted conceptions have shifted from a content-centric system that merely supports the organization and provision of access to particular collections of data and information, to a person-centric system that delivers innovative, evolving, and personalized services to users. Conceptions of the role of Digital Libraries have shifted from static storage and retrieval of information to facilitation of communication, collaboration, and other forms of dynamic interaction among scientists, researchers, or the general public on themes that are pertinent to the information stored in the Digital Library. Moreover, expectations of the capabilities of Digital Libraries have evolved from handling mostly centrally located text to synthesizing distributed multimedia document collections, sensor data, mobile information, and pervasive computing services.
This vision of Digital Libraries seems to resonate well with the concept of "Information Space" as established within the field of Computer Supported Cooperative Work (CSCW). Snowdon, Churchill, and Frecon  have developed future visions about "Connected Communities" and "Inhabited Information Spaces", with the latter being closely related to the vision of Digital Libraries, in that ubiquitous information is a prerequisite for CSCW.
The variety of conceptions of what a Digital Library is has had a substantive impact on attempts to define and bound the term 'Digital Library'. Since 2006 the term has been generally used to refer to systems that are heterogeneous in scope and provide diverse types of functionality. These systems include digital object and metadata repositories, reference-linking systems, archives, content administration systems (mainly developed by industry), and complex systems that integrate advanced digital library services (mainly developed in research environments). This "overloading" of the term 'Digital Library' results in Digital Library services and systems that do not deliver interoperability and reuse of content and technologies.
Given the current level of maturity and substantial knowledge and experience that have been accumulated in recent years, the time is certainly ripe to lay the necessary foundations for overcoming these challenges by facilitating integration of relevant research results and improving system development methodologies. In order to respond to this need, and following the example of other IT research areas,2 the members of the DELOS Network of Excellence on Digital Libraries have published the Digital Library Manifesto. The Manifesto establishes principles to underpin the field and lead to the development of reference documents that will capture the full spectrum of concepts playing a role in Digital Libraries. The Manifesto exploits the collective understanding of Digital Libraries developed by European research groups, including those that are partners in DELOS, and the results of DELOS working meetings (e.g., San Cassiano in 2001, Corvara in 2004, and Frascati in 2006).
In this article, we present the core elements of the Manifesto and introduce central aspects of the Digital Library framework. The discussion begins with an examination of the three types of relevant "systems" in this area: Digital Library, Digital Library System, and Digital Library Management System. It also explains how they interrelate. The discussion then moves on to examine three other core topics: the key concepts that characterize these systems encompassing content, user, functionality, quality, policy, and architecture; the range of roles that actors play in digital libraries from application developer to administrator, to designer, and finally to end-user; and clarification of the different levels of abstraction that help us to talk intelligently about the DL Universe.
2 The Digital Libraries Universe: A Three-tier Framework
A Digital Library is an evolving organization that comes to existence through a series of development steps that bring together all necessary constituents. Figure 1 presents this process pictorially and indicates three distinct notions of "systems" developed along the way: Digital Library, Digital Library System, and Digital Library Management System. These correspond to three different levels of conceptualization of the universe of Digital Libraries.
These three system notions are often confused and are used interchangeably in the literature; this terminological imprecision has produced a plethora of heterogeneous entities and contributes to making difficult the description, understanding and development of digital library systems. As Figure 1 indicates, all three systems play a central and distinct role in the Digital Library development process. To clarify their differences and their individual characteristics, the explicit definitions that follow may help:
Digital Library (DL)
Digital Library System (DLS)
Digital Library Management System (DLMS) A generic software system that provides the appropriate software infrastructure both (i) to produce and administer a Digital Library System incorporating the suite of functionality considered foundational for Digital Libraries and (ii) to integrate additional software offering more refined, specialized, or advanced functionality.
A Digital Library Management System belongs to the class of "system software". As is the case in other related domains, such as operating systems, databases, and user interfaces, DLMS software generation environments may provide mechanisms to be used as a platform to produce Digital Library Systems. Depending on the philosophy it follows, a DLMS belongs to one of the following three types:
Although the concept of Digital Library is intended to capture an abstract system that consists of both physical and virtual components, the Digital Library System and the Digital Library Management System capture concrete software systems. For every Digital Library, there is a unique Digital Library System in operation (possibly consisting of many interconnected smaller Digital Library Systems), whereas all Digital Library Systems are based on a handful of Digital Library Management Systems.3 For instance, through DILIGENT it is possible to build and run a number of DLSs, each realising a DL serving a target community. The DL is thus the abstract entity that "lives" thanks to the software system constituting the DLS.
3 The Digital Libraries Universe: Main Concepts
Despite the seeming richness and diversity of existing digital libraries,4 in actuality, there is only a small number of core concepts defined by all systems. These concepts are identifiable in nearly every Digital Library currently in use. They serve as a starting point for any researcher who wants to study and understand the field, for any system designer and developer intending to construct a Digital Library, and for any content provider seeking to expose its content via digital library technologies. In this section, we identify these concepts and briefly discuss them.
Six core concepts provide a foundation for Digital Libraries. Five of them appear in the definition of Digital Library: Content, User, Functionality, Quality, and Policy; the sixth one emerges in the definition of Digital Library System: Architecture. All six concepts influence the Digital Library framework, as shown in Figure 2.
The concepts of Content, User, Functionality, and Policy share many similar characteristics and are all concepts referring to internal entities of a Digital Library that can be sensed by the external world. Introducing the Resource concept, which subsumes the above four, enables us to reason about these characteristics in a consistent manner.
The six core concepts (Content, User, Functionality, Quality, Policy and Architecture) that lie at the heart of DLs need to be considered in conjunction with the four main ways that actors interact with digital library systems, as discussed in the next section.
4 The Digital Library Universe: The Four Main Roles of Actors
We envisage actors interacting with Digital Library Systems playing four different and complementary roles: DL End-Users, DL Designers, DL System Administrators, and DL Application Developers.
As shown in Figure 3, each role is primarily associated with one of the three "systems" in the three-tier framework.
DL System Administrators
DL Application Developers
The four roles described above encompass the whole spectrum of actors interacting with digital libraries. Their models of the DL Universe are linked together in a hierarchical fashion, as depicted in Figure 4. This hierarchy is a direct consequence of the above definitions, since DL End-Users act on the Digital Library, whereas DL Designers, DL System Administrators and DL Application Developers operate on the DLMS and, consequently, on the DLS and DL as well. This inclusion relationship ensures that co-operating actors share a common vocabulary and knowledge. For instance, the DL End-User expresses requirements in terms of the DL model and, subsequently, the DL Designer understands these requirements and defines the DL accordingly.
5 Reference Frameworks
The digital library universe is complex and comprises multiple elements (see Figure 5). The representation of the details of these elements depends upon the introduction of frameworks supporting different levels of abstraction:
The relationship of these three frameworks to the general digital library environment is shown in Figure 5. At the top is the most abstract Reference Model, which guides the more specific Reference Architecture and Concrete Architecture further down. In turn, these should constrain the development and implementation of any actual system. The three reference frameworks are the outcome of an abstraction process that has taken into account the goals, requirements, motivations and, in general, the digital library market, as shown in the left-hand side of Figure 5, and the best practices and relevant research shown on the right-hand side of the same figure. When these frameworks are adopted and followed by the community, the resulting systems will be largely compatible with each other; the interoperability thus afforded will open up significant new horizons for the field.
This article has presented the core parts of The Digital Library Manifesto produced by members of the DELOS Network of Excellence on Digital Libraries to be used as a springboard for future foundational research and development in the domain of Digital Libraries.
The Digital Library Manifesto is accompanied by two other works currently available in their first release as a DELOS deliverable , introducing the DL Reference Model and a DLS Reference Architecture. We hope that these documents will provoke discussion and debate within the digital library community that will promote their improvement. Clearly, diversity of needs among different digital library "systems" will continue to introduce new concepts that will require incorporation into the Reference Model and in the Reference Architectures. Hence, these documents should be considered as first versions of otherwise dynamic documents that will keep evolving, having the Manifesto as a firm foundation.
The Digital Library Manifesto is built on the experience and knowledge gained by many previous efforts in the Digital Library area. In particular, the authors wish to acknowledge the considerable input to the drafting of the Manifesto received from the participants of the DELOS Reference Model Workshop held in Frascati, Italy, in June 2006. More details about this Workshop can be found at <http://www.delos.info/ReferenceModel>.Note on website citations: All citations of websites were validated on 16 February 2007.
1. DELOS: Network of Excellence on Digital Libraries (G038-507618) is co-funded by the European Union under the 6th Framework Programme runs from January 2004 through December 2007.
3. To the extent that it is helpful, one may draw an approximate analogy between the world of Digital Libraries and the world of Databases. A DBMS (e.g., the DB2, Oracle system, MySQL or PostgreSQL) corresponds to a DLMS, offering general data management services. A DBMS together with all application software running on top of it at an installation corresponds to a DLS. Finally, a DL corresponds to a so-called "Information System" that consists of the above software, its data, and its users.
4. From here on, we shall use the terms "Digital Library" (or its acronym "DL"), "Digital Library System (DLS)" and "Digital Library Management System (DLMS)" to denote the systems identified in Sec.2, while by the term "digital libraries" we shall refer to the whole field of digital library research and applications.
6. This picture was inspired by the "Reference Model for Service Oriented Architecture" document .
(All citations of websites were validated on 16 February 2007.)
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Copyright © 2007 Leonardo Candela, Donatella Castelli, Yannis Ioannidis, Georgia Koutrika, Pasquale Pagano, Seamus Ross, Hans-Jörg Schek, Heiko Schuldt, and Constantino Thanos