European Library Automation Group
ELAG 2007
Barcelona (Spain), 9-11 May 2007
http://elag2007.upf.edu/
  
  
  

The DELOS Reference Model for Digital Libraries
Vittore Casarosa (ISTI-CNR, Pisa, Italy)
   

A reference model is a set of inter-related concepts that collectively circumscribe and capture the essence of a distinct part of human knowledge, be it a whole scientific field or an individual idea or anything in between. It is a framework that helps in the understanding of the basic elements of its concepts of concern, and it can be established at various levels of abstraction, from very high-level and flexible to very concrete and precise. The field of Digital Libraries is increasingly showing the need of a reference model in order to facilitate their interoperability and their further evolution.
   

Digital Libraries constitute a relatively young scientific field, whose life spans roughly the last fifteen years. They represent the meeting point of a large number of disciplines and fields, i.e., data management, information retrieval, library sciences, document management, information systems, the web, image processing, artificial intelligence, human-computer interaction, and others. These first fifteen years have been mostly spent on bridging some of the gaps between the disciplines (and the scientists serving each one), defining along the way what "Digital-Library functionality" is supposed to be, and integrating solutions from each separate field into digital library systems. The result has been a very complex universe, driven by the growth and the evolution of the digital library community, with a variety of approaches, solutions and systems. The field is now mature enough to attempt the definition of an overarching framework capable to comprehend all of those different aspects and facilitate further evolutions.
   

The framework proposed by DELOS, the Network of Excellence on Digital Libraries, consists of (i) the Digital Library Manifesto, (ii) the Digital Library Reference Model, (iii) the Digital Library Reference Architectures, (iv) the Digital Library Concrete Architectures, and (v) the Digital Library Implementations. This presentation will focus on the Digital Library Manifesto, which sets the ground and lays the high-level guidelines for the other items, and on the Digital Library Reference Model, which identifies the set of concepts that characterize the essence of the digital library universe. It will then conclude with a brief overview of the Digital Library Reference Architecture, which tackles the problem of identifying the appropriate design patterns in implementing, in different application contexts, systems described and identified according to the Reference Model.
   

The Manifesto introduces the three main concepts underlying Digital Libraries. A Digital Library is a live organization that comes to existence through a series of development steps that bring together all necessary constituents. There are three major milestones in this process, which result in three distinct notions of a "system", respectively: Digital Library Management System, Digital Library System, and Digital Library. Although these are often confused and used interchangeably, they play a central and distinct role in the world of Digital Libraries.
   

­A Digital Library (DL) is a (potentially virtual) organization that comprehensively collects, manages, and preserves for the long term rich digital content and offers to its user communities specialized functionality on that content, of measurable quality, and according to prescribed policies.
­A Digital Library System (DLS) is a software system that is based on a (potentially distributed) architecture and provides all functionality that is required by a particular Digital Library. When operating in a Digital Library environment, users interact with the corresponding Digital Library System.
­A Digital Library Management System (DLMS) is a generic software system that incorporates all functionality that is considered foundational for Digital Libraries and provides the appropriate software infrastructure to both produce a basic Digital Library System and integrate additional software offering more refined, specialized, or advanced functionality. An integral part of a DLMS is a "Digital Library Administrative Tool" that is used to choose the appropriate subset of its functionality, e.g., through relevant parameters of its components, and then (potentially automatically, to some degree) install, deploy, and (re)configure a Digital Library System.
   

It has to be noted that, while the concept of Digital Library is intended to capture an abstract system consisting of both physical and virtual components, the remaining two capture concrete software systems. For every Digital Library, there is a unique Digital Library System in operation (possibly consisting of many interconnected smaller DLSs in the most general case), whereas it is expected that most of the Digital Library Systems will be derived from (generated by) a handful of Digital Library Management Systems.
  

The Reference Model consists of a set of inter-related concepts and their related definitions along with the set of relationships that collectively capture the intrinsic nature of the various entities of the digital libraries universe. Six main high-level concepts common to each of these systems have been introduced, i.e. Content, User, Functionality, Quality, Policy, and Architecture. Their relationships and characteristics are shown through the use of Concept Maps, a simple and immediate representation of the conceptual model based on the well known usage of graphs, where nodes are concepts and arcs represent their relationships.
  

Finally, the Reference Architecture proposes an architectural philosophy that promotes the development of DL systems not as single units with tightly coupled components, but rather as loosely coupled components, each of which performs a logical, discrete function. It stresses the added value of loosely-coupled components that: (i) are ready for being assembled in a variety of ways; (ii) can be reused in different application contexts to satisfy the needs of different and heterogeneous user communities; (iii) can be used to add new building blocks and replace others as the user needs change.