A Digital Library is an evolving
organisation that comes into existence through
a series of development steps that bring together
all the necessary constituents. Figure shown
below presents this process and indicates three
distinct notions of ‘systems’ developed
along the way forming a three-tier framework:
Digital Library, Digital Library System, and
Digital Library Management System. These correspond
to three different levels of conceptualization
of the universe of Digital Libraries.
DL, DLS and DLMS: A Three-tier Framework
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 the description, understanding
and development of digital library systems difficult.
As Figure I.2-1 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)
An organisation, which might be virtual, 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 codified policies.
Digital Library System
(DLS)
A software system that is based on a defined
(possibly distributed) architecture and provides
all functionality required by a particular Digital
Library. Users interact with a Digital Library
through the corresponding Digital Library System.
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 fundamental for Digital Libraries
and (ii) to integrate additional software offering
more refined, specialised 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 may belong to one of the
following three types:
• Extensible Digital Library System
A complete Digital Library System that is fully
operational with respect to a defined core suite
of functionality. DLs are constructed by instantiating
the DLMS and thus obtaining the DLS. Thanks
to the open software architecture, new software
components providing additional capabilities
can be easily integrated. The DelosDLMS [184][3]
is a prototypical example of a system based
on this philosophy.
• Digital Library System Warehouse
A collection of software components that encapsulate
the core suite of DL functionality and a set
of tools that can be used to combine these components
in a variety of ways (in Lego®-like fashion)
to create Digital Library Systems offering a
tailored integration of functionalities. New
software components can be easily incorporated
into the Warehouse for subsequent combination
with those already there. BRICKS [41] and DILIGENT
[68] are two prototypical examples of systems
that are based on this philosophy.
• Digital Library System Generator
A highly parameterised software system that
encapsulates templates covering a broad range
of functionalities, including a defined core
suite of DL functionality as well as any advanced
functionality that has been deemed appropriate
to meet the needs of the specific application
domain. Through an initialisation session, the
appropriate parameters are set and configured;
at the end of that session, an application is
automatically generated, and this constitutes
the Digital Library System ready for installation
and deployment. The MARIAN framework equipped
with the 5SL specification language represents
an example of this process [96].
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.5 For
instance, through DILIGENT it is possible to
build and run a number of DLSs, each realizing
a DL serving a target community. The DL is thus
the abstract entity that ‘lives’
thanks to the software system constituting the
DLS.
