\section{Vision of Digital Libraries}   

In our view, the final purpose of a digital library is to improve the exploitation of the globally
networked information universe with a clear focus on the particular information needs of an individual
user and his task at hand \cite{Hou95}. In other words, a digital library is the software needed to bridge the gap
between the virtually infinite amount of information out there on the net and the very specific
information needs of a particular person and task.

With this goal in mind, we distinguish on the level of \e{library content} two kinds of \e{library
entities}:

\begin{description}     

\item[{\em Information tokens}] represent the information content \linebreak which is collectively provided by the net; information
tokens are multi-medial and may represent texts, data, images, sound, videos etc.
\cite{McNSmiWit+96,LiGauGau+96}.  In addition to information tokens digital libraries also contain software tokens,
like applets or generic functions (such as filters, constructors, evaluators, presentors, binders or wrappers).
     
\item[{\em Information artifacts}] are (recursively) created views over information tokens. Selecting, structuring,
combining, and annotating are typical steps in this process. Artifact construction aims at adding value to
information and thereby increasing the satisfaction of information consumers. Information artifacts may be
image tokens associated with texts on their content as well as entire multi-media libraries including sound
and videos.


\end{description} 

Two main library processes can be identified: value-adding artifact construction and personalized exploitation. Although
this paper concentrates on information artifacts it should be emphasized that the software required for 
personalized library processes puts heavy demands on software customization. Indeed, there is a striking
similarity between processes which personalize the library's information content and those which customize
its software required for doing so. Such software is also represented by software tokens out of which
customized software artifacts can be constructed which are finally bound and applied to information
artifacts. In our closing remarks we will come back briefly to this similarity between information and
software libraries.


These \e{library processes} are supported -- on the information as well as on the software level -- by two
kinds of library environments:
  

\begin{description}          

\item[{\em Binding environments}] enable artifact construction and exploitation on the two intertwined levels
of information artifacts and information handling software artifacts.  Binding environments provide a
wide range of binding capabilities and support their safe and disciplined use in heterogeneous, open,
networked and multi-medial settings.

\item[{\em Tracing environments}] are highly desirable because library processes themselves are sources of valuable
information on artifacts. Tracing such processes and extracting context information (on the who and when, the
what and where, the why?, but also on software versions, hardware platforms etc.) may substantially
augment the artifact's value.

\end{description}


\subsection{Information Tokens} 

Information tokens represent the basic multi-media content of digital libraries and are provided by
a digital and networked information universe. Their main purpose is to conceptualize information
on an elementary level completely independent of any specific contexts in which information is referenced, aggregated and consumed.

Examples of information tokens are 

\begin{itemize}

\item pictures in digital formats such as JPEG or GIF;
\item texts in formats such as ASCII, Postscript or Word; 
\item time dependent tokens such as audio and video sequences in QuickTime or MPEG formats; 
\item software tokens such as JAVA applets or generic sorting functions in source or compiled bytecode or
even native (machine-dependent) code. 

\end{itemize} 

% BEGIN CHANGE

To best serve their role as basic building blocks in value-adding digital libraries, tokens should be unbiased in their representation.  Furthermore they should be 
self-describing, equipped at least with basic methods for presentation, copying, migration,
storage etc. Note that the information tokens may inherently provide further more specific methods. A Word document, for example, may come with methods
for reformatting or generating a table of content.  
For software tokens the basic methods may be compilation, execution or interpretation. 

%END CHANGE


Independent of their content and the platform on which they reside, tokens have to share the following
basic properties:

% BEGIN CHANGE

\begin{itemize} 
\item digital representation
\item atomic view, i.e., context-independent interpretation
\item unconstrained content described by automorphic typing.
\end{itemize}

Automorphic typing allows the basic classification of information tokens, 
stresses their self-contained character of the information tokens and
avoids restriction to predefined formats. 

%END CHANGE

Information tokens have to be handled uniformly, independent of their content, lifetime or location in the network space. Therefore, the following additional properties are requested uniformly over all kinds of information tokens:

\begin{itemize}
\item network-wide, stable identity  
\item universal referentiability via the net.
\end{itemize}

Any specific demands on information which are derived from the needs of individual users or user
groups are captured by the concept of information artifacts.



\subsection{Information Artifacts}

Information artifacts are constructed (recursively) from tokens with the intent to add, step by step, value to the
information on its way from the network of information providers to the individual information consumer. 

The structure of artifact constructing processes is shown in Figure~\ref{Fig:InformationUniverse}.
Information tokens provided by a networked information universe are composed into information artifacts, a
process guided by the two related principles that value be added by information composition, but also by
personalization \cite{RoeWinPae95,RoeMogWin94}. Conceptually, a digital library has manifold artifacts visible to information consumers
with high emphasis on personalized artifacts customized for the consumers preferences and tasks.

\begin{figure*}
\begin{center} 
\leavevmode
\epsfxsize=0.9\textwidth 
\epsffile{Pictures/InformationUniverse.epsf} 
\end{center} 
\caption{Constructing Personalized, Value-Added Information Artifacts}
\label{Fig:InformationUniverse}
\end{figure*}

%BEGIN CHANGE
Typical examples of personalized information artifacts are individual collections of text and image
tokens (references or copies) augmented by hyper-text artifacts and personal annotations.  Artifacts
may be attached to agents which search autonomously for further material on the net by employing text
retrieval systems or SQL engines. Other examples are video sequences augmented by data on title, producer,
costs, list of actors, comments and ratings etc. 
%END CHANGE

Examples of software artifacts are application packages such as Oracle's graphical query tools with GUI and SQL components or Microsoft's application package \e{Office} composed of basic
modules such as Word, Excel, and Powerpoint. 

After all, effective exploitation of information artifacts relies on additional and more traditional
library services \cite{Gra95,CouKetPae+95,Pae96} such as 

\begin{itemize}
\item information filtering capabilities (static and dynamic views) 
\item mobility in heterogeneous environments (platform independence)
\item exchange between libraries (external gateways)
\item persistent storability, multiple user support and recovery (database system support)
\item security and accounting services (authorization and authentication). 
\end{itemize}

However, it is the \e{binding technology} which, in our view, provides
the core capabilities for artifact construction and use -- on the
information as well as on the software level. Therefore, binding
requires particularly strong support in any modern digital library
system.




\subsection{Binding Environments}
\label{SubSec:BindingEnvironments}

% BEGIN CHANGE
On the technical level, extended \e{binding capabilities} are most
crucial for our approach to digital libraries. It is through binding that selected information entities can be
organized into artifacts and that information consumers can access the artifacts of their choice. It is also
bindings which connect information artifacts to software artifacts when value-adding software services are
requested for artifact exploitation. 
% END CHANGE


As outlined above, library entities may be found on any medium, anywhere in the net, privately
owned or shared, short-lived or persistent, mutable or not, etc. Consequently, our demands on binding
capabilities are multiple \cite{MABD90}: Bindings

\begin{itemize}
\item have to be possible between all kinds of entities on an \e{open set of media};
\item must also be complete in the sense that all computational entities,  \e{data}, \e{code} and \e{threads} can
be freely combined by bindings; 
\item may be \e{fixed} over time or \e{mutable} on demand;
\item may refer to \e{transient} or \e{persistent} entities; 
\item may involve \e{local} entities or reach out to \e{remote} sites of the net.
\end{itemize}

Bindings to entities \e{internal} to a system have to be defined and managed as well as \e{external} bindings to
entities spread over heterogeneous systems.  Artifact construction may require both kinds of semantics -- \e{copy
semantics} and \e{reference semantics} -- via bindings; the transition between both has to be supported.



\subsection{Tracing Environments}


People, when working with libraries, usually follow certain patterns of use. Frequently they work top
down, from vague ideas via keywords and catalogues to volumes and individual texts. Library users
work continuously in the sense that they often resume sessions at the point of interrupt some time
earlier. There also exist distinguished modes of operation which people apply when using a library: 

\begin{itemize} 
\item quick look-up of references to dictionaries or texts, or
\item evaluation and memorizing of a concrete text performed in a series of sessions, or
\item construction, maintenance and use of topic-oriented personal reference libraries over years.
\end{itemize}

The individual context in which such patterns are applied varies, of course, with the user and his task
(the who and when, the what and where etc.). With digital libraries we are in a position to trace
library processes on an individual level. We can discover individual patterns behind processes and
provide environments to support them. Individual process execution contexts can be recorded and  the
information gained can be used for improving the library content as well as its usability \cite{BoeRak94,KasShaShe96}. 
 
Typical information gained from process traces answers questions such as

\begin{itemize} 
\item who is involved in artifact construction? when is it constructed? 
\item which digital libraries are consulted to find information? where do they reside? 
\item which queries are posted and what are the results of these queries? 
\item what alternatives or versions exist for artifact construction?
\item which design decisions lead to the artifact?  
\end{itemize} 

Some of this information (see Figure~\ref{Fig:Context}) can only be given explicitly by the person who
constructs the artifact (e.g. information on the why?). Other information can be deduced automatically by
the tracing process attributes (e.g. owner, time), statement sequences (e.g. retrieval operations) or
process bindings (retrieval results). 

\begin{figure*}
\begin{center} 
\leavevmode
\epsfxsize=0.7\textwidth 
\epsffile{Pictures/ContextGeneration.epsf} 
\end{center} 
\caption{Context Information for Information Artifacts}
\label{Fig:Context}
\end{figure*}

\hspace*{3em}

% BEGIN CHANGE
We conclude our vision of digital libraries by contrasting them with
traditional database systems:  databases contain homogeneous data (e.g.
relations) and database architectures provide a centralized view on
data and software (global schema; distribution transparency).
Furthermore, databases are accessed by isolated transactions (ACID
principle). Digital libraries differ substantially from databases in
all three dimensions just mentioned: they access heterogeneous
information represented by multiple models on multiple media; their
architecture is a dynamic and highly personalized construction of
information and services, and their users work with contexted processes
which are traced and evaluated by the library environment. 
Some of these extended requirements of digital libraries are addressed in 
the research area of information integration and heterogeneous databases 
(see e.g. \cite{GarHamIre+95,HamMcL93}).
%END CHANGE

In the following section we discuss some of the linguistic and architectural
demands on software platforms on which digital libraries are best
developed, maintained and used.

