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66th IFLA Council and General

Jerusalem, Israel, 13-18 August


Code Number: 029-142-E
Division Number: VI
Professional Group: Information Technology
Joint Meeting with: -
Meeting Number: 142
Simultaneous Interpretation: Yes

Digital libraries on the Internet

Taly Sharon


Ariel J. Frank

Bar-Ilan University,
Ramat-Gan, Israel
E-mail: taly.ariel@cs.biu.ac.il


The Internet and the Web have been growing in leaps over the past few years, accelerating the problem of information explosion, a well-known phenomenon to all of us. Indeed, search engines (SEs) that have popped up everywhere enable us to access the cyberspace, but they flood us with vast amounts of irrelevant information. Nonetheless, considering the vast amount of information, the Web is considered by many to be the world's ultimate virtual library - but is this solution the right one?


In any case, the Web and the SEs do not substitute the classical, loved libraries. Looking backwards, libraries can be classified into 3 types:
  1. Analog/Paper Library (PL) - the classical paper library with its card catalog.

  2. Automated/Hybrid Library (AL) - an analog library with a computerized catalog.

  3. Digital Library (DL) - a computerized library in which most of the information is digital.

The problems of our regular libraries are well known and need not be detailed here. On the other hand, it is less clear to us what a digital library is and what are its various characteristics.

First, we classify the digital libraries into three categories:

  1. Single Digital Library (SDL) - the regular classical library implemented in a fully computerized fashion.

  2. Federated Digital Library (FDL) - this is a federation of several independent libraries, centered on a common theme, on the network.

  3. Harvested Digital Library (HDL) - this is a virtual library providing summarized access to related material scattered over the network.
Consequently, we compare the various types of libraries and focus on a comprehensive comparison between HDLs and SEs on the Web. To demonstrate, we show exemplary digital libraries. In particular, we mention the Katsir HDL, based on the Harvest system, which is currently being developed in Bar-Ilan University.

1 Introduction

The Internet and the Web have been growing in leaps and bounds over the past few years, accelerating the problem of information explosion, a well-known phenomena to all of us. According to Nature 1, the publicly indexable Web contains an estimated 800 million pages as of February 1999. Indeed, the growing amount of Search Engines (SEs) that have popped up everywhere, reaching more than 2400 different SEs, enable us to access the cyberspace, but they also flood us with vast amounts of irrelevant information. Search engine coverage, relative to the estimated size of the publicly indexable Web, has recently decreased substantially, with no engine indexing more than about 16% of the estimated size of the publicly indexable Web 1.

The article is structured as follows. This section presents the resource repository hierarchy, defines the notion of a library and the development from paper to digital libraries. The following section classifies digital libraries, compares between the different types and introduces the logical harvesting model. We conclude with a discussion.

1.1 Resource Repositories Hierarchy

Both SEs and Digital Libraries (DLs) are Internet Resource Discovery (IRD) Tools. We introduce a resource repositories hierarchy with two major paradigms: search engines and digital libraries, where each branches to categories. SEs can be classified into three categories: Basic-SE, Directory, and Meta-SE. All the categories support search user interfaces, but with significant differences in their construction method:
  1. Basic-SE/Index - a tool that uses an automatic robot/crawler to gather metadata on items.

  2. Directory/Catalog/Guide - a tool that uses human judgement to collect and catalog items.

  3. Meta-SE - a tool that holds no database of its own, but rather queries Basic-SEs upon a user request.

A detailed discussion about digital libraries, including DL categories, will be presented in section 2.

1.2 What is a library

Before we delve into digital libraries, we define the notion of a library in general and of a digital library specifically. We define a library as having six major characteristics:
  1. Collection of data objects - A library holds a collection of data objects, also called holdings, items, resources, or just material. The items can be: books and journals, documents (e.g., HTML pages), and multimedia objects (such as pictures or images, tapes or video files, etc.). The library objects can be available locally in the library, or indirectly, by using a network to access them.

  2. Collection of metadata structures - A library contains a collection of metadata structures, such as catalogs, guides, dictionaries, thesauri, indices, summaries, annotations, glossaries, etc.

  3. Collection of services - A library provides a collection of services, such as: various access methods (search, browse, etc.) for different users, management of the library, logging/statistics and Performance Measurement Evaluation (PME) and Selective Dissemination of Information (SDI) or as called Push mode.

  4. Domain focus - A library has a domain focus and its collection has a purpose. For example: art, science, or literature. Also, it is usually created to serve a community of users, and therefore is finely grained. For example: academic, public, special, school, national, or state library.

  5. Quality control - A library uses quality control in the sense that all its material is verified and consistent with the profile, or stereotype, of the library. The material is filtered before it is included in the library, and also its metadata is usually enriched (e.g., annotated), etc.

  6. Preservation - Libraries and archives have served as the central institutional focus for preservation, and both types of institutions include preservation as one of their core functions. The purpose of preservation 2 is to ensure protection of information of enduring value for access by present and future generations. Preservation includes regular allocation of resources for persistence, preventive measures to arrest deterioration of materials, and remedial measures to restore the usability of selected materials.

1.3 From Paper to Digital Libraries

In any case, the Web and the SEs do not substitute the classical, loved libraries. Looking backwards, libraries can be classified into 3 types 3:
  1. Paper/Analog Library (PL) - the classical paper library with its card catalog.

  2. Automated/Hybrid Library (AL) - a paper library with a computerized catalog.

  3. Digital Library (DL) - a computerized library in which most of the information is digital.
No one questions or disputes the long and lasting contribution of existing classical libraries 4. The concept of the paper library and the various services it provides are well established. The idea is that DLs should provide all these and more 5 6. We use the term 'integrated services' in DLs to allude to that. These integrated services will add services that are made possible by use of the digital medium such as: varied search techniques resulting in focused results, faster provision of relevant resources, and access also to multimedia resources.

The problems of our regular libraries are well known and need not be detailed here. On the other hand, it is less clear to us what a digital library is and how it works - this is the subject of this paper.

2 Digital Libraries

2.1 Classifying DLs

We classify the digital libraries into three categories: Stand-alone Digital Library (SDL), Federated Digital Library (FDL), and Harvested Digital Library (HDL). We now detail:

1) Stand-alone Digital Library (SDL)

This is the regular classical library implemented in a fully computerized fashion. SDL is simply a library in which the holdings are digital (scanned or digitized). The SDL is self-contained - the material is localized and centralized. In fact, it is a computerized instance of the classical library with the benefits of computerization. Examples of SDLs are the Library of Congress 7, and the Israeli K12 Portal Snunit 7.

2) Federated Digital Library (FDL)

This is a federation of several independent SDLs in the network, organized around a common theme, and coupled together on the network. A FDL composes several autonomous SDLs that form a networked library with a transparent user interface. The different SDLs are heterogeneous and are connected via communication networks. The major challenge in the construction and maintenance of a FDL is interoperability (since the different repositories use different metadata formats and standards). Examples of FDLs are the NCSTRL 9 and NDLTD 10.

3) Harvested Digital Library (HDL)

This is a virtual library providing summarized access to related material scattered over the network. A HDL holds only metadata with pointers to the holdings that are "one click away" in Cyberspace. The material held in the libraries is harvested (converted into summaries) according to the definition of an Information Specialist (IS). However, a HDL has regular DL characteristics, it is finely grained and subject focused. It has rich library services, and has high quality control preserved by the IS, who is also responsible for annotating the objects in the library. Examples of HDLs are the IPL 11 and WWW Virtual Library 12.

2.2 Comparison

To emphasize the different aspects of this DL categorization, let us get into the various DL types. In SDL and FDL, the items are electronically purchased or fully digitized/scanned. These items are stored in the local repository (in SDL), or in separate SDL repositories accessed using a network protocol (in FDL). Each SDL holds a huge repository containing both the items and some metadata structures to enable efficient retrieval. This material is updated every now and then, in a process similar to the one in classical library. It is important to note that composing a FDL out of SDLs requires interoperability capabilities, and the use of a common protocol.

In contrast to the SDL and FDL, the HDL's items are gathered from the network. These items are scattered on many servers, and accessed via direct retrieval using standard protocols such as HTTP, FTP, etc. The HDL holds only metadata on the items, and therefore its repository is small and compact. Because the items that belong to the HDL can be updated any time by their authors, their summaries have to be dynamically refreshed in the HDL using computerized procedures that are triggered automatically or initiated explicitly by the IS. An interesting point is that the profile of a HDL can be changed by the IS to enhance the library contents.

2.3 Harvesting Model for HDLs

We will now describe our developed logical model for constructing HDLs 13. The model includes processes, data repositories and auxiliary repositories. The initiating IS invokes the Harvester with the DL harvesting request. The Harvester generates the initial DL profile and passes this as the harvesting query to the Locator component. The Locator uses various network search techniques to enrich the initial collection of URLs to be harvested. The next component to be invoked is the Gatherer. It uses each top-level URL, in a recursive manner, to gather all referenced resources from the network providers, and passes them to the Filtering component.

The Filtering component is responsible for blocking the non-relevant documents from reaching the focused DL. It uses various levels of filtering that all remaining documents have to pass to be considered relevant. A first level, for example, can use 'regular expressions' to match query keywords with the URL string tokens. A second level can use statistical techniques on the document itself, based on keyword counts and frequencies. A third level might use a Categorizer to classify the document and check if it belongs to the gathered DL categories. More levels or any geared combination of levels can ensure a cleaner DL devoid of 'noises'. All relevant documents are passed now to the Summarizer. It extracts a summary of the document, and passes a stream of summaries to the Broker. The Broker indexes the summaries and organizes the DL. The IS builds for the DL a relevant topics-tree, possibly using advanced IR tools for categorization and clustering. The Retriever provides the DL user with a user-friendly interface.

2.4 Implementation of Harvest/Katsir System

To demonstrate HDLs, we mention the Katsir HDL 13, based on the Harvest system 14, an initial/partial implementation of the harvesting model for HDL 15. Katsir is currently being developed in Bar-Ilan University (BIU) as cooperation between the Mathematics and Computer Science department and the department of Information Studies.

3 Discussion

Considering the vast amount of information, the Web is considered by many to be the world's ultimate virtual digital library - but is this solution the right one?

We already confronted SEs with DLs to realize the differences between them. We can compare the different categories of SEs with the different categories of DLs. Basic-SE is similar to all DLs in the basic user interface, IR tools, and network access. Furthermore, Basic-SE is similar to HDL since they both hold metadata repositories rather than full items. A directory is even closer to a DL than a Basic-SE, since it is humanly compiled and therefore has quality control. But lets not forget it does not have domain focus and DL integrated services. Meta-SE is somewhat similar to FDL in the sense that both generate on-the-fly queries to other SEs/DLs to answer user queries. We elaborate more on the differences between SEs and DLs in the next section.

3.1 Search Engines vs. Digital Libraries

The SE paradigm and the DL one are really located at the extremes of a spectrum of data repositories and types of search. There are two sides to each of these coins: the data repository construction side and the user information search side. We will now discuss and contrast these aspects.

As regards to the construction of SEs, this is a complex undertaking. It is clearly a long-term effort that is (eventually) supported by commercial companies. The SE aims to build a quantitative global repository that represents as much information available on the Internet as possible or at least a large amount of it. The SE maintains various data structures, to represent its repository, like indices, directories, and catalogs. It also provides an elaborate user interface for search purposes. The SE continuously employs various types of robots to search out and index pages on the Internet and to dynamically update its provided repository.

Let's look now at the user side of SEs. Assume that a user needs some information on a certain topic. So he summons on a whim of a second his favorite SE to search for any relevant information. The SE is invoked with an ad-hoc query composed of a supposedly appropriate combination of keywords. The SE will certainly return a lot of information (with low precision and recall), which is bound to overload the user. He will then have to tediously sift through it all and manually filter the supplied references. The relevant information found will then be immediately used or temporarily kept in a cache for a short-term period of use.

Consider now the process of harvesting (i.e., constructing) a DL. A user, say an Information Specialist (IS), realizes a well-thought out need to build a qualitative data repository on an important focused topic. He decides to invest by harvesting and maintaining a long-term DL, described by a set of specific categories. So he interacts with an IS interface to carefully define his DL harvesting request. The DL is then harvested and made available to its users. It supports transparent user access methods using various data structures to enable efficient keywords search, touring a DL via a topics tree, and DB/SQL oriented views of the DL contents. The contents of the DL are continuously kept current and the DL can be annotated and enhanced with additional relevant material.

Let's check now on the use of DLs. A serious user will tend to often need information on a topic included in his areas of interest. There is a good chance then that he already has access to a relevant DL, previously harvested. So he invokes the high-level DL interface and chooses an appropriate way to search this DL. The DL will return a reasonable amount of information (with high precision and recall) that the user can readily digest. The returned results will be made available at three levels of detail: first, a high-level summary (metadata) 16 17; then, if requested, an additional abstract; and finally, if relevant, the referenced resource itself will be fetched and presented. Not much sifting will be necessary in any case. The relevant information can be further annotated by the user and later rediscovered whenever needed.

So, to summarize, SEs necessitate a huge organizational effort, provide the user with too much noisy information, but are useful for a one-time shot for quickly needed information. DLs, on the other hand, require a modest support effort, provide the user with focused information, but have to be made available beforehand while excelling in quality and ease of use. It is important to note that these two paradigms are neither conflicting nor exclusive, but are complementary in nature.

4 Conclusion

Digital libraries and search engines on the Internet are similar in many ways yet differ in others. The direction they are all going seems somewhat alike, yet, more research should be carried out to determine the real trends. Further research can also probe into the versatile types of SEs and DLs and their generations. More exploration into additional aspects, like multimedia, composition of libraries, and DL profiles should take place to promote these issues for the benefit of the millions of users surfing the net.


1 Lawrence S., Giles L., Accessibility and Distribution of Information on the Web, Nature, 400, 107-109, 1999, http://www.wwwmetrics.com/.

2 Hedstrom M., Digital Preservation: a Time Bomb for Digital Libraries, http://www.uky.edu/~kiernan/DL/hedstrom.html.

3 Chen H., A. L. Houston, "Digital Libraries: Social Issues and Technological Advances", Advances in Computers, Academic Press, vol. 48, pp. 257-314.

4 Tock S., Can the Library Survive in a Digital Age?, http://comm1.uwsp.edu/302x/jan98projects/tock.htm.

5 Kessler J., Internet Digital Libraries, Artech House, Boston, 1996.

6 Lesk M., Practical Digital Libraries, Morgan Kaufmann, San Francisco, 1997.

7 Library of Congress (LC) and its National Digital Library (NDL), http://www.loc.gov/.

8 The Israeli K12 Portal Snunit http://www.snunit.k12.il/.

9 Networked Computer Science Technical Reference Library (NCSTRL), http://cs-tr.cs.cornell.edu/.

10 Networked Digital Library of Theses and Dissertations (NDLTD), http://www.ndltd.org/.

11 Internet Public Library (IPL), ,http://www.ipl.org/.

12 WWW Virtual Library, http://www.vlib.org/.

13 Katsir, http://bicsir.cs.biu.ac.il:8088/katsir/.

14 Based on the Harvest system, http://www.tardis.ed.ac.uk/harvest/.

15 Hanani U., A. Frank, Intelligent Information Harvesting Architecture: an Application to a High School Environment, Online Information 96, London, December 1996, pp. 211-220.

16 Lassila O., Web Metadata: A Matter of Semantic, IEEE Internet Computing, July/August 1998, 2, 4, 30-37.

17 Rust G., Metadata: the Right Approach, D-Lib Magazine, July/August 1998, http://www.dlib.org/dlib/july98/rust/07rust.html


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