Network Working Group W. Turner
Request for Comments: 1691 LTD
Category: Informational August 1994

The Document Architecture for the Cornell Digital Library

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Abstract

This memo defines an architecture for the storage and retrieval of
the digital representations for books, journals, photographic images,
etc., which are collected in a large organized digital library.

Two unique features of this architecture are the ability to generate
reference documents and the ability to create multiple views of a
document.

IntrodUCtion

In 1989, Cornell University and Xerox Corporation, with support from
the Commission on Preservation and Access and later Sun Microsystems,
embarked on a collaborative project to study and to prototype the
application of digital technologies for the preservation of library
material. During this project, Xerox developed the College Library
Access and Storage System (CLASS), and Cornell developed software to
provide network access to the CLASS Digital Library.

Xerox and Cornell University Library staff worked closely together to
define requirements for storing both low- and high-resolution
versions of images, so that the low-resolution images could be used
for browsing over the network and the high-resolution images could be
used for printing. In addition, substantial work was done to define
documents with internal structures that could be navigated. Xerox
developed the software to create and store documents, while Cornell
developed complementary software to allow library users to browse the
documents and request printed copies over the network.

Cornell has defined a document architecture which builds on the
lessons learned in the CLASS project, and is maintaining digital
library materials in that form.

Document Architecture Overview

Just as a conventional library contains books rather than pages, so
the electronic library must contain documents rather than images.
During the scanning process, images are automatically linked into
documents by creating document structure files which order the image
files in the same way the binding of a book orders the pages. Thus,
the digital book as currently configured consists of two parts: a set
of individual pages stored as discrete bit map image files, and the
document structure files which "bind" the image files into a
document. In addition, a database entry is made for each digital
document which permits searching by author and title (i.e.,
bibliographic information). Beyond the order of the pages, the
arrangement of a physical book provides information to readers. The
title page and publication information come first; the table of
contents usually precedes the text; the text is divided into sections
or chapters; if there is an index, it follows the text. The reader
often refers to these components of a book when browsing the library
shelves, in order to determine whether to read the book.

The document structure provides direct access to the components of an
electronic document, storing the information that would otherwise be
lost when the book is disbound for scanning.

Document Architecture Requirements

Listed below are the requirements that were initially set down for
the Cornell Digital Library Architecture.

1. The architecture must be open (i.e., published and freely
available).

2. The architecture should be as simple as possible (to facilitate
product development).

3. The architecture should assume data storage in UNIX file systems.

4. The architecture should allow for standard data usage, such as via
FTP and Gopher servers (i.e., pages of a document must exist in a
single Directory, and the naming convention used must order them
in the standard collating sequence, such as the series "0001.TIF,
0002.TIF,..., 0411.TIF" (NOTE: a series such as "1.TIF, 2.TIF,...,
10.TIF" would be ordered "1.TIF, 10.TIF, 2.TIF, ..." which is not
acceptable).

5. The architecture should provide for storing the same information
in different formats. For example, when a page of a document is
available at several different resolutions.

6. Low-resolution "thumbnail" images of each page must be stored to
facilitate browsing and sharing of data.

7. The architecture must support distribution of files so that
similar files may be stored together, permitting optimization of
storage use and performance.

8. The architecture must support documents that are composed of
references to all or part of other documents.

9. The architecture must support document components which are
stored on separate servers distributed across the network.

10. The architecture must support not only an hierarchical structure
for each document, but the ability to define multiple views of
each document.

11. The architecture should accept, rather than dictate, directory
structures in which documents will be stored. This will permit
documents created in other ways to be added to the Digital
Library simply by adding database information rather than by
copying or moving files.

Document Architecture Description

A digital library consists of a Digital Library Server, networked
storage, and a referencing database. A single digital library will
contain one or more collections. Each collection will contain one or
more documents.

The referencing database allows searching for documents by author,
title, and document ID. In the current implementation, the
referencing database is a relational SQL database, and each
collection is epresented by a table in the database. It is planned
to migrate to Z39.50 database searching as the preferred method, as
this protocol has been established as the standard for library
applications.

Authorization will be primarily collection-based, although the design
will permit authorization checking at any level down to the
individual file. Notification would come only when the patron
attempted to open the document or access the particular component.

Each document consists of three components: the logical structure;
the physical references; and the data files.

The logical structure is a logical description of the document.
Conceptually, a document is a tree, with the leaves being the data
files (pages). At a minimum, all documents have a logical structure
which lists the pages in the document and the order in which they
appear. Usually, documents will have a more elaborate structure.
The logical structure relates the logical structure of a document to
the physical references which make up the document.

These physical references map the lowest levels of the document's
logical structure (the leaves of the tree) to the files that contain
the data. Where there are multiple representations of a page, such
as images at various resolutions, these are linked together in the
physical references file.

The data files contain the data making up a document. Any format can
be accommodated: image files, ASCII text, PostScript, etc. However,
one-to-one correspondence between data files for a given physical
reference is assumed. That is, if there are multiple file types for
a single page, these files should represent exactly the same
information.

Physical References File

The Physical References file is the component of the document which
relates logical structures (logical components of documents) to
physical files. Document references, by which a document can be
composed of all or part of other documents possibly residing on
different servers, are handled in the Physical References file.

A document may contain multiple document objects, each of which
contains one or more data objects. When a document contains actual
physical data (for example, it is created by scanning or importing
images), a Master Document Object is created. When a document
incorporates components of other documents, a Reference Document
Object is created for each of the other documents. The Document
Objects are numbered with internal reference numbers, which are
included in the corresponding Data Object lines.

Data Object lines include the Document Object number, the file
reference number, and the file type. The Document Object number
refers to a Document Object line, from which the library name,
collection name, and document ID can be retrieved. The tuple

<libraryID>+<collectionID>+<documentID>+<filetype>+<file reference>

is guaranteed to locate a file. Each Data Object line refers to a
single file; where multiple file types of a single document page
exist, there will be multiple Data Object lines for that page.

In the file, all Document Object lines will preceed all Data Object
lines for a given document. Document Object lines may be either
grouped together at the beginning of the file, or may immediately
preceed the first Data Object line for the Document Object. Document
Object lines will appear in order by Document Object number. Data
Object lines will appear in order by sequence number, NOT by Document
Object number.

The fields in the Physical References file are delimited by vertical
bars.

Document Object Lines

Field Description Comments
----- ---------------------- ----------------------------
1 Document Object number 0 => Master Document Object
1-9 => Reference Document Object
2 Library name Server name
3 Collection name
4 Document ID 8-digit number
5 Author name
6 Volume
7 Title
8 Edition

Data Object Lines

Field Description Comments
----- ---------------------- ----------------------------
1 Document Object number Corresponds to above
2 Sequence number
3 File reference Reference number used to locate
file in filing system
4 Physical reference number Equal to Logical Structure file
5 File type 1 = TIFF 600dpi
2 = TIFF thumbnail
3 = ASCII version of page
(i.e., OCR output)
4 = ASCII notes
5 = Other
6 = TIFF 300dpi
6 Note

Physical References File Example

+0CORNELLOLINLIB00000001Boole, Mary EverestPhilosophy Of Algebra

010000000251 (File ref. #2 = Phys. ref. #5 = 600dpi TIFF image)
020000000352 (File ref. #3 = Phys. ref. #5 = 100dpi TIFF image)
030000000461 (File ref. #4 = Phys. ref. #6 = 600dpi TIFF image)
040000000562 (File ref. #5 = Phys. ref. #6 = 100dpi TIFF image)

Note that in the above, it is guaranteed that file references 2 and 3
are two different versions of the same page, as are file references 4
and 5.

Logical Structure File

The Logical Structure file is the component of the document structure
which offers "views" of a document and links images together
logically to define documents. The file is actually an unloaded tree;
when a document is "opened", the file is read and the tree
reconstructed. By convention, all Logical Structure files contain one
logical structure "PAGES" which defines the document by listing the
pages in the order in which they appeared in the original document.

Document Structure lines

Field Description Comments
----- ---------------------- ----------------------------
1 Parent structure number Structure is a child of...
2 Sequence number
3 Logical Structure name Label for this structure
4 Structure number Equal to Physical Reference file
5 Logical Children # of logical children of this
structure
Document Structure lines (continued)

Field Description Comments
----- ---------------------- ----------------------------
6 Physical Children # of physical children of this
structure
7 References # of references to this
structure within this document
(for how many structures is this
a substructure)

Logical Structure File Example

00ROOT0400 Structure 0, ROOT, has 4 logical children
01PAGES110001 Str. 1, PAGES, has 100 logical children
02CONTENTS22201 Str. 2, CONTENTS, has 22 logical children
...has no physical children
...
11Production note5022 Str. 5 is child of structure 1
...has a label "Production note"
...has no logical children
...has 2 physical references
...is referenced twice in this document
126021 Str. 6 has no label
137021 Str. 7 has 2 physical references
148021 Str. 8 is referenced only here
159021 Str. 9 is 5th sequential child of PAGES
...
199103022
1100104022
21Production note105101 Str. 105 is a child of str. 2
22Title page106101 Str. 106 has 1 logical child
23Table of contents107201
24Chapter 1. From Arithmetic to Algebra108601
25Chapter 2. The Making of Algebras109401
26Chapter 3. Simultaneous Problems110401
27Chapter 4. Partial Solutions...111301
28Chapter 5. Mathematical Certainty...112301
29Chapter 6. The First Hebrew Algebra113801
210Chapter 7. How to Choose our Hypotheses114901
211Chapter 8. The Limits of the Teachers Function115501
212Chapter 9. The Use of Sewing Cards116401
...
220Chapter 17. From Bondage to Freedom124501
221Appendix125211
222advertisements126412
1051Production note5022 Str. 5 is a child of str. 105
1061Title page11022 2nd reference to str. 11
1071715022
1072816022
...
1264104022

Implementation Details

The tuple <library ID>+<collection ID>+<document ID>+<filetype>+
<file reference> is guaranteed to locate a file. A file locator
program will translate between this tuple and the fully-qualified
path and file name in the underlying file system. While a library
will always have a hierarchical nature corresponding to UNIX file
systems, the order of the hierarchy will be flexible to accommodate
optimization efforts. Each level of the hierarchy will have an INFO
file that describes the order of the lower levels of the hierarchy.
The file locator program will read these files as it navigates the
directory structure of the file system when a library, collection, or
document is opened. Two examples follow:

Example 1. Hierarchy is LIBRARY, COLLECTION, DOCUMENT, FILETYPE.

/<library name>
LIBINFO.TXT Description of library
/<collection name>
COLINFO.TXT Description of collection
/<document ID>
DOCINFO.TXT Description of document
LOGSTR.000 Logical structure file
PHYSREF.000 Physical reference file
/<filetype1>
00001.TIF
00002.TIF
...
/<filetype2>
00001.TIF
00002.TIF
...

Example 2. Hierarchy is LIBRARY, FILETYPE, COLLECTION, DOCUMENT.

/<library name>

LIBINFO.TXT Description of library
/<filetype1>
/<collection name>
COLINFO.TXT Description of collection
/<document ID>
DOCINFO.TXT Description of document
LOGSTR.000 Logical structure file
PHYSREF.000 Physical reference file
00001.TIF
00002.TIF
...
/<filetype2>
/<collection name>
COLINFO.TXT Description of collection
/<document ID>
DOCINFO.TXT Description of document
LOGSTR.000 Logical structure file
PHYSREF.000 Physical reference file
00001.TIF
00002.TIF
....

This implementation involves some redundancy, but it permits complete
copies of a collection to be mounted on different file systems for
performance considerations. In particular, the second scheme would
facilitate storing all low-resolution images on high-speed magnetic
disk for fast access, and all high-resolution images on slower, less
eXPensive storage. This will also facilitate authorizing access to
low-resolution images by other software systems (FTP, Gopher) while
restricting access to high-resolution images.

Security Considerations

Security issues are not discussed in this memo.

References

[1] Turner, W., "Cornell Digital Library Document Architecture,
Version 1.1 - 3/22/94", Library Technology Department, Cornell
University.

Author's Address

William Turner
Library Technology
502 Olin Library
Cornell University
Ithaca, NY 14853

Phone: 607-255-9098
Fax: 607-255-9346