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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.


   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


   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.

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RFC 1691               CDL Document Architecture             August 1994

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

   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

   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.

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RFC 1691               CDL Document Architecture             August 1994

   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

   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.

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RFC 1691               CDL Document Architecture             August 1994

   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

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.

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RFC 1691               CDL Document Architecture             August 1994

   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

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

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RFC 1691               CDL Document Architecture             August 1994

Physical References File Example

+0|CORNELL|OLINLIB|00000001|Boole, Mary Everest||Philosophy Of Algebra||

|0|1|00000002|5|1||   (File ref. #2 = Phys. ref. #5 = 600dpi TIFF image)
|0|2|00000003|5|2||   (File ref. #3 = Phys. ref. #5 = 100dpi TIFF image)
|0|3|00000004|6|1||   (File ref. #4 = Phys. ref. #6 = 600dpi TIFF image)
|0|4|00000005|6|2||   (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
Document Structure lines (continued)

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

Turner                                                          [Page 6]

RFC 1691               CDL Document Architecture             August 1994

Logical Structure File Example

|0|0|ROOT|0|4|0|0|            Structure 0, ROOT, has 4 logical children
|0|1|PAGES|1|100|0|1|         Str. 1, PAGES, has 100 logical children
|0|2|CONTENTS|2|22|0|1|       Str. 2, CONTENTS, has 22 logical children
                              ...has no physical children
|1|1|Production note|5|0|2|2| 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
|1|2||6|0|2|1|                Str. 6 has no label
|1|3||7|0|2|1|                Str. 7 has 2 physical references
|1|4||8|0|2|1|                Str. 8 is referenced only here
|1|5||9|0|2|1|                Str. 9 is 5th sequential child of PAGES
|2|1|Production note|105|1|0|1|          Str. 105 is a child of str. 2
|2|2|Title page|106|1|0|1|               Str. 106 has 1 logical child
|2|3|Table of contents|107|2|0|1|
|2|4|Chapter 1. From Arithmetic to Algebra|108|6|0|1|
|2|5|Chapter 2. The Making of Algebras|109|4|0|1|
|2|6|Chapter 3. Simultaneous Problems|110|4|0|1|
|2|7|Chapter 4. Partial Solutions...|111|3|0|1|
|2|8|Chapter 5. Mathematical Certainty...|112|3|0|1|
|2|9|Chapter 6. The First Hebrew Algebra|113|8|0|1|
|2|10|Chapter 7. How to Choose our Hypotheses|114|9|0|1|
|2|11|Chapter 8. The Limits of the Teachers Function|115|5|0|1|
|2|12|Chapter 9. The Use of Sewing Cards|116|4|0|1|
|2|20|Chapter 17. From Bondage to Freedom|124|5|0|1|
|105|1|Production note|5|0|2|2|          Str. 5 is a child of str. 105
|106|1|Title page|11|0|2|2|              2nd reference to str. 11

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RFC 1691               CDL Document Architecture             August 1994

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

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RFC 1691               CDL Document Architecture             August 1994


  /<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
                  /<collection name>
                         COLINFO.TXT          Description of collection
                         /<document ID>
                               DOCINFO.TXT    Description of document
                               LOGSTR.000     Logical structure file
                               PHYSREF.000    Physical reference file

   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.

Turner                                                          [Page 9]

RFC 1691               CDL Document Architecture             August 1994

Security Considerations

   Security issues are not discussed in this memo.


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

Author's Address

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

       Phone: 607-255-9098
       Fax:   607-255-9346
       EMail: wrt1@cornell.edu

Turner                                                         [Page 10]