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     The Xanadu Hypertext System is a tool for storing, retrieving, and
organizing information, although it is not a "database management system" in
the traditional sense of the term.  It differs both in its form and its
function.

     In the Xanadu System, information is stored in the form of documents:
letters, memos, articles, engineering specifications, scribbled notes,
textbooks, manuals, computer programs, contracts, reports, and so on.  These
can vary greatly in their size and in the nature and complexity of their
internal structure.  This contrasts sharply with database management systems,
where the information stored consists of aggregates of records holding chunks
of data in fairly fixed formats and in fairly homogeneous arrays.  In short,
DBMS's store lists of items, where every item in the list has the same
structure.  Clearly, these two forms of information storage have different
purposes and are appropriate for different kinds of information.

     DBMS's arose out of traditional record keeping systems in which
information was stored on paper in the form of ledgers, lists, card files,
etc., and the storage, retrieval and update functions were performed by hand,
often by armies of clerks.  As these functions were computerized, it was
discovered that the computer made possible a variety of capabilities that had
not previously been considered.  In addition to allowing vastly larger bodies
of data to be accessed and manipulated far faster and more accurately than
ever before, entirely new functions could be performed.  It was found that if
the data was stored according to certain kinds of generalized schemes, one
could impose different patterns of organization on the same body of data,
allowing appropriately different organization schemes to be used for different
applications.  Also, it was realized that one could store information about
the relationships between different chunks of data, in addition to storing the
data itself, and then manipulate this relational information as well, using
the very same techniques.  One could even store relational information about
other relational information, ena-bling the most general organizational
structures to be built.

     Today, DBMS's are a standard tool for handling information.  Research
goes on to make them ever more powerful, efficient, and general-purpose.
Tomorrow's DBMS's will be even more sophisticated.

     Techniques for handling information in the form of documents, however,
have not kept pace with database technology.  While there are numerous systems
available which allow one to store, retrieve, and to some extent manipulate
document type data using a computer, systems that allow one to organize and
interrelate this information, in the sense that DBMS's allow one to organize
and interrelate bulk data, have been crude to non-existent.  The Xanadu
Hypertext Systems ends this state of affairs.

     The Xanadu Hypertext System is the first and so far the only effective
system for document management which allows one to organize and interrelate
document type information in a totally general and application independent
fashion.


This is what the Xanadu Hypertext System can do:

     --It can store, retrieve, and update documents on command.

     --On command, it can create explicit linkages between arbitrary sets
of arbitrary portions of arbitrary documents and other linkages.  It can
retrieve these linkages at will, including information about what is being
linked to what and about the nature and type of the link itself.  It can
correctly maintain these linkages in the midst of changes to the very data
being interlinked.

     --It can maintain multiple versions of any given document, efficiently
storing the common portions in common, with separate portions of storage
required only for those parts of the documents which are actually different.
It can provide historical traceback information in dated chronological order
about any and all changes to a given document, for purposes of error recovery
or auditing.

     --It can do all of this efficiently, with logarithmic overhead in terms of both storage space and response time.  This means that storage requirements
will grow as a constant multiple of the volume of data stored (as opposed to
many systems of various sorts in which storage overhead increases much faster
than the volume of data does).  Response time will remain effectively constant
no matter how vast the body of stored documents may become.  It can make
effective use of ultra-high density write-once storage technology, such as
video disks. It can effectively and efficiently operate in a totally
distributed storage environment in which the data is spread over a network of
intercommunicating computers which may be located all over the country or all
over the world.

     That is what the Xanadu Hypertext System can do.  No other system is
available that can do these things.  Quite probably none will be for some time
to come.
