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An application of FPIs

From: Jon Bosak <bosak@atlantic-83.Eng.Sun.COM>
Date: Sun, 8 Dec 1996 19:49:55 -0800
Message-Id: <199612090349.TAA04073@boethius.eng.sun.com>
To: w3c-sgml-wg@w3.org
cc: bosak@atlantic-83.Eng.Sun.COM
In this message I'm going to attempt to fulfill promises made in
earlier postings to the SGML WG by sketching the FPI system we're
putting into place at Sun for Solaris documentation.  The point is to
give a concrete example of a bottom-up FPI resolution system that is
well on its way toward implementation and may serve as an initial test
case for FPI/URN resolution.  Despite the fact that not everyone on my
team at Sun agrees with me that FPIs should be included in XML 1.0,
please note that I am speaking here as an interested party and
consider what I have to say in that light.


The AB2 daemon
--------------

To understand why my publishing group is interested in
location-independent addressing, you have to know a little bit about
how we intend to distribute documents in Solaris 2.6.  In that
release, we are going to replace our old PostScript-based AnswerBooks
with a new SGML-based system.  The new system will be HTTP-based
rather than NFS-based and will deliver data to generic HTML clients
rather than to specialized viewers.  We are still looking for a
properly slick market-oriented name for this system, but at the moment
we are just calling it AnswerBook2, or AB2 for short.

AB2 is implemented as a daemon, ab2d, that is (at administrator
option) installed on a server, along with some set of compiled SGML
documents, and started up when Solaris is booted.  The ab2d daemon is
not a full Web server but just a lightweight HTTP process that sits in
the background and waits for document requests.  When a Web client
makes such a request (using a complex URL driven by a forms-based
interface of our construction), ab2d hands it off to a custom DynaWeb
plugin that turns the complex URL into a query against the compiled
SGML database, retrieves the data, and converts the requested document
-- or by autochunking, an appropriate document fragment -- into HTML
on the way out.  (DynaWeb implementors will understand that the server
may also return a generated table of contents, among other things, and
that it is trivially easy for the same server to generate XML instead
of HTML from the same compiled SGML files if a client identifies
itself as XML-aware.)


A distributed document space
----------------------------

This system has a number of pretty cool advantages, not least among
which is the ability of system administrators to make their own
tradeoffs between performance and disk usage in corporate networks.
Let's suppose that in order to conserve local disk space you have
installed just the basic Solaris user documentation on your
workstation.  The ab2d daemon is running on your workstation, and you
can consult the documentation you've installed there using HotJava,
Navigator, lynx, or whatever HTML browser you happen to have handy.

Now suppose that you want to consult some documentation relating to
the C compiler.  You don't have that available on your system, but you
know that Sally down the hall has installed that on the departmental
server.  If you know the name of that server, a simple request to its
ab2d lets you access that server's list of document resources, and you
can navigate, read, and bookmark the documents available there just as
easily as you can the ones on your own machine.  To find even less
frequently used documentation, you may have to go as far as the
corporate library machine, located at some distant node on the
corporate WAN.  And to get some really obscure piece about an obsolete
version of one of Sun's products, you may have to go through the
Internet to Sun's AB2 server, which has copies of everything we
publish.

But in every case, you are accessing a unified document space.  If
your AB2 installation has been set up correctly (which we make very
easy to do), you are not even aware where specific documents are
located; the only perceptible difference between getting a document
from your own machine or some other machine is a performance
difference resulting from network latency and the difference in CPU
utilization.  If you go clear out to Sun's document repository you
will, of course, notice a perceptible lag due to Internet transmission
delays, but within the kind of corporate network for which this system
is primarily designed, such delays will be far less noticable.  If you
find yourself using a distant collection often enough, you can always
request that it be copied to a machine that is faster or closer to
you, or you can just access Sun's master server directly and download
the compiled SGML files to your own workstation.


Location independence
---------------------

Anyone who has tried to set up a distributed system like the one I've
just described knows that it is virtually impossible to do if URLs are
the only way to identify documents.  What happens to the
cross-reference links between your partial document set and the
compiler documentation if Sally's machine goes down?  What happens to
your bookmarks if she reorganizes the file system on her machine?

We realized very early on in the AB2 effort that the whole idea hinges
on the use of location-independent document identifiers.  Two things
are needed to make the AB2 system work:

1. All documents must be referred to through a location-independent
naming system, and

2. This system cannot depend on centralized name resolution, because
(a) the corporate network cannot be assumed to be connected to the
Internet, (b) an internal resolution server cannot be assumed to be
available at all times, and (c) users and system adminstrators must be
free to install and remove documents from the bottom up, without
having to check things in and out of some central tracking database.
We also wanted our customers to be able to use AB2 to distribute their
own documents with an absolute minimum of organizational overhead.


FPIs and socats
---------------

The most important design decision we made in constructing a
location-independent naming system was a negative one: we were not
going to attempt a global, all-encompassing solution, we were just
going to implement the simplest possible system that could work for us
and our customers.  Consequently, a number of features that one would
want in a more general solution, such as the ability of the system to
find the best possible copy of a resource at any given moment, have
been deliberately omitted from the design.  The assumption of
relatively modest goals has resulted in a system that is simple,
robust, and easily managed.

The whole thing is based on FPIs and on SGML Open catalogs that use
Tauber's proposed DELEGATE extension.  We call such catalogs "socats"
for short.

Whenever a document is checked into our corporate document system, it
is assigned a unique identifier of the form

   -//Sun::SunSoft//DOCUMENT SPARCINSTDESK Version 1//EN

The uniqueness of the identifier is verified by the check-in process
that all publications have to go through on their way to our master
document database.  The same database is referred to by the link
editor that all SunSoft authors use for making links between books,
for example

   <!ENTITY SPARCINSTDESK PUBLIC "-//Sun::SunSoft//DOCUMENT
   SPARCINSTDESK Version 1//EN" NDATA SGML>
   [...]
   <olink TargetDocEnt="SPARCINSTDESK">Installation Instructions for
   Solaris 2.6 (SPARC Platform Edition)</olink>

Thus, all book authoring, management, compilation, and distribution is
done using identifiers that are completely independent of physical
location.  When a book fragment containing the example link above is
finally converted from SGML to HTML at the moment of its transmission
to the client browser, the FPI reference in the olink is translated to
a URL containing encoding that an AB2 server will understand as a
request to resolve the FPI if the user traverses the link.

To resolve the link, all AB2 servers maintain a "local socat"
consisting only of PUBLIC and DELEGATE entries (other entries are
ignored).  The PUBLIC entries provide a lookup table for the FPIs of
all books installed on the local system, and the DELEGATE entries
point to a list of other AB2 servers that have been designated as
alternative sources of information by the system administrator.  The
local socat is automatically updated whenever a documentation package
is installed on or removed from the system.  By default it only
contains one DELEGATE entry, which points to the master document
repository at Sun; a utility allows the system administrator to add or
remove other DELEGATE entries at will, keeping the default entry
always at the bottom of the list (because a reference to one of our
FPIs will always find a match there and never fall through to other
entries).

The local socat is typically cached in server RAM.  If there were a
requirement to solve the global URN resolution problem with this
mechanism, it would obviously fail, but since the local socat consists
only of locally installed books and manually entered DELEGATE entries,
it remains small and fast, even if (as in the case of our own master
document server) it contains entries for every document we publish.

If we were to extend the DELEGATE mechanism to allow multiple levels
of indirection, as suggested in Tauber's original proposal, we could
run into a number of interesting complications, starting with circular
references.  But again, we're not trying to solve the world's problems
with this system, so we feel perfectly comfortable in arbitrarily
limiting the DELEGATE process to just one level.  If an attempt to
resolve a given FPI fails, an AB2 server queries (in list order) the
other AB2 servers pointed to by the DELEGATE entries in its local
socat.  These AB2 servers know how to respond to the special query by
returning their own local socats, but their DELEGATE entries are
ignored.

The result, therefore, is that every AB2 document on the local system
and every AB2 document on systems explicitly pointed to in entries
made by the system administrator is available to the user in a
seamless, unified document space.  In a large network, a careful admin
will make sure to include servers that have duplicate copies of
various document sets.  The order in which to seek alternative copies
of a resource is not determined by some insanely complex algorithm but
simply by the order in which the admin has decided to list them based
on his or her own knowledge of the network environment.

Thus, if the user has bookmarked a publication that typically comes
from a machine at the other end of the building, and that machine
happens to be down at the moment, then the link does not fail but
rather continues down the list until it finds another copy.  If the
user is hooked up to the Internet, then in the worst case the FPI
falls through to the bottom of the list and finds resolution on Sun's
master document repository.  The user will notice a delay, but the
link won't fail, and as soon as the usual machine comes back up again,
performance will return to normal.

There are lots of interesting details that I have left out of this
brief description because the folks on the AB2 team who are
implementing the system plan to tell you all about it at the WWW6
conference in April and at SGML Europe in May.  Suffice it to say that
this system solves our problem and will (we think) solve similar
problems for Solaris customers who wish to use the same infrastructure
for distributing their own documents.


Relevance to FPIs in XML
------------------------

As I stated in an earlier posting, nothing in our scheme requires the
addition of a single line of code to existing HTML browsers or future
XML browsers; the implementation hit on browser vendors is exactly
zero.  In our system this all takes place on a server.

Furthermore, nothing in our scheme requires changes to the existing
XML draft if XML is used only as a delivery mechanism, because XML
generated from our SGML data can use exactly the same generated URLs
that we're using now in HTML generated from that data.  The issue of
including FPIs in XML arises only if XML is going to be used as an
authoring format.  Our current DocBook-based publishing system doesn't
need FPIs in XML, but at some point I would like to use XML as a
migration path for other groups within Sun that are using HTML on an
ad hoc basis to provide documentation for unbundled products.  At that
point, we will need XML to include FPIs as syntactic objects.

Some people (including the AB2 developers!) have pointed out to me
that FPIs need not be included in XML 1.0 to serve Sun's immediate
needs, which is true.  But I do think that they have to be included at
some point if I'm going to be able to use XML as a way to get current
HTML writers into our unified document space, and since the only
requirement for an application like ours is that FPIs have a syntactic
specification, I think that they should be included now rather than
later.

As I made clear at the beginning of this message, my argument is far
from disinterested, but I suspect that the application I've described
is not unrepresentative of other schemes that use FPIs to uniquely
identify documents.

Jon
Received on Sunday, 8 December 1996 22:52:00 EST

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