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Re: Permitting non-indirect links



At 12:15 19/1/97 -0800, Joe English wrote:

>Martin, would it serve your needs if instead of providing a
>mechanism for specifying URL components separately, XML
>allowed system identifiers to be specified relative to
>some base identifier, a la FSIs or the HTML3/HTML+ "base"
>attribute?

The HTML Base option works fine for links to locally owned data sets. These
are relatively well catered for as far as shortcuts are concerned. Local
links are also fairly easy to manage - you know their sources and their
likely update cycles, or can arrange to track them. 

What are much more difficult to manage, because you can get no knowledge of
them, are pointers to external data sources. The example I quoted earlier
was NASA's decision to move the site for the Planatery Data System
information, which required all links to the existing data set to be changed
without being able to warn linkers of the change. The SGML Formal System
Identifier use of SOIbase to identify the system source of the relevant file
is a vast improvement here, but if it is used as specified in the standard
you still have to change every entity reference pointing to the site when
the site or directory moves.

One way to alleviate this is to replace the SOIbase with a pointer to a
HyTime location address. Now you only have one point at which you manage one
address for each unique path at the site. You still have problems if the
whole site moves.

What I was trying to get people to look at is what it takes to manage web
links, which introduce a special set of problems. The problem is that the
time for directory name changes is shorter than that for site name changes,
and that the two overlap. (PDS changed site without changing all of its
directory structure, but did introduce some new structures.) The only way I
can see to alleviate this is to set up location ladders so that you can
manage the site name independently from the directory name. For XML I was
suggesting that these names be separated at capture time so that we could
set up implicit location ladders rather than have to use the full power of
HyTime.

----
Martin Bryan, The SGML Centre, Churchdown, Glos. GL3 2PU, UK 
Phone/Fax: +44 1452 714029   WWW home page: http://www.u-net.com/~sgml/