Response to Simon St.L. on Entities v. XLL

David G. Durand dgd at
Tue Dec 2 03:06:01 GMT 1997

>	From: Peter Murray-Rust <peter at>

>XML(SGML) entities (NOTATION) have traditionally used PUBLIC and FPIs
>(Formal Public Identifier) for adding type information. This works if there
>is a registry of FPIs for this purpose. Without it is not much use. My
>impression - and I'm happy to be corrected - is that there are few
>useful FPIs for Typing objects.

This is a real problem. Steve DeRose and I provided a list of PUBLIC
IDs for a variety of data formats in our book Making Hypermedia Work.
We did this because of the lack of FORMAL public identifiers. We based
our IDs on the ISBN of the book, so that we met the letter as well as
the spirit of the ISO rules. Some have actually used these IDs.

>Using a SYSTEM Id is subject to the problem of permanence and
>uniqueness of URLs.

The proposal to use MIME types for NOTATION system IDs failed, but you
could easily make a URL that contained the MIME type, and would thus
be easy to resolve for the knowledgeable, without actually following
the URL. The URL itself could even be a CGI script returning a page
describing the convention (and possbily the mime type), for those who
did resolve it.

for example:

could return a document saying "use the 'text/application' mime type
for the referenced entity".

>>*  The XLL mechanism (well, I should say the MIME mechanism really) is
>>based on the entity being self-identifying as to type (aided by
>>any additional attributes you like on the linking element).
>Unfortunately, not all targets of XLL HREFs will be self-identifying. This
>is true of local files and not-very-smart-servers.  It is therefore useful
>for the author to be able to add MIME types to the target.
>As yet, MIME is not part of the XLL mechanism. I wish it was, and keep
>squeaking for it. If it isn't I suggest we use XDEV:MIME as a FUA
>'frequently used attribute' in XML-LINKs.

I don't understand why (if you are putting the information in the
source document) you don't simply use NOTATION, which works very well
with XLL without the need to invent your own private attribute convention.
SGML entity declarations allow the association of a type with a
destination in the source document. Untyped XLL links should only be
used in cases (and they exist) where the HTTP MIME type information is
dependably available and thus preferable to static in-document declaraions.

     -- David

David Durand                 dgd at| david at
Boston University Computer Science        | Dynamic Diagrams  |
                                          | MAPA: mapping for the WWW

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