SML References

TAG members:

During a recent face to face discussion with Noah Mendelsohn, he suggested 
that members of the TAG might wish to take note of the extensible means 
provided in SML for encoding references to documents.  In particular, as 
explained in more detail below, SML encourages use of URIs as a standard 
and interoperable means of making such references, but it also provides 
for other forms of reference as well.  This email is being sent on behalf 
of the SML working group to give the TAG a “heads-up” on this question and 
to provide the TAG with the opportunity to voice its feedback. 

As part of the XML Activity of the Architectural Domain, the Service 
Modeling Language (SML) Working Group is proposing markup, including 
extensions to XML Schema, in order to facilitate the use of XML for 
modeling purposes.  The SML specification defines the concept of SML 
references, allowing elements to refer to other elements in the same 
and/or in distinct document(s).  Each SML reference element constitutes a 
single reference that may have multiple alternative syntactic 
representations.  Each of the representations is an instance of an SML 
reference scheme described by a set of rules for recognizing its 
representation in an SML reference element and a set of rules for 
determining the referred-to document and element. The SML specification 
defines one reference scheme, the SML URI reference scheme, to facilitate 
wide interoperability, and allows others to define additional reference 
schemes as an extensibility mechanism. 

The SML URI reference scheme, as defined by the SML specification [1], 
uses a URI to identify the target document of an SML reference; the URI’s 
fragment component is interpreted based on the document’s Internet Media 
Type [2].  The SML-IF specification notes that this is the only SML 
reference scheme likely to result in documents that are widely understood 
[3], as well as the only SML reference scheme that is required to be 
understood by all SML-IF producers:

Use of the SML URI Reference Scheme as defined in the SML specification is 
the only guaranteed way of achieving interoperability for all SML 
references in the model. Use of any other reference scheme requires that 
the consumer know about its use in the document and understand how to 
dereference it.

Since the SML specification allows users or dependent specifications the 
ability to define their own reference schemes, it is possible that non-URI 
document identifiers could be used, with all the implications that 
involves [4].  User-defined reference schemes may prove to be more 
efficient within certain architectures or constrained usage contexts than 
URIs.  Note that the specification makes no claims that the use of such 
user-defined reference schemes will be interoperable over the Web (see 
statement above).  They would be essentially meaningless outside the 
system specifically designed to handle them.  Because we cannot anticipate 
what all of these systems will be like, we feel the need to accommodate 
reference scheme extensibility.
 


[1] SML URI reference scheme 
http://www.w3.org/TR/2008/WD-sml-20080303/#URI_Scheme

[2] Fragment identifiers http://www.w3.org/TR/webarch/#fragid 
[3] SML-IF interoperability 
http://www.w3.org/TR/2008/WD-sml-if-20080303/#interoperability

[4] Architecture of the World Wide Web 
http://www.w3.org/TR/webarch/#uri-benefits


Best Regards, John

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