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   Re: [xml-dev] Are people really using Identity constraints specif ied in

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On Aug 24, 2004, at 12:25 PM, Bullard, Claude L (Len) wrote:

> Some background:  a public safety system can be seen
> as multiple information ecosystems and technologies that
> exchange information to prevent crime, solve crime, and
> predict and mitigate incidents that threaten the public.
> They can be ranked by their relationship to real time
> events, aka, a Call For Service dispatched from a 911
> system (a call center).  Once a CFS event is closed,
> an incident is created in the police records management
> system and/or the court systems.

Just curious ... hypothetically what could/should a public safety 
agency do with a CFS event that didn't conform to some future 
structural contract / schema that it was supposed to conform to?   
Obviously "nyah, nyah, invalid message, let the poor sucker bleed" is 
not the right answer.  On the other hand, letting the bug in somebody's 
procedures or code go un-reported is not a great idea either.  I guess 
this echoes the eternal RSS/Atom debate over draconian error 
processing, and I suspect that all the myriad ways that RSS gets 
ill-formed in practice will be revisited as XML becomes pervasive in 
other situations.

I'm thinking these days that schema validation has a big role to play 
in test-driven development but is of highly doubtful value in 
operational situations, or at least ones in which people can die or 
fortunes be lost if a message that is otherwise meaningful is rejected 
for "mechanical" XML reasons.

Also, to address one of the issues that came up in this thread, I think 
that declarative vs procedural definition of the validation rules is a 
question that is orthogonal to this one.  In general I agree with 
Roger's summary,  but we can imagine "structural" constraints that 
could only be validated procedurally ['the value in field X must be a 
prime number' is the classic, if contrived example]. We can also  
imagine "semantic" constraints that could be validated with a 
declarative rule-based system, and anyway query languages such as SQL 
and XQuery seem to live in the fuzzy middle ground between declarative 
and procedural.


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