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   RE: [xml-dev] Are people really using Identity constraints specified in

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In a one way flow to a source, the business rules can be 
concentrated in the back end fairly successfully.  In a 
blind exchange between potentially hundreds, thousands, 
or millions of actors/services, this doesn't work.

Without the application design properly scoped, one can't 
make the rules of thumb work.   That is what scaling 
is all about.  As the number of service peers goes up, 
one might want to actually increase the work done by 
the schema, but at some point, one will hit a boundary, 
eg, cultural, legislative, etc., and it will throw an 
exception.  Exceptions are not errors.   They are feedback. 

One does not measure to manage; one measures to improve.


From: Michael Champion [mailto:mc@xegesis.org]

On Aug 23, 2004, at 9:05 AM, Roger L. Costello wrote:
> Would you mind elaborating more upon what you see as
> the role of validation?  Should there be levels of
> validation as I suggested last week?  Or perhaps you
> are suggesting that there should be no validation?

I'm not the Michael you asked to respond, but here's the way I see it.  
"Syntactical" or "structural" validation of the sort that Schemas, 
XForms, or client-side Javascript do has a role in eliminating a 
certain number of mechanical data entry errors, such as leaving out 
required items or not putting numbers in fields that require numbers 
(e.g. phone numbers, dates, etc.)   XML schemas (of one sort or 
another) are potentially useful ways to define such constraints, 
although as Michael Kay's anecdotes remind us, this is hard to do in a 
culture-neutral way, e.g, US postal codes are all numeric, most other 
countries' are not.  But this is at best a rough check on "validity" in 
some business sense.

"Semantic" or "business rule" validation, on the other hand, 
essentially cannot be done without accessing a database / knowledge 
base to make sure that the request being made can be fufilled.  An 
obvious example is credit card validation, and for a variety of 
technological and security reasons it is going to be done towards the 
back end of a dataflow.  Trying to force-fit this into schema 
validation seems more or less hopeless.

One question I've always had is whether it makes sense to bother with 
the structural validation if you are going to have to do the business 
rule validation anyway.  It comes down to a design decision -- since 
it's generally cheaper to recover from errors detected earlier in the 
process than later, if you can catch them early with schema validation, 
go ahead.  But if that is going to be harder to do than it is worth, 
e.g. making the data entry screen  understand the co-occurrence 
constraints between a country code and the format of a postal code, I'm 
not at all sure why one would bother.


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