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Re: [xml-dev] Stability of schemas -- frequency of versioning


Your points are interesting and form an interesting basis for a 
conversation.  But I disagree that you have been successful in 
persuading me (or anybody else) that a schema is not a good idea for 
describing a type of information that changes frequently.

To me, it's far more important to ask whether the information for the 
domain in question is important enough that it justifies ensuring 
that its form (structure) and content (values) are appropriate.  As 
others have mentioned, many domains are rooted in laws passed by 
Congress and various legislatures, or in regulations promulgated in 
response to such laws.  Consider income tax forms as an extreme 
example.  The IRS surely benefits from being able to accurately 
determine whether a value for line X on form Y is required, optional, 
or prohibited based on the value placed on line A of form B, and from 
being able to determine instantly whether the value entered on line R 
of form S is within the specified range for those values.

Schemas are the primary way of making such determinations, both in 
the XML domain and in the relational domain.  While the cost of 
re-doing all of those schemas every year is obviously high, the 
benefits are vastly greater than the costs (just ask Intuit if you 
have any doubts).

Frequency of change, and even cost of change, are not the proper 
criteria by themselves.  The cost of *not* making the changes is 
certainly another important criterion.  But the most important 
criterion, IMHO, is the value of having correct data as opposed to 
unreliable data.

Hope this helps,

At 11/21/2011 05:58 AM, Costello, Roger L. wrote:
>Hi Folks,
>How frequently should schemas be allowed to change?
>Let "schemas" refer to XML Schema, Relax NG, DTD, or Schematron.
>Let "change" refer to non-backward compatible changes such as 
>requiring a new element.
>I will attempt to persuade you of the following:
>      To be effectively deployed, schemas require a certain amount 
> of stability.
>     That is, they shouldn't change too often. Further, any changes 
> that do occur
>     should be backward compatible.
>That says, for example, that if your domain is Books then the kind 
>of information that goes into Books is stable; if your domain is 
>financial contracts -- swaps, options, futures -- then the kind of 
>information that goes into financial contracts is 
>stable.  Consequently your schemas are stable. Conversely, if your 
>Book or financial contract schemas are constantly changing then your 
>schema development and software development will thrash and users 
>will be constantly confused.
>An example of a rock-solid schema is the XML Schema for XML Schemas. 
>It hasn't changed in 10 years. And the new version is backward 
>compatible with the old. Ditto for the Relax NG schema for Relax NG schemas.
>Suppose, however, that the information for a domain is required to 
>frequently change, say, three times a year. I have attempted to 
>persuade you that a schema may not be a good fit for describing that 
>type of information. But I am at a loss for what is a good fit. What 
>is a good fit?
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Jim Melton --- Editor of ISO/IEC 9075-* (SQL)     Phone: +1.801.942.0144
   Chair, ISO/IEC JTC1/SC32 and W3C XML Query WG    Fax : +1.801.942.3345
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=  Facts are facts.   But any opinions expressed are the opinions      =
=  only of myself and may or may not reflect the opinions of anybody   =
=  else with whom I may or may not have discussed the issues at hand.  =

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