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Re: Stability of schemas -- frequency of versioning

• From: Mike Sokolov <sokolov@ifactory.com>
• To: "Costello, Roger L." <costello@mitre.org>
• Date: Mon, 28 Nov 2011 08:37:56 -0500

Since Roger hasn't yet weighed in with a summary of this informative 
discussion, here's what I gleaned:

Schema change can be managed with careful planning and coordination, but 
schemas are best at representing constraints that change slowly, and in 
an accretive fashion.

Constraints that are fluid are best thought of as "value constraints" 
and can be imposed using value-based validation systems like Schematron.

If you insist on using schemas for things they're not good at, you might 
end up eating sand barefoot.

-Mike

On 11/21/2011 07:58 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?
>
> /Roger
>
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• References:
  • Stability of schemas -- frequency of versioning
    • From: "Costello, Roger L." <costello@mitre.org>