Stability of schemas -- frequency of versioning

["Costello, Roger L." <costello@mitre.org>]

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