Re: The <any/> element: bane of security or savior of versioni
At 04:39 PM 2007-10-19 -0700, Thomas Lord wrote: >..... >Too late for those systems already failing but the solution >is to impose a discipline of language versioning. Let's suppose >we have a schema that (today) *strictly* defines a language >X (no "any" foo - no handling of future updates). Tomorrow, >someone invents the similarly strict language Y and we >all realize "X should become Y!". There's a difference between saying "X should span a set of X-schemas that are proper elements of Y" and "X should become Y". The whole idea is to preserve the validity of X-instances in the space of Y, which is expected to be defined in such a way as to be a proper superset of X. The "problem" comes only when Y misses out on certain aspects which makes it fail in certain ways to include certain X-instances as proper elements. So it is not just a translation issue. >To make Y the next version of X, we should be obligated to >define two transforms: one that converts X to Y, the other >for Y to X. Economically, the need is almost always to convert X-schema to Y-schema. If, by definition, a reverse-relation is necessary, then that definition almost for sure imposes excessive overhead that is not needed in practice. Furthermore, since Y is expected to be an "improved version" of X, it would contain extra features which permit elements not recognizable in X. Such reverse translation of Y to X must fail by implication. >So, the solution is that programs shouldn't simply check >inputs against a schema, if they want an extensible input >language. Rather, programs should first transform inputs >to a familiar type, then check those, and optionally transform >outputs to some externally requested type. It's not yet a solution, but basically deferring the "problem" to the new intermediary called "a familiar type (AFT)". One has to ask how X-schemas associate with elements in AFT, and how Y-schemas associate with elements in AFT. The battle of resolving compatibility then goes into AFT but not going away. cheers. Melvin Chin
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