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Melvin Chin wrote: > > Economically, the need is almost always to convert X-schema > to Y-schema. But I'm responding to the scenario another poster offered earlier up. That person has a million lines of legacy code that uses X and the Y to X morphism is very important to keeping that system running while making upgrades incrementally. > 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. > Well, no, not really. It depends entirely on how well designed X is. My transform-based approach tells you generally how to avoid/solve problems like this, but yes, you must *still* design X with extensions in mind. It's just that future formats should be transformable into X extensions, not that they necessarily have to be represented as X extensions. That's why it's "too late" for systems already suffering this problem -- you have to design X with extensibility in mind in the first place, it's just that designing such an X is simplified if you think of type conversions rather than simply sub-types and super-types. -t
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