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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: A multi-step approach on defining object-oriented nature o
 Tim Bray wrote: > I have a good solution but it's probably not generally applicable - I > interchange information with XML but I usually mash it as quickly as > possible into application native data structures, which have their own APIs > aimed at their own needs. > > I have no problem saying in public that XML is really really good for > interchange and really really irritating for in-memory manipulation. I > think we all ought to be more up-front about this. For several years now I have advocated this approach as the basis of an XML processing model. Logically implemented, however, it contradicts what is in practice a central dogma of XML orthodoxy. The question is whether an XML-consuming application ought to be built around the data structures as interchanged or should rather use a private local data structure best suited to the application's own implementation of some particular expertise. This is actually a fundamental choice, which does not permit the answer 'do both', if that means to use an agreed interchange data structure for data acquisition and then use a more appropriate structure for the application internals. Validation, whether by DTD, schema, or otherwise, is grounded in the expectation that an XML-consuming application adheres to a contract to process only input which conforms to a pre-agreed schematic. This is a legacy SGML notion that has never successfully translated to the very different environment 'on the Web'. XML 1.0 recognized this problem and indeed offered the best solution to it in simple well-formedness. WFness acknowledges the autonomy of any processing node in the internetwork topology of the Web by recognizing that such a node will make its own decisions about what to process, and how, regardless of the semantics apparently intended, or hoped to be conveyed, by the particular structure to which a document instance might conform. The real problem is actually the belief, whether explicitly acknowledged or not, that moving from simple well-formedness to validity--which is to say, casting an instance document in a particular structure--adds semantics and, what is more, semantics of consequence to an XML-consuming application receiving that document. There is a fundamental divide between believing in one case that the first priority of an XML-consuming application is to adhere to, and to enforce, the schematic contract which is perceived as the basis of document interchangeability, and believing in the other case that the point of an application is to apply specific expertise in process, which includes particular, and probably private, judgments about what input data to accept, and generally about where and to get the data that the application's expertise requires. Transaction protocols and remote procedure invocations depend ultimately on the premise that presenting a particular data structure to an application will cause it to execute a particular process. That assumption depends on the homogeneity of an enterprise network where application nodes understand each other's processes intimately, and is utterly invalid in the internetwork topology of the Web. The autonomous processing nodes of the internetwork topology are of value because of what they produce, which is to say the expertise which they implement. But they must also be designed around that expertise, one consequence of which is that they apply their private expertise to their own data acquisition and data instantiation. By design, such applications cannot be invoked by the presentation of a particular data structure if by 'invoked' we mean be made somehow to respect the intent or otherwise accept the semantics of whoever would invoke them. This is why the answer to the question of whether an application uses a public or a private data structure cannot be 'do both'. Respectfully, Walter Perry 
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