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Francis Norton wrote: > We are watching a transition from "close-coupled" comms to "loosely-coupled" > comms. I'm using "coupling" here to refer to how much the send and receive > code has to know about the fine structure of the data being sent, from an > RPC-style list of individually-typed parameters at one extreme to a single > XML document at the other. I agree with this and believe that it is the inevitable consequence of the greater diversity and node-to-node differences which are possible in the internetwork topology. > SOAP, the exciting XML contender, has a spec which is focussed on > close-coupled calls. The SOAP EncodingStyle allows you to serialise an > RPC-style parameter set. I believe that, given my understanding of node-to-node diversity and autonomy in the internetwork topology, these 'close-coupled' premises are SOAP's chief design flaw. > There's nothing to stop you sending single literal XML documents, there is > just no explanation of how literal XML > content should be signalled A weakness. > WSDL makes explicit provision for both RPC-style and document-style messages > - in fact for the SOAP binding, the default values for the > soap:operation/@use and soap:body/@style attributes appear to make unadorned > literal XML content the default. A strength. > We already use XML Schema structured messages to interface between major > components, internal and external, of our retail finance systems. I believe that this is too fine-grained for communication with arbitrary counterparties across the Internet. The business strategy question, of course, is whether that is what you want to do. > My view is that [a] single document style is far more productive than RPC > style, Agreed. > [b] that XML Schema means that the message can use declarative validation in > the comms layer rather than procedural validation in the application (and > no, DTDs don't cut it for us) IMHO this presumes that the application knows more of the schema, or that the schema designer knows more of the application, than I believe is a healthy or reasonable assumption in an internetwork topology. > and [c] that XML Schemas will be the units of selection by which industry > standard common message structures will evolve. I believe that selection by structure, or schematic, is too brittle and fine-grained for the limited knowledge of one node by another in the internetwork topology. I specifically believe that, over a large population of diverse nodes, there will be only a small percentage whose interaction is standardized by the structure of the message exchanged. Respectfully, Walter Perry
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