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changed once and the change propagated throughout However, with a schema, reuse makes inheritance hazardous: the ripples due to small changes may avalanche into massive modifications without the developer s knowledge Changes in a distributed environment can have unpredictable consequences The safe approach is to make all changes manually in the beginning of the development cycle Whenever a schema is developed, freeze it and put it under configuration management Some XML document or application depends heavily on that schema If a schema is modified, make an entirely new copy that does not depend on the original If the new one does not work with a particular application or XML document, go back to the old one and debug the new one Do not simply discard the new one Instead, try to learn from the mistakes A few corrected mistakes result in dramatic increases in experience These are good learning exercises When creating a schema, plan it like an XML document Rather than building a collection of apparently unrelated definitions, try to build a collective whole that fits into a hierarchy One of the advantages of using schemas over DTDs is the coherence and relationships among elements Another advantage is the ability to build XSLT scripts that can transform the schemas as needed, automatically

One of the significant issues about XML DTDs and schemas is how to convert an XML document with one vocabulary to a document with another vocabulary

DTDs are not easy to transform, but schemas are a different story Because a schema is an XML document, it can be manipulated just like any other XML document XSLT was designed to handle just these kinds of manipulations and transformations The XSL Transformations (XSLT) specification defines an XML-based style sheet scripting-type language for expressing transformation rules that map one XML document into another When XML schemas become the dominant meta-data for XML documents, XSLT will be help provide the basic infrastructure for building interoperable systems Because XSLT can

transform XML documents and because schemas are XML documents, XSLT is a superb vehicle for mapping the schema of one corporation into that of another corporation For example, imagine that Ford wants to make fuel cells for Boeing Ford sends its XML documents to Boeing, and Boeing s XML translation Web server compares the Ford schemas to the standard Boeing schemas The server then runs an XSLT translation engine that maps the Ford vocabulary to the Boeing vocabulary XSLT is a scripting or programming language, so this kind of mapping is not much more difficult than a table lookup Then the modified Ford document is sent to the appropriate departments for review Either Boeing or Ford may reverse the process for documents sent from Boeing to Ford Designing the XSLT translation engine may require some clever artificial intelligence programming, but maintaining and running the engine should be straightforward Developing this XSLT translation engine or providing this type of service is probably a window of opportunity for some clever entrepreneur

XSLT can address the significant challenge caused by using multiple XML schemas describing similar data within different XML documents With XSLT, a developer can map XML documents to one another, and create a new XML document that conforms to the schema of choice Of course, simply mapping documents and vocabularies doesn t ensure that they can work together in the way expected Manual intervention and direct communications are still required to interpret the meaning of the data While the mechanics of meta-data transformation may be worked out, the logistics are a completely different story The syntax may be portable, but corporate semantics are not For example, if you ask for a glass of water and I bring you a glass of warm water from the tap, you may be disappointed that I did not understand you You may have wanted: cold water, water with ice, water with a slice of lemon, or bottled water The syntax was clear; the meaning was not This is the same situation with data exchange between corporations One corporation may identify a customer by using <name> However, does <name> refer to last name, or full name The same dilemma occurs for <address>, for <product>, for <inventory number>, etc The basic business vocabulary for a corporation varies greatly across industries and may vary