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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: DTDs, W3 XML Schema, RELAX, or Schematron?
 Or None Of The Above. Use DTDs if you don't need extensive datatypes, derivation of types within a schema, precision in defining ranges, or you dislike the cognitive overhead of element:Element, schema:Schema. If you want to process declarations using the same tools used for the instances of declarations, use schema or something like XDR if you want a simpler validation. But don't forget the well-formed option (none of the above). Consider the environment (application to application routing) and if you need an XML Schema or DTD to make that work. It isn't database vs document. To usefully apply XML, consider your existing technology. For example, namespaces can be useful for keeping up with which table is a source for a view as defined in a SQL query, but relational schemas don't share namespaces. An XML schema defining a mixed namespace is doing the work a relational programmer does in the query syntax of SQL. While one can use the namespace to prevent name collisions, in practice, there are other ways to do this. Names collide in aggregates (views) and in relational systems, views are temporary, so there isn't a need to declare the constraints for a view in other than procedural terms. In SQL, the query is used JIT to alias the name for the scope of the operation (select, insert, delete, update, etc.). The kind of reuse described in the XML Primer may not prove to be all that useful in some operational contexts. So you can consider the environment and what kind of process you want to support not simply the types of artifacts (documents, data, etc). If a relational schema is the record of authority for transactions, your use of the XML schema or a DTD can be safely limited to simple applications but your ability to safely control this and communicate it to other trading partners is limited. If on the other hand, you have an open system in which dissipative effects brought on by decentralized control (lots of players, no overall controlling authority, new players entering at irregular intervals - an open dynamic web), then you have the problem of not being able to easily scope local decisions that affect global decisions (aggregate rippling). I can see how using SOAP could lead to some interesting problems. Think of the current American election dilemma: lots of little different voting systems all contributing to a single decision. It works fine until you get a close vote and then the uneven precision brought on by lots of different rules in different political scopes is exposed. SNAFU. A system of communication that mixes DTDs, Schemas, etc. could be unmanageable unless noise is smoothed at the interfaces, and in that case, local decision scope is not exposed. DTDs won't go away soon but fewer and fewer developers will use them. I expect XML Schemas to become the dominant instrument as soon as the tool support is ubiquitous and the practice is established. Today we are still wrestling with the spec, the lack of clarity in the explanations, and the mindFUD of learning how to use them. I expect the Schematron work, RELAX, etc to contribute to the next version of XML Schema. OTOH, the insertion of the technology will be slow and tedious and during that time frame, niches will become entrenched. Len http://www.mp3.com/LenBullard Ekam sat.h, Vipraah bahudhaa vadanti. Daamyata. Datta. Dayadhvam.h 
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