[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Proliferation of validation options
Dear xml-dev'ers: I'm only now beginning to appreciate just how many specifications have sprouted up for XML document validation mechanisms. In the face of the limitations of DTDs, we have no shortage of proposals for approaches to the problem of specifying constraints on document content: XML Schema, DSD, SOX, RDF, Schematron, RELAX, to name some of the options. Unfortunately, the only one of these which has the backing of the W3C behind it -- XML Schema -- has a number of significant drawbacks. In spite of the frequent complaints (not difficult to understand) about the complexity of the draft specification, XML Schema does not address all of the basic requirements commonly needed in document validation. For example, we could not use XML Schemas to specify that the acceptable content for a given element differs based on the presence or value of one of its attributes. We've looked at RELAX, but the only one of the three existing implementations which fully supports the spec is not open source, and we understand that all three will need to be rewritten from the ground up to accommodate a newly published algorithm. We're looking at some of the others (Schematron, for example, appears to have a very nice mechanism for specifying context-specific rules for content constraints), but the real problem is that we have no way of knowing which -- if any -- of these options will still be around a year from now. If we're going to pick something that's going to disappear in six months we might as well roll our own specification and implementation, throwing yet another option onto the heap. XML Schema seems to be in that worst-of-both-worlds state. On one hand the tool vendors say they can't support it because it's not an approved standard (so we'll have to produce DTDs for the XML editor, since none of the candidate products with a customization API sufficient for our requirements has any Schema support). On the other hand, the working group says the specification documents are in the "Last-Call" stage, and that no further changes in functionality can be expected. So, two questions: 1. How did Schema get so far, with a spec that's harder to read than any of the others, and still leave out functionality the absence of which is causing all of these competing specs to appear? 2. Are projects really expected to choose between (a) rolling their own validation grammar and implementation; (b) adopting possibly ephemeral proposals; and (c) omitting support for required functionality? Cheers, Bob Kline
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