RELAX NG by Eric van der Vlist will be published by O'Reilly & Associates (ISBN: 0596004214)
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I wrote in the introduction to this chapter that the notions of "extensible" and "open" are largely independent. After all we have seen, we might even think that opening a schema may be an impediment to its extensibility. Let's say we have written an open model for the content of our book element that will allow foreign nodes:
<define name="book-content">
<interleave>
<attribute name="id"/>
<attribute name="available"/>
<element name="isbn">
<ref name="isbn-content"/>
</element>
<element name="title">
<ref name="title-content"/>
</element>
<zeroOrMore>
<element name="author">
<ref name="author-content"/>
</element>
</zeroOrMore>
<zeroOrMore>
<element name="character">
<ref name="character-content"/>
</element>
</zeroOrMore>
<ref name="foreign-nodes"/>
</interleave>
</define> |
or:
book-content =
attribute id { text }
& attribute available { text }
& element isbn { isbn-content }
& element title { title-content }
& element author { author-content }*
& element character { character-content }*
& foreign-nodes |
We have independently applied the tips to build an extensible schema (using interleave and containers) and also to define an open schema (referencing a wildcard to allow foreign nodes). Unfortunately, if our schema is open, it's no longer very extensible.
Imagine that I want to add a couple of XLink attributes to define a link to a web page. I can't combine this new attribute with the existing schema using interleave. This new attribute would be considered a duplicate of the implicit definition of "xlink:href" already contained in our "foreign-nodes" wildcard.
The situation is similar for the addition of new elements. If I want to add an optional dc:copyright element, for intance, I can, but the constraint applied to this element will be in conflict with the lax definition of dc:copyright implicitly contained in our "foreign-nodes" wildcard. If our new constraint is not met, RELAX NG will still find a match for a bogus dc:copyright element in the wildcard.
Does that mean that open schemas cannot be extensible? Yes and no. While wildcards make open schemas less extensible, we can overcome that problem by extending our schemas before opening them. To come back to our example, we'd better write a closed schema first, closed-schema.rng:
<define name="book-content">
<interleave>
<attribute name="id"/>
<attribute name="available"/>
<element name="isbn">
<ref name="isbn-content"/>
</element>
<element name="title">
<ref name="title-content"/>
</element>
<zeroOrMore>
<element name="author">
<ref name="author-content"/>
</element>
</zeroOrMore>
<zeroOrMore>
<element name="character">
<ref name="character-content"/>
</element>
</zeroOrMore>
</interleave>
</define> |
or, in the compact syntax, closed-schema.rnc:
book-content =
attribute id { text }
& attribute available { text }
& element isbn { isbn-content }
& element title { title-content }
& element author { author-content }*
& element character { character-content }* |
We can then carefully keep this closed schema in a first document and extend it by inclusion and combination to become open:
<include href="closed-schema.rng"/>
<define name="book-content" combine="interleave">
<ref name="foreign-nodes"/>
</define> |
or:
include "closed-schema.rnc" book-content &= foreign-nodes |
Applications would then use the open schema (the one produced by inclusion and combination) and derive the benefit as if the schema was natively open. The "closed-schema" would be available if we needed to extend the content model, redefine the foreign-node wildcard, and open the schema again in different ways.
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All text is copyright Eric van der Vlist, Dyomedea. During development, I give permission for non-commercial copying for educational and review purposes. After publication, all text will be released under the Free Software Foundation GFDL.