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Re: XML Schemas: Best Practices
- From: "Roger L. Costello" <email@example.com>
- To: Jeff Rafter <firstname.lastname@example.org>
- Date: Tue, 06 Mar 2001 06:29:51 -0500
Thanks for all your comments Jeff. Let me try to reply to your
questions. [Note: this week I feverishly working on summarizing all
these issues and posting them on my web site. I hope to have it
completed by Friday.]
> > <?xml version="1.0"?>
> > <schema xmlns="http://www.w3.org/2000/10/XMLSchema"
> > targetNamespace="http://www.library.org"
> > xmlns:lib="http://www.library.org"
> > elementFormDefault="qualified">
> > <include schemaLocation="BookCatalogue.xsd"/>
> > <element name="Library">
> > <complexType>
> > <sequence>
> > <element name="BookCatalogue">
> > <complexType>
> > <sequence>
> > <element ref="lib:Book"
> > minOccurs="0"
> > maxOccurs="unbounded"/>
> > </sequence>
> > </complexType>
> Should there be a closing element tag here? I don't think this is
> important for sake of the example-- only if this exact example is
> published as a Best Practice. (The WF error also occurs in
> Approach 2).
Thanks, I'll make certain that the online version is correct.
> > Before describing why I believe that approach 2 is better, we
> > need to first revisit Chameleon components.
> > Components that are in a schema with no targetNamespace have a very
> > interesting property: when they are <include>d in a schema that has
> > a targetNamespace then those no-namespace components take on the
> > namespace of the <include>ing schema. They "blend into the
> > background" just like a Chameleon. Thus, the no-namespace
> > components are called Chameleon components.
> > In the above schemas there is an <include> element to bring in the
> > components in BookCatalogue.xsd. Let's consider how the above
> > approaches behave when the schema being <include>d
> > (BookCatalogue.xsd) has no namespace. Here is BookCatalogue.xsd:
> Wow, what a great description. Do you happen to have any other
> references for this?
Thanks! Yes, we discussed the Chameleon design approach quite a bit in
the Zero, One, or Many Namespaces issue. See:
> It seems as though stuff like this may in fact affect other Best
> Practices or decisions therein. This also seems like a good one for
> the Schema FAQ-- What is a chameleon component?
A Chameleon component is a component (element, simpleType, complexType,
attribute, attributeGroup, or modelGroup) in a no-namespace schema.
They are called "Chameleon" because they have the property of taking on
the namespace of a schema which <include>s them (just like a Chameleon
animal is able to blend into its surroundings) (Good question Jeff, I
have never explicitly defined this. Thanks.)
> (are you going to give a prefix as best practice? e.g. "xsd:"
Uh, no (but I like xsd).
> Is there an Approach 3? (a.k.a. is this even right?)
Yes, good point. I forgot about that. As you mention, Approach 3 is to
have no default namespace, and explicitly qualify everything. Below is
shown this approach:
<xsd:element name="LocalElement" type="xsd:string"
> Notice niether Book or LocalType are prefixed because there is no
> default namespace.
No, with Approach 3 you need to explicitly qualify all references -
lib:Book, lib:LocalType - since there is no default namespace.
Thanks for the comments! /Roger