Re: XML Schemas: Best Practices

[Eddie Robertsson <eddie@allette.com.au>]

> PublicationType - the base type:
>
>     <complexType name="PublicationType">
>         <sequence>
>             <element name="Title" type="string"/>
>             <element name="Author" type="string" maxOccurs="unbounded"/>
>             <element name="Date" type="year"/>
>         </sequence>
>     </complexType>
>
> BookType - extends PublicationType by adding two new elements, ISBN
> and Publisher:
>
>     <complexType name="BookType">
>         <complexContent>
>             <extension base="c:PublicationType" >
>                 <sequence>
>                     <element name="ISBN" type="string"/>
>                     <element name="Publisher" type="string"/>
>                 </sequence>
>             </extension>
>         </complexContent>
>     </complexType>
>
> MagazineType - restricts PublicationType by dropping the Author
> element:
>
>     <complexType name="MagazineType">
>         <complexContent>
>             <restriction base="c:PublicationType">
>                 <sequence>
>                     <element name="Title" type="string"/>
>                     <element name="Author" type="string"
>                              minOccurs="0" maxOccurs="0"/>
>                     <element name="Date" type="year"/>
>                 </sequence>
>             </restriction>
>         </complexContent>
>     </complexType>
>
> - Oftentimes the variable content section will contain elements that
>   are conceptually related but structurally vastly different. The base
>   type (the abstract element's type) should contain items common to all
>   the variable content elements.  To allow for elements that may be
>   very dissimilar the base type would need to have very little
>   structure. This defeats the purpose of inheritance.
>
> Question:
>
> - In the second disadvantage above I state: "This defeats the purpose
>   of inheritance."  This seems like a very weak statement.  Can you
>   provide a stronger statement telling why it is bad that the base
>   type has little structure?

I agree with Roger here. Sometimes the base type will have very little
structure but that doesn't mean you can't use extension if that suit your
purpose. I think it's better to have a small base type instead of having to
use the restriction functionality but maybe that's just me not being used to
derivation by restriction. I'm still very new to XSD and I haven't made up
my mind about restriction yet.
For example, in the above example I would much rather see that the Author
element is removed from the base type and then added in the BookType.

/Eddie