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Re: XML Schemas: Best Practices

• From: Jon Cleaver <j.cleaver@e...>
• To: xml-dev@l...
• Date: Wed, 18 Oct 2000 10:52:47 +0100

Roger, All,

Names for the two design paradigms: Componentized vs. Heirarchical?

1. The Componentized approach

This makes global definitions of elements and types, then combines
these in an arbitrary manner locally. This makes the first level of
the schemas more complicated but this is offset by greater simplicity
deeper into the schema where components can be reused.

The trade-off with this approach is that it means that any change to
one of the components can propagate throughout the schema, or other
schemas that the component. This necessitates the identification of
dependancies and how they might be affected by any structural change.

2. The Heirarchical approach

This defines all components and structure locally. As such, the first
order level of the schema is simpler but deeper down there may be
additional complexity, due to having to redefine components whenever
they are used.

The advantage of this is that any changes to structure will only have
an impact within the scope of the parent element of the component
being changed. As such, rigorous dependancy checking is not required
outside of the sub-system being modified, since it is not visible
outside of its scope.

--

As such, I would suggest that should a sub-component be liable to
change then I would be inclined to use the second approach, because
the administration overhead of managing change could become
prohibitive. Should it be fixed, it may be part of a standard or
specification, then it should be declared globally as the
administration cost will be negligible compared to the benefits of
allowing it to be reused elsewhere.

Hope this helps!

Jonathan

--- In xml-dev@e..., "Roger L. Costello" <costello@m...> wrote:
> Hi Folks,
>
> I would like to move on to the next schema design issue:
>
> Issue: When should an element or type be declared global versus when
> should it be declared local?
>
> [Recall that a component (element, complexType, or simpleType) is
> "global" if it is an immediate child of <schema>, whereas it
is "local"
> if it is not an immediate child of <schema>, i.e., it is nested
within
> another component.]
>
> If someone were to ask you, "In general, when should an element or
type
> be declared global versus when should it be declared local?", what
> advice would you give them?
>
> A month ago I would have answered, "as a general rule, make things
> global".  However, after the discussions that we have had, I would
have
> a very different answer today.
>
> Example.  Below is a snippet of an instance document.  Let's
explore the
> different design strategies for defining <Book> and its components.
>
>     <Book>
>         <Title>Illusions</Title>
>         <Author>Richard Bach</Author>
>     </Book>
>
> One design approach is to mirror the instance document - declare a
Book
> element and within it declare a Title element followed by an Author
> element:
>
> First Design:
>
>     <element name="Book">
>         <complexType>
>             <sequence>
>                 <element name="Title" type="string"
>                          minOccurs="1" maxOccurs="1"/>
>                 <element name="Author" type="string"
>                          minOccurs="1" maxOccurs="1"/> />
>             </sequence>
>         </complexType>
>     </element>
>
> That's one end of the design spectrum.  At the other end of the
design
> spectrum: we disassemble the above instance document into its
individual
> components, define each component, and then assemble them together:
>
> Second Design:
>
>     <element name="Title" type="string"/>
>
>     <element name="Author" type="string"/>
>
>     <complexType name="Publication">
>         <sequence>
>             <element ref="cat:Title"
>                      minOccurs="1" maxOccurs="1"/>
>             <element ref="cat:Author"
>                      minOccurs="1" maxOccurs="1"/>
>         </sequence>
>     </complexType>
>
>     <element name="Book" type="cat:Publication"/>
>
> These approaches represent both ends of the design spectrum.
>
> For this issue, I like to think in terms of boxes, where a
represents an
> element or type.  Thus,
>
> - The first design approach corresponds to having a single box, and
it
> has nested within it boxes, which in turn have boxes nested within
them,
> and so on.
> - The second design approach corresponds to having many separate
boxes
> which are composed together.  The composition of the boxes creates
the
> whole.
>
> I believe that it will be useful to create a name for the two design
> approaches:
>
> - What name would you give to the design strategy where the
components
> (i.e., element declarations and type definitions) are nested within
each
> other?  It is the "xxxxx" design approach for schema construction.
> - What name would you give to the design strategy where components
are
> defined individually and then composed together?  It is the "yyyyy"
> design approach for schema construction.
>
> Let's examine the characteristics of each of the two design
approaches.
> (Perhaps the characteristics will yield insights into appropriate
names
> for the two design approaches?)
>
> First Design Characteristics:
>
> [1] Opaque content. The content of Book is opaque to other schemas,
and
> to other parts of the same schema. The impact of this is that none
of
> the types or elements within Book are reusable.
>
> [2] Localized scope. The region of the schema where the Title and
Author
> element declarations are applicable is localized to within the Book
> element.  The impact of this is that if the schema has set
> elementFormDefault="unqualified" then the namespaces of Title and
Author
> are hidden (localized) within the schema.
>
> [3] Compact.  Everything is bundled together into a tidy, single
unit.
>
> Second Design Characteristics:
>
> [1] Transparent content. The components which make up Book are
visible
> to other schemas, and to other parts of the same schema.  The
impact of
> this is that the types and elements within Book are reusable.
>
> [2] Global scope. All components have global scope.  The impact of
this
> is that, irrespective of the value of elementFormDefault, the
namespaces
> of Title and Author will be exposed in instance documents
>
> [3] Verbose. Everything is laid out and clearly visible.
>
> I am sure that there are other characteristics that I am missing.
Can
> you please help to list them?
>
> As I see it, the major tradeoff between the two design approaches
is
>
> - The first design approach facilitates hiding (localizing)
namespace
> complexities
> - The second design approach facilitates component reuse.
>
> (I find it interesting that this issue is relating back to the
issue we
> discussed earlier on when to hide (localize) namespace complexities
> within the schema versus when to expose the namespaces in instance
> documents.)
>
> Here's a summary of things to be resolved for this issue:
>
> (1) What name do we give to (what I have been calling) First Design?
> (2) What name do we give to (what I have been calling) Second
Design?
> (3) What are the other characteristics of the two design approaches?
> (4) What do you see as the main tradeoffs in the two design
approaches?
>
> Thanks!  /Roger








--
Programming today is a race between software engineers striving
to build bigger and better idiot-proof programs, and the Universe
trying to produce bigger and better idiots.  So far, the Universe is
winning.

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