W3C home > Mailing lists > Public > xmlschema-dev@w3.org > June 2001

RE: Global Vs Root Element.

From: Tom Wason <wason@mindspring.com>
Date: Tue, 12 Jun 2001 09:42:09 -0400
To: "Venkateswar Wunnava" <wvsvenkat@worldnet.att.net>, "Jeff Rafter" <jeffrafter@definedweb.com>
Cc: <xmlschema-dev@w3.org>
Message-ID: <NEBBJIDAALNJAOPHOLJJKEIBEOAA.wason@mindspring.com>
Venkateswar--

Yes, it would sometimes be nice if one could designate an element as the
"root".  I am encouraged that XML Spy can deduce the logical "root" element,
although this can become ambiguous when another schema is imported or
included.  Good schema design should make the root logically apparent.  It
is probably not possible to make a specification that forces "good design".
Better we all have an awareness of the issues that constitute good design.

In XML-Schema an element does not "know" what its "parent(s)" is (are),
consequently, a declaration of "root" would imply that the element "knows"
its parent, namely, "none".  A root element in one schema may be a child
when it is imported or included into another.  One should consider each
schema as potentially part of another schema.  Often libraries of schemas
will be created.  A key here is reuse of schemas and their components.  One
may wish to use fragments of another schema in your schema.  The fragment
may not start at the root.  On the other hand, if the fragment does start at
the root, then including or importing that external root element could end
up with a schema that has more than one root element if "root" can be
declared by an element.  Thus, at best is root only local to the particular
schema that includes it.  It would seem that schema design also includes
design of the library of (sub) schemas.  In this way, one can harmonize all
of the schemas within a domain.  This is not a trivial exercise.

You speak of limiting the use of global elements.  Overriding elements
through the use of substitutionGroup can be used to create controlled
extensions.  I have been working with a situation (IMS) in which extensions
are normal.  A controlled extension through an override requires access to
elements and their types, hence they must be global.  In other circumstances
I can see that you might want to prevent the overriding of elements.  There
is probably no general rule on global versus local definitions of
components; it is a function of the particular application.  Generally I
will use a "global" definition as it provides for reuse, future evolution
and controlled extension.  That one can tailor XSDs to this level is one of
the strengths of XML-Schema.

Your thoughtful comments are stimulating.

Regards,
  --Tom

Thomas D. Wason
e-Learning & Meta-Data Consultant
+1 919.839.8187
wason@mindspring.com
http://www.twason.com
1421 Park Drive
Raleigh, North Carolina 27605 USA

< -----Original Message-----
< From: Venkateswar Wunnava [mailto:wvsvenkat@worldnet.att.net]
< Sent: Tuesday, June 12, 2001 2:40 AM
< To: wason@mindspring.com; Jeff Rafter
< Cc: xmlschema-dev@w3.org
< Subject: Re: Global Vs Root Element.
<
<
< That is a good suggestion Tom.
<
<  But I suppose XMLSpy is doing more ingenuine check above and beyond what
< XMLSchema Spec., stipulates. I suppose XMLScema specificatoin should have
< given a way to specify one of the global elements as root element in the
< form of a constraint, (some thing, XMLSpy is trying to infer). I
< personally
< do not like to hang around more than one global elements within a Schema
< unless I cannot do away with it. Particularly, because I do not want my
< Parser to punt on me by validating an instance that has a
< particular global
< element as a root element while I really did not intend to make it one,
< which, can be solved by a mechanism to specify one of those
< global elements
< as root element. I do not understand why this piece is being left so loose
< in the spec., while all possible forms of neat mechanisms have
< been provided
< in XMLSchema specification. Is there any specific reason for doing that,
< which I am missing?
<
< Best Regards,
<
< Venkateswar.
<
< ----- Original Message -----
< From: "Tom Wason" <wason@mindspring.com>
< To: "Jeff Rafter" <jeffrafter@definedweb.com>; "Venkateswar Wunnava"
< <wvsvenkat@worldnet.att.net>
< Cc: <xmlschema-dev@w3.org>
< Sent: Monday, June 11, 2001 1:53 PM
< Subject: RE: Global Vs Root Element.
<
<
< > Jeff--
< >
< > I agree with you in concept, as a schema may be a fragment.  However (ah
< > yes, however...), a general rule to use is that:
< >
< >  The root element can directly or indirectly contain all of the other
< > elements in the schema (presuming this is a complete schema).
< >
< > Therefore, the only "true" root element in this schema is <paragraph>.
< > Indeed, if you create a new XML instance with XML Spy, it will
< automatically
< > create an XML instance in which the root is <paragraph>.
< >
< > If you wanted to make all of the elements potentially root elements, you
< > could add the <paragraph> element to the complexType's <choice> list.
< >
< > --Tom
< >
< > Thomas D. Wason
< > e-Learning & Meta-Data Consultant
< > +1 919.839.8187
< > wason@mindspring.com
< > http://www.twason.com
< > 1421 Park Drive
< > Raleigh, North Carolina 27605 USA
< >
< > < -----Original Message-----
< > < From: xmlschema-dev-request@w3.org
< > < [mailto:xmlschema-dev-request@w3.org]On Behalf Of Jeff Rafter
< > < Sent: Monday, June 11, 2001 12:42 PM
< > < To: Venkateswar Wunnava
< > < Cc: xmlschema-dev@w3.org
< > < Subject: Re: Global Vs Root Element.
< > <
< > <
< > <
< > < Typically when referring to root elements you are referring to the
< > < first/outer element within an XML document (aka XML instance).
< > < For example:
< > <
< > < <?xml version="1.0" encoding="UTF-8"?>
< > < <root>
< > <   <child/>
< > < </root>
< > <
< > < The element <root> is the root element.  Global elements are referring
< to
< > < the element definitions within a schema document that appear as direct
< > < children of the <schema> element.  Within the schema definition
< > < there can be
< > < many global elements:
< > <
< > < <xsd:schema xmlns="paragraph" targetNamespace="paragraph"
< > < xmlns:xsd="http://www.w3.org/2000/10/XMLSchema"
< > < elementFormDefault="qualified">
< > <   <xsd:element name="paragraph" type="paraType"/>
< > <   <xsd:element name="bold" type="paraType"/>
< > <   <xsd:element name="italic" type="paraType"/>
< > <   <xsd:element name="underlined" type="paraType"/>
< > < ...
< > < <xsd:schema>
< > <
< > < (Quoting Tom Wason's recent example).  In this sample the elements
< > < paragraph, bold, italic and underline are all global.  Any of
< these can
< be
< > < used in an XML document as the root (however there is still only one
< root
< > < element per XML document).
< > <
< > < So the following XML documents would all be okay:
< > <
< > < <?xml version="1.0" encoding="UTF-8"?>
< > < <paragraph xmlns='paragraph'>
< > <   This is some <bold>text</bold>
< > < </paragraph>
< > <
< > < <?xml version="1.0" encoding="UTF-8"?>
< > < <bold xmlns='paragraph'>
< > <   This is some text
< > < </bold>
< > <
< > < <?xml version="1.0" encoding="UTF-8"?>
< > < <italic xmlns='paragraph'>
< > <   This is some <bold>text</bold>
< > < </italic>
< > <
< > < In each of these documents a different global element was used as the
< root
< > < element in an XML instance.
< > <
< > < Hope this helps,
< > < Jeff Rafter
< > < Defined Systems
< > < http://www.defined.net
< > < XML Development and Developer Web Hosting
< > <
< > < ----- Original Message -----
< > < From: "Venkateswar Wunnava" <wvsvenkat@worldnet.att.net>
< > < To: <xmlschema-dev@w3.org>
< > < Sent: Sunday, June 10, 2001 11:44 AM
< > < Subject: Global Vs Root Element.
< > <
< > <
< > < Hi,
< > <
< > <  I am not too familiar with XML. Can some one tell me what is the
< > < difference
< > < between Global Elements and Root Elements. From what I understand from
< > < XMLSchema specification, we cannot impose any cardinality
< constraints on
< > < Global Elements. They can occur with multiple instances as top level
< > < elements. I am confused how they are different from Root Element,
< > < which can
< > < be only one element. Thanks in advance.
< > <
< > < Best Regards,
< > <
< > < Venkateswar.
< > <
< > <
< > <
< >
<
<
Received on Tuesday, 12 June 2001 09:44:06 GMT

This archive was generated by hypermail 2.2.0+W3C-0.50 : Tuesday, 11 January 2011 00:14:21 GMT