Ishan,
Difference between a schema vs. a schema document:
* Schema
I am a schema, I have a global element declaration named "root" in
namespace "ns1", and its type is "myType" in namespace "ns2". "myType" is
derived from the schema built-in type "int" by specifying a maxInclusive
facet of 100.
* Schema document
I am an <xs:schema> element, I have an attribute named "targetNamespace"
whose value is "ns1". I have a child element <xs:element>. The child has an
attribute named "name" whose value is "root". It also has an attribute
named "type" whose value is "p2:int". It also has a namespace declaration
that associates the prefix "p2" with the namespace "ns2".
I am another <xs:schema> element, with attribute targetNamespace="ns2",
child <xs:simpleType>. <xs:simpleType> has name="myType", child
<xs:restriction>, with attribute base="xs:int", ...
Now you can see the difference. Schema is an abstraction: it's a collection
of schema components. Each component has its properties, like name, type,
etc. And schema document is one way to represent a schema using XML syntax.
A schema may be assembled from one or more schema documents (or it could be
built without schema documents).
For a SCD/SCP that says "give me the global element declaration for name
{ns1}root", we should return the element declaration in the schema (an
XSElementDeclaration object), and not the <xs:element> element in the
schema document.
Now about the absolute part of the SCD for identifying the schema. For a
given absolute URI (e.g. http://abc.def/xyz), there is no standard way to
know which schema it's pointing to. Dereferencing the URI will return you a
sequence of bytes (maybe some kind of file), but that's not a schema. The
association of the URI with a schema has to be done through magic: in my
system, I *know* this URI corresponds to that schema (e.g. that XSModel
object).
In the example from the spec, it's assuming that, in that particular
environment, http://example.org/schemas/po.xsd corresponds to a particular
schema. It may be that that schema is assembled from loading the schema
document at "http://example.org/schemas/po.xsd";, but that's the knowledge
only meaningful in that system, and not an established standard.
This is exactly why I suggested
"It'll be useful, IMO, to focus more on relative SCDs (and SCPs in
particular), than absolute SCDs."
e.g. our SCD interface could have methods like:
String getCanonicalSCP(XSObject component, XSModel schema, NamespaceContext
nc);
XSObjectList resolveSCP(String scp, XSModel schema, NamespaceContext nc);
instead of
String getCanonicalSCD(XSObject component, XSModel schema);
XSObjectList resolveSCD(String scd);
The first 2 methods assume that the association between the absolute URI
and the schema is done (hence the XSModel is available). The latter 2
methods operate on absolute SCDs, and will suffer from the problem of not
knowing how to construct/resolve the absolute URI for the schema.
Thanks,
Sandy Gao
XML Technologies, IBM Canada
Editor, W3C XML Schema WG.
Member, W3C SML WG
(1-905) 413-3255 T/L 313-3255
Ishan Jayawardena <[email protected]> wrote on 2010-02-27 05:49:42 AM:
> [image removed]
>
> Re: SCD implementation
>
> Ishan Jayawardena
>
> to:
>
> j-dev
>
> 2010-02-27 05:50 AM
>
> Please respond to j-dev
>
> Hi Sandy,
> I think more clarification is needed about the association among the
> terms, "the schema", "a schema document", and an absolute SCD. The
> spec defines the absolute SCD in the following way,
> "An absolute schema component designator identifies a particular
> schema component; it consists of two parts: a designator for the
> assembled schema (a schema designator), and a designator for a
> particular schema component or schema components relative (a relative
> schema component designator) to that assembled schema."
> But at the same time, it gives
> "http://example.org/schemas/po.xsd#xscd(/type::purchaseOrderType)" as
> an example for an absolute SCD. Obviously, the first URI part refers
> to a schema document, not to a schema but the definition mentions it
> to be an assembled schema.
> Also, you say that,
>
> > - Differences between SCD and SCP. SCD could be absolute, with an URI
to
> > identify the schema, and a fragment for the component(s) in that
schema.
> > Given that there is no defined way to resolve an URI to a schema (note:
not
> > a schema document), it'll often be difficult (and not very useful) to
work
> > with absolute SCDs.
>
> So according to you, the URI part doesn't need to be something like
> 'http://example.org/schemas/po.xsd' because it has to be a URI for an
> assembled schema. Can you please help me clarify the correct
> interpretation of absolute SCD? Why do you say that there's no defined
> way to resolve a URI to a schema? Here, are you reffering to the
> capability of Xerces of resolving a URI to a schema?
> Thanks in advance.
>
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