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This document captures the result of disucussions by the Web Services Description Working Group regarding WSDL 2.0 type system extensibilty at the time of its publication. The Working Group normatively defines the use of XML Schema 1.0 as a type system in the WSDL 2.0 Core specification. This document sketches out the basics of extensions for Document Type Definitions (DTDs) and Relax NG.
This document is an editors' copy that has no official standing.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This document is a W3C Working Group Note. It has been produced by the Web Services Description Working Group, which is part of the Web Services Activity.
The material in this note was previously published as an Appendix of the Web Services Description Language (WSDL) 2.0: Core Language Last Call specification. In response to Last Call comments, the Working Group agreed to remove this material from that specification and publish it separately as a Working Group Note. Current versions of WSDL 2.0 Core no longer contain this material. This publication differs from the previous material in that it also includes some expanded discussion of issues that should be given consideration by type system extension designers.
No further work on this topic is planned at this point. Errors in this document can be reported to the public public-ws-desc-comments@w3.org mailing list (public archive).
Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
1. Introduction
2. Issues facing multiple schema languages/type systems
3. Examples of Specifications of Extension Elements for Alternative Schema Language
Support.
A. References
1. Introduction
2. Issues facing multiple schema languages/type systems
3. Examples of Specifications of Extension Elements for Alternative Schema Language
Support.
3.1 DTD
3.1.1
namespace
attribute information item
3.1.2
location
attribute information item
3.1.3 References to Element Definitions
3.2 RELAX NG
3.2.1 Importing RELAX NG
3.2.1.1
ns
attribute information item
3.2.1.2
href
attribute information item
3.2.2 Embedding RELAX NG
3.2.2.1
ns
attribute information item
3.2.3 References to Element Declarations
A. References
WSDL 2.0: Core Language [WSDL 2.0 Core] describes Web Service interaction in terms of exchanges of typed messages. WSDL only provides general support for type systems based on the XML Infoset [XML Information Set] and specific support for the W3C XML Schema Description Language [XML Schema: Structures]. Describing messages with WSDL using schema languages other than XML Schema or non-XML Infoset type systems requires extending the WSDL component model. While the Web Services Description Working Group has not defined any such extensions, there were discussions in the Working Group about how those extensions might be defined and used. This document is the result of those disucussions and captures part of the Working Group's thinking about schema language and type system extensibilty at the time of its publication.
Without the use of an extension, a WSDL document can only use a single type system, XML Schema. If extensions are defined to support alternative schema languages or non-XML type systems, then issues regarding the mixing of type systems in a single document arise. Part 1 does not define the behavior of mixed type system documents, so it is incumbent on extension authors to do so.
For example, suppose a WSDL author used a extension supporting Relax NG along side the native support for XML Schema. Further suppose that there is an element component which has a definition in both the referenced XML Schema and Relax NG schema. There are several possibilities for interpreting such a document:
Multiple definitions in distinct type systems is always an error
Multiple definitions must be in some sense equivalent, for example, if XML Schema type and an Relax NG production validate exactly the same set of Infoset fragments, otherwise, an error
Multiple definitions are legal, and are interpreted as a union type constraint
The last interpretation suggests a further general possibility: being able to define a union type (or other compound type) that spans distinct type systems (and, to further generalize, where the unioned types had distinct QName identifiers). The Data Access Working Group had a use case wherein their return message could either be in RDF/XML, which cannot have an interesting XML Schema but does have an interesting Relax NG schema, and their other results format, which they would prefer to specify with an XML Schema. This example is little artificial, as the Data Access Working Group could easily describe the entire results format in Relax NG.
The first interpretation is most in the spirit of WSDL and was strongly preferred by the Working Group. Since WSDL extensibility points are generally quite unrestricted, the Working Group did not try to enforce the first option, but the general belief of the Working Group was that the other options were confusing and unwise.
A Document Type Definition (DTD) as defined in [XML 1.0] may be used as the schema language for WSDL. It may not be embedded; it
must be imported. A namespace must be assigned. DTD types appear in the {element
declarations} property of the Description Component and may be
referenced from the wsdl:input , wsdl:output and
wsdl:fault elements using the element
attribute information item.
The prefix, dtd, used throughout the following is mapped to the namespace URI "http://www.example.org/dtd/".
The dtd:import
element information item references an external Document Type
Definition, and has the following Infoset properties:
A [local name] of import.
A [namespace name] of "http://www.example.org/dtd".
One or two attribute information items, as follows:
A REQUIRED namespace
attribute information item as described below.
An OPTIONAL location
attribute information item as described below.
namespace
attribute information item
The namespace
attribute information item sets the namespace to be used with
all imported element definitions described in the DTD. It has the following
Infoset properties:
A [local name] of namespace.
A [namespace name] which has no value.
The type of the namespace
attribute information item is xs:anyURI.
The WSDL author should ensure that a prefix is associated with the namespace at the proper scope (probably document scope).
location
attribute information item
The location
attribute information item, if present, provides a hint to the
processor as to where the DTD may be located. Caching and cataloging
technologies may provide better information than this hint. The
location
attribute information item has the following Infoset
properties:
A [local name] of location.
A [namespace name] which has no value.
The type of the location
attribute information item is xs:anyURI.
The element
attribute information item MUST be used when referring to an
element definition (<!ELEMENT>) from a Message Reference
component; referring to an element definition from a Interface Fault
component is similar. The value of the element definition MUST correspond to
the content of the namespace
attribute information item of the dtd:import
element information item. The local name part must correspond
to an element defined in the DTD.
Note that this pattern does not attempt to make DTDs namespace-aware. It applies namespaces externally, in the import phase.
A RELAX NG [Relax NG] schema may be used as the schema language for WSDL. It may be embedded
or imported; import is preferred. A namespace must be specified; if an imported
schema specifies one, then the [actual value] of the namespace
attribute information item in the import
element information item must match the specified namespace. RELAX
NG provides both type definitions and element declarations, the latter appears
in the {element declarations} property of Description Component
respectively. The following discussion supplies the prefix rng which is mapped
to the URI "http://www.relaxng.org/ns/structure/1.0".
Importing a RELAX NG schema uses the rng:include mechanism defined by RNG,
with restrictions on its syntax and semantics. A child element
information item of the types
element information item is defined with the Infoset properties
as follows:
A [local name] of include.
A [namespace name] of "http://www.relaxng.org/ns/structure/1.0".
Two attribute information items as follows:
A REQUIRED ns
attribute information item as described
below.
An OPTIONAL href
attribute information item as described
below.
Additional attribute information items as defined by the RNG specification.
Note that WSDL restricts the rng:include
element information item to be empty. That is, it cannot
redefine rng:start and rng:define
element information items; it may be used solely to import a
schema.
ns
attribute information item
The ns
attribute information item defines the namespace of the
type and element definitions imported from the referenced schema. If the
referenced schema contains an ns
attribute information item on its grammar
element information item, then the values of these two
attribute information items must be identical. If the
imported grammar does not have an ns
attribute information item then the namespace specified
here is applied to all components of the schema as if it did contain
such an attribute information item. The ns
attribute information item contains the following Infoset
properties:
A [local name] of ns.
A [namespace name] which has no value.
The type of the ns
attribute information item is xs:anyURI.
href
attribute information item
The href
attribute information item must be present, according to
the rules of the RNG specification. However, WSDL allows it to be empty,
and considers it only a hint. Caching and cataloging technologies may
provide better information that this hint. The href
attribute information item has the following Infoset
properties:
A [local name] of href.
A [namespace name] which has no value.
The type of the href
attribute information item is xs:anyURI.
Embedding an RNG schema uses the existing top-level rng:grammar
element information item. It may be viewed as simply cutting
and pasting an existing, stand-alone schema to a location inside the
wsdl:types
element information item. The rng:grammar
element information item has the following Infoset properties:
A [local name] of grammar.
A [namespace name] of "http://www.relaxng.org/ns/structure/1.0".
A REQUIRED ns
attribute information items as described below.
Additional attribute information items as specified for
the rng:grammar
element information item in the RNG specification.
Child element information items as specified for the
rng:grammar
element information item in the RNG specification.
ns
attribute information item
The ns
attribute information item defines the namespace of the
type and element definitions embedded in this schema. WSDL modifies the
RNG definition of the rng:grammar
element information item to make this attribute
information item required. The ns
attribute information item has the following Infoset
properties:
A [local name] of ns.
A [namespace name] which has no value.
The type of the ns
attribute information item is xs:anyURI.
Whether embedded or imported, the element definitions present in a schema may be referenced from a Message Reference or Interface Fault component.
A named rng:define definition MUST NOT be referenced from the Message Reference or Interface Fault components.
A named Relax NG element declaration MAY be referenced from a Message
Reference or Interface Fault component. The QName is constructed from the
namespace (ns
attribute information item) of the schema and the content of
the name
attribute information item of the element
element information item An element
attribute information item MUST NOT be used to refer to an
rng:define
element information item.