Discussion of Alternative Schema Languages and Type System Support in WSDL

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http://www.w3.org/TR/2005/@@
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Editors:
Amy Lewis, TIBCO
Bijan Parsia, Maryland Information and Network Dynamics Laboratory at the University of Maryland at College Park

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Abstract

Status of this Document

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/.

Short Table of Contents

1. Introduction
2. Issues facing multiple schema languages/type systems
3. Examples of Specifications of Extension Elements for Alternative Schema Language Support.

Table of Contents

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

1. Introduction

WSDL describes Web Service interaction in terms of exchanges of typed messages. WSDL 2.0: Part 1 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. 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 WSDL 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.

2. Issues facing multiple schema languages/type systems

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:

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 group did not try to enforce the first option, but the general belief of the group was that the other options were confusing and unwise.

3. Examples of Specifications of Extension Elements for Alternative Schema Language Support.

3.1 DTD

A DTD 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 <specref ref="Definitions_details"/> 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.

3.1.1 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).

3.1.2 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.

3.1.3 References to Element Definitions

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.

3.2 RELAX NG

A 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 <specref ref="Definitions_details"/> respectively. The following discussion supplies the prefix rng which is mapped to the URI "http://www.relaxng.org/ns/structure/1.0".

3.2.1 Importing RELAX NG

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.

3.2.1.1 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.

3.2.1.2 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.

3.2.2 Embedding RELAX NG

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.

3.2.2.1 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.

3.2.3 References to Element Declarations

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.