XML News from Friday, April 13, 2007

The W3C Web Services Description Working Group has posted four working drafts and two last call working drafts for WSDL 2.0:

Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language

"Web Services Description Language Version 2.0 (WSDL 2.0) provides a model and an XML format for describing Web services. WSDL 2.0 enables one to separate the description of the abstract functionality offered by a service from concrete details of a service description such as 'how' and 'where' that functionality is offered. This specification defines a language for describing the abstract functionality of a service as well as a framework for describing the concrete details of a service description. "

Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts

WSDL is an XML format for describing network services as a set of endpoints operating on messages containing either document-oriented or procedure-oriented information. Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts defines predefined extensions for use in WSDL 2.0:

  • Message exchange patterns

  • Operation styles

  • Binding Extensions

Web Services Description Language (WSDL) Version 2.0 Part 0: Primer

"This document is a companion to the WSDL 2.0 specification (Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language [WSDL 2.0 Core], Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts [WSDL 2.0 Adjuncts]). It is intended for readers who wish to have an easier, less technical introduction to the main features of the language."

Web Services Description Language (WSDL) Version 2.0 SOAP 1.1 Binding

"WSDL SOAP 1.1 Binding (this specification) describes the binding extension for SOAP 1.1 [SOAP11] protocol. This binding is intended to ease the migration from WSDL 1.1 to WSDL 2.0 for implementers describing services that use SOAP 1.1 protocol. And, this binding allows users to continue using SOAP 1.1 protocol."

Web Services Description Language (WSDL) Version 2.0: Additional MEPs

"This document defines additional message exchange patterns (MEPs) to be used in WSDL 2.0 and are provided as examples of the extensibility of WSDL 2.0. This document is the product of the Web Services Description Working Group, but its contents are non-normative."

Web Services Description Language (WSDL) Version 2.0: RDF Mapping

Web Services Description Language is defined in XML, because XML is the standard format for exchange of structured information. The use of XML brings better interoperability to WSDL generators and parsers, and the use of XML Schema makes the structure of WSDL well constrained, yet extensible. On the other hand, XML vocabularies in general don't have clear composition rules, so combining for example the WSDL description of a Web service, the service's policies and other information (presumably expressed in XML) can be done in many significantly different ways (e.g. extending WSDL, extending the policy language, creating a special XML container for all the information etc.), and little interoperability can be expected when such combined documents are used.

For example, a policy can be combined with WSDL by adding the policy elements in WSDL service element. Equally, a WSDL description can be combined with a policy by adding the WSDL description as part of the policy. While the results should be similar (WSDL with policy information), they are in fact very different for the processing software, and a policy in WSDL cannot easily be used by software that doesn't know WSDL.

In contrast, the Semantic web requires knowledge from many different sources to be easily combined so that unexpected data connections can be used. For this purpose there is the Resource Description Framework (RDF), whose graph structure together with the use of URIs for identifying nodes makes it very easy for different documents to be brought together. If a WSDL document describes a Web service, a policy document attaches constraints to the service and a general description specifies the author of the service, all this information can be merged and the resulting document will contain all the three kinds of information associated with the single service.

The main objective of this specification is to present a standard RDF ([RDF]) and OWL ([OWL]) vocabulary equivalent to WSDL 2, so that WSDL 2 documents can be transformed into RDF and merged with other Semantic Web data.

Comments are due by April 15.

The W3C Web Services Policy Working Group has posted a working draft of WSDL 1.1 Element Identifiers. "This section defines a fragment identifier syntax for identifying elements of a WSDL 1.1 document. This fragment identifier syntax is compliant with the [XPointer Framework]. This document is primarily based upon [WSDL 2.0 Core]. There is a substantial difference between the WSDL 1.1 and WSDL 2.0 fragment identifiers.WSDL 2.0 defines fragment identifiers with respect to the WSDL 2.0 component model, whereas WSDL 1.1 defines XML element and attribute syntax only. Because there is no WSDL 1.1 component model, the WSDL 1.1 fragment identifiers identify WSDL 1.1 elements."

The W3C Web Services Activity. has sent Semantic Annotations for WSDL back to last call. According to the draft,

Semantic Annotations for WSDL and XML Schema (SAWSDL) defines how to add semantic annotations to various parts of a WSDL document such as input and output message structures, interfaces and operations. The extension attributes defined in this specification fit within the WSDL 2.0 [WSDL 2.0] and WSDL 1.1 [WSDL 1.1] extensibility frameworks. For example, it defines a way to annotate WSDL interfaces and operations with categorization information that can be used to publish a Web service in a registry. The annotations on schema types can be used during Web service discovery and composition. In addition, SAWSDL defines an annotation mechanism for specifying the structural mapping of XML Schema types to and from an ontology such mappings could be used during invocation, particularly when mediation is required. To accomplish semantic annotation, SAWSDL defines extension attributes that can be applied both to WSDL elements and to XML Schema elements.

Semantic annotations are references from an element within a WSDL or XML Schema document to a concept in an ontology or to a mapping. This specification defines annotation mechanisms for relating the constituent structures of WSDL input and output messages to concepts defined in an outside ontology. Similarly, it defines how to annotate WSDL operations and interfaces. Further, it defines an annotation mechanism for specifying the structural mapping of XML Schema types to and from an ontology by means of a reference to a mapping definition. The annotation mechanism is independent of the ontology expression language and this specification requires no particular ontology language. It is also independent of mapping languages and does not restrict the possible choices of such languages.