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.
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."
