Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 112

(Erl 2005). SOAP also provides precise Header blocks for the management
of intermediaries that have been taken into account since the early stages of
the protocol design. Intermediaries are software systems capable either to
receive or to forward a SOAP message, and are usually used to provide value
added services or to support the scalability of a distributed environment.
The set of a SOAP sender, the zero or more SOAP Intermediaries and the
ultimate SOAP receiver is known as SOAP message path.
The Web Services Description Language
The Web Services Description Language (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. The operations and messages are
described abstractly, and then bound to a concrete network protocol
and message format to defi ne an endpoint.'
(Christensen et al. 2001)
In other words, WSDL is an XML based language for Web service
description and accessing. It uses XML Schema for both the defi nition of
the type system and the defi nition of SOAP messages (although their use
is not compulsory) and, furthermore, it separates the abstract aspects of a
service description from the concrete aspects, such as the binding with a
certain network protocol. Essentially, a WSDL description contains the three
fundamental properties of a Web service (What it does, how it is accessed,
where it is located) through the following elements: Types, Message,
Operation, Port Type, Binding, Port, and Service.
The former four elements refer to the Abstract Description of a Web
service, which specifi es the interface characteristics with no references
to any specifi c technology platform; the latter three refer to its Concrete
Description which allows to connect the abstract interface of a Web service
to a real technology and to transport protocol. One of the main advantages
of this distinction is that the “public interface” of the Web service can be
preserved by changes in the underlying technology. Figure 3 depicts a
WSDL Document structure as made up of an Abstract Description and a
Concrete Description.
The Open Geospatial Consortium Web Services
The Open Geospatial Consortium (OGC) is 'an international industry
consortium of more than 479 companies, government agencies and
universities participating in a consensus process to develop publicly
available interface standards.' The main goals of this consortium are to