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Fig. 4.1.
Web service architecture
This figure illustrates the basic building blocks of a service-oriented archi-
tecture. In order for a service to be discovered, a service provider must first
publish the service. In most of the current proposals, this is done in some
central repository. A service requester is then able to issue queries to obtain a
reference to the desired service. This interaction is regulated by, for example,
the UDDI (Universal Description, Discovery and Integration) standard.
To invoke a service the requester needs to know how to do this (i.e. it needs
the interface definition). This information is provided by WSDL (Web Service
Description Language)[27]. Together with the SOAP standard, used for trans-
mitting data, this gives us three core standards (UDDI, WSDL, and SOAP)
that enable the basic interactions required by a service-oriented architecture.
It has to be emphasized that so far mainly syntactical aspects have been
addressed. Therefore, these technologies provide only a set of rigid services
that cannot adapt to changing environments without human intervention.
The human programmer has to be kept in the loop, and the scalability and
economy of Web services are limited [41].
Nevertheless, Web service technologies represent a milestone in making
the development of distributed applications in an SOA possible on large scale.
They provide, for the first time, widely accepted and widely used standards to
govern the communication between applications, facilitating their combination
into more complex entities. This is a major step beyond previous and existing
middleware standards, which have not reached the same level of consensus
and acceptance. This failure was partly due to a lack of agreement between
global players in the industry, and partly due to the lack of simple widespread,
application-independent protocols and standard formats for data exchange -
protocols and standards such as HTTP and XML upon which Web services
are built.
In this chapter, we first give an overview of the terminology used and the
underlying principles in Section 4.1, followed by a brief history and overview
of Web service technologies in Section 4.2. We proceed to a more detailed
analysis of UDDI, WSDL, SOAP, and other relevant Web service standards
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