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The first component can, in principle, be realized by any of the common
transport protocols such as SMTP or FTP. The most popular protocol in the
context of Web services is HTTP. HTTP has the advantage of allowing the
reuse of existing infrastructure while catering for the preexisting know-how of
potential users.
The second component is realized by SOAP [92]. SOAP is a specifica-
tion for standardization of the exchange of XML-coded messages. Moreover,
SOAP specifies the binding to HTTP as an underlying communication proto-
col between two addressable service partners (“endpoints”). Communication
viaSOAPoverHTTPis,inasense,asolutiontoexistingproblemswith
approaches such as RMI and CORBA. The shortcomings of the latter two
include imposing tight coupling between service partners and the inability to
reuse the open “Web infrastructure”: firewalls usually block special propri-
etary communication protocols in common IT across enterprise boundaries.
The Web Service Description Language (WSDL) [27] serves as the third
component for XML-based description of interfaces. WSDL allows a distinc-
tion between the description of the abstract functionality (operations) that
a Web service provides and the details of how to access the service. On the
one hand, operations, along with their input and output parameters, are de-
scribed; on the other hand, the addresses at which the service is invocable
(“endpoints”) and the supported transport protocols can be described inde-
pendently.
Finally, Universal Description, Discovery and Integration (UDDI) [10] pro-
vides interfaces to publish (advertise), browse, and query existing Web ser-
vices. UDDI thus realizes the fourth main component in Web-services-based
middleware solutions.
Excellent overviews of the predecessors of current Web service technologies
and the current state of the art of “syntactical”
2
Web service standards are
provided by Alonso et al. [2] and Weerawarana et al. [133].
4.3 The Web Service Technology Stack
In this section we give further details on the current Web service technology
stack, covering the three main standards: SOAP, WSDL and UDDI.
4.3.1 SOAP
The overall goal of the Simple Object Access Protocol (SOAP), as the first
version of this protocol, published as a W3C Note in May 2000, was called,
is the provision of a messaging framework for peers communicating messages
2
We talk about “syntactical” standards here, to emphasize the fact that these
technologies do not yet tackle the semantic annotation of (Web) services.
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