An Introduction to SOAP,WSDL, and UDDI
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Unraveling the
Web Services Web
An Introduction to SOAP,
WSDL, and UDDI
O
ver the past few years, businesses have
interacted using ad hoc approaches that
take advantage of the basic Internet infra-
structure. Now, however, Web services are emerg-
ing to provide a systematic and extensible frame-
work for application-to-application interaction,
built on top of existing Web protocols and based
on open XML standards.
Say, for example, that you want to purchase a
vacation package using an online travel agent. To
locate the best prices on airline tickets, hotels, and
rental cars, the agency will have to poll multiple
companies, each of which likely uses different,
incompatible applications for pricing and reserva-
tions. Web services aim to simplify this process by
defining a standardized mechanism to describe,
locate, and communicate with online applications.
Essentially, each application becomes an accessible
Web service component that is described using open
standards. An online travel service could thus use
the same Web services framework to locate and
reserve your package elements, as well as to lease
Internet-based credit check and bank payment ser-
vices on a pay-per-use basis to expedite fund trans-
fers between you, the travel agency, and the vendors.
The Web services framework is divided into
three areas — communication protocols, service
descriptions, and service discovery — and specifi-
cations are being developed for each. In this arti-
cle, we look at the specifications that are current-
ly the most salient and stable in each area:
■ the simple object access protocol (SOAP,
www.w3.org/2000/xp) which enables commu-
nication among Web services;
■ the Web Services Description Language (WSDL,
www.w3.org/TR/wsdl.html), which provides a
formal, computer-readable description of Web
services; and
■ the Universal Description, Discovery, and Inte-
gration (UDDI, www.uddi.org) directory, which
is a registry of Web services descriptions.
At this point, Web services technology is still
emerging, and researchers are still developing
important pieces, including quality of service
descriptions and interaction models. Because the
Web services framework is modular, however, you
can use just the parts of the stack you need. There-
fore, developers can take advantage of the avail-
able specifications and tooling now and incorpo-
rate more modules as the technology matures.
Communication: SOAP
Given the Web’s intrinsically distributed and het-
erogeneous nature, communication mechanisms
must be platform-independent, international, secure,
and as lightweight as possible. XML is now firmly
established as the lingua franca for information and
data encoding for platform independence and inter-
nationalization. Building a communication protocol
using XML is thus a natural answer for Web services.
Enter SOAP, which was initially created by
Microsoft and later developed in collaboration with
Developmentor, IBM, Lotus, and UserLand. SOAP
is an XML-based protocol for messaging and
remote procedure calls (RPCs). Rather than define
a new transport protocol, SOAP works on existing
transports, such as HTTP, SMTP, and MQSeries.
At its core, a SOAP message has a very simple
86 MARCH • APRIL 2002 http://computer.org/internet/ 1089
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7801/02/$17.00 ©2002 IEEE IEEE INTERNET COMPUTING
Francisco Curbera, Matthew Duftler, Rania Khalaf,William Nagy,
Nirmal Mukhi, and Sanjiva Weerawarana • IBM T.J.Watson Research Center
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