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756 WEBSERVICESpriceline.com (http://www.priceline.com) and Expedia.
com (http://www.expedia.com) act as a broker for airlines,
hotels, and car rental companies. They offer through their
portal sites statically composed Web services that have
prenegotiated an understanding with certain airlines and
hotels. These are mostly a business-to-consumer (B2C)
kind of Web services. A large number of technologies
and platforms have appeared and been standardized so
as to enable the paradigm of Web services to support
business-to-business (B2B) and B2C scenarios alike in a
uniform manner. These standards enable creation and de-
ployment, description, and discovery of Web services, as
well as communication amongst them. We describe some
preeminent standards below.
The Web Services Description Language (WSDL) is a
standard to describe service interfaces and publish them
together with services’ access points (i.e., bindings) and
supported interfaces. Once described in WSDL, Web ser-
vices can be registered and discovered using the Univer-
sal Description, Discovery, and Integration (UDDI). Af-
ter having discovered its partners, Web services use the
Simple Object Access Protocol (SOAP), which is in fact
an incarnation of the Remote Procedure Call (RPC) in
XML, over the HyperText Transfer Protocol (HTTP) to ex-
change XML messages and invoke the partners’ services.
Though most services are implemented using platform-
independent languages such as Java and C#, development
and deployment platforms are also being standardized;
J2EE and .NET are two well known ones. Web services
and their users often expect different levels of security
depending on their security requirements and assump-
tion. The primary means for enforcing security are dig-
ital signature and strong encryption using the Public
Key Infrastructure (PKI). SAML, XKMS, and XACML are
some of recently proposed security standards. Also, many
secure payment mechanisms have been defined. (See
Figure 1).Web Services Description
In traditional distributed software architectures, devel-
opers use an interface definition language (IDL) to de-
fine component interfaces. A component interface typi-
cally describes the operations the component supports by
specifying their inputs and expected outputs. This enables
developers to decouple interfaces from actual implemen-
tations. As Web services are envisaged as software acces-
sible through the Web by other Web services and users,.NetUDDIIWSDLSOAPJ2EE
HPPM/
MQSeriesWeb MethodsFigure 1: Web services.Web services need to be described so that their interfaces
are decoupled from their implementations. WSDL serves
as an IDL for Web services.
WSDL enables description of Web services indepen-
dently of the message formats and network protocols
used. For example, in WSDL a service is described as a set
of endpoints. An endpoint is in turn a set of operations.
An operation is defined in terms of messages received or
sent out by the Web service:Message—An abstract definition of data being communi-
cated consisting of message parts.
Operation—An abstract definition of an action supported
by the service. Operations are of the following types:
one-way, request–response, solicit–response, and noti-
fication.
Port type—An abstract set of operations supported by one
or more endpoints.
Binding—A concrete protocol and data format specifica-
tion for a particular port type.
Port—A single endpoint defined as a combination of a
binding and a network address.
Service—A collection of related endpoints.As the implementation of the service changes or evolves
over time, the WSDL definitions must be continuously
updated and versioning the descriptions done.Web Services Discovery
When navigating the Web for information, we use key
words to find Web sites of interest through search engines.
Often times, useful links in search results are mixed with
a lot of unnecessary ones that need to be sifted through.
Similarly, Web services need to discover compatible
Web services before they undertake business with them.
The need for efficient service discovery necessitates some
sort of Web services clearing house with which Web
services register themselves. UDDI (http://www.uddi.org)
supported by Ariba, IBM, Microsoft, and HP, is an ini-
tiative to build such a Web service repository; it is now
under the auspice of OASIS (http://www.oasis-open.org).
These companies maintain public Web-based registries
(operator sites) consistent with each other that make
available information about businesses and their techni-
cal interfaces and application program interfaces (APIs).
A core component of the UDDI technology is registra-
tion, an XML document defining a business and the Web
services it provides. There are three parts to the regis-
tration, namely awhite pagefor name, address, contact
information, and other identifiers; ayellow pagefor clas-
sification of a business under standard taxonomies; and
agreen pagethat contains technical information about
the Web services being described. UDDI also lists a set of
APIs for publication and inquiry. The inquiry APIs are for
browsing information in a repository (e.g., findbusiness,
getbusinessDetail). The publication APIs are for business
entities to put their information on a repository.
E-marketplaces have been an important development
in the business transaction arena on the Internet. They
are a virtual meeting place for market participants
(i.e., Web services). In addition to the basic registration