40 WebSphere and .Net Interoperability Using Web Services
4.1 Web services concepts
Web services are self-contained applications that are published, located,
described and invoked over the Internet or intranet.
Web services perform business functions, ranging from a simple query to
complex business process interactions. A Web service can be built from the
ground up as a new application or an existing legacy system can be
re-engineered to make it Web service enabled.
There are three main participants in the generic Web service pattern.
Figure 4-1 Web services roles and operations
1. The service provider creates and hosts a Web service and possibly publishes
its interface and access information to the service registry, such as to a UDDI
The service provider decides which services to expose to the Internet, to
extranets or to intranets, manages the access to the service, and provides
information about the service to the service broker.
2. The service broker (also known as the service registry) is responsible for
making the Web service interface and implementation access information
available to any potential service requestor. Typically, a broker, such as a
UDDI registry, contains information to identify the service and points back to
the service provider for obtaining details about the service interface.
The implementers of a broker have to decide about the scope of the broker.
Public brokers are available all over the Internet, while private brokers are
only accessible to a limited audience, for example, users of a company-wide
Chapter 4. Web services primer 41
intranet. Furthermore, the width and breadth of the offered information has to
be decided. Some brokers will specialize in breadth of listings. Others will
offer high levels of trust in the listed services. Some will cover a broad
landscape of services, and others will focus within a given industry. Brokers
also exist that simply catalog other brokers. Depending on the business
model, a broker may attempt to maximize look-up requests, number of
listings, or accuracy of the listings.
3. The service requestor (the Web service client) locates entries in the broker
registry using various find operations and then connects to the service
provider in order to invoke one of its Web services.
One important issue for users of services is the degree to which services are
statically chosen by designers compared to those dynamically chosen at
runtime. Even if most initial usage is largely static, any dynamic choice opens
up the issues of how to choose the best service provider and how to assess
quality of service. Another issue is how the user of services can assess the
risk of exposure to failures of service suppliers.
Web services are sometimes created as brand new software components, but
more frequently are composed of existing (“legacy”) systems. The Web service
provider may expose the Web service interface on a gateway server and connect
the gateway to the legacy systems using an enterprise service bus.
4.1.1 What is a Web service?
Let’s turn to the W3C Web services Architecture Working Group. In
http://www.w3c.org/TR/ws-arch/#whatis, they define a Web service succinctly
“... a software system designed to support interoperable machine-to-machine
interaction over a network. It has an interface described in a
machine-processable format (specifically WSDL). Other systems interact with
the Web service in a manner prescribed by its description using SOAP
messages, typically conveyed using HTTP with an XML serialization in
conjunction with other Web-related standards.
What does this definition mean? Let’s examine the definition in more detail.
򐂰 “[A Web service is] a software system ...
Any software system can be a Web service. Web services are technology-,
language- and platform-independent. The software system could be a
WebSphere Enterprise JavaBean, a Microsoft .Net class, a CICS®
transaction, a WebSphere MQSeries Workflow; virtually any existing software
system can be turned into a Web service.

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