Web Services Concepts, Architectures And Applications

5
Web Services

In previous chapters we have discussed the architecture of information systems
(Chapter 1), middleware and enterprise application integration (Chapters 2
and 3), and the basics of Web technology (Chapter 4). These chapters have
shown a chronological evolution of the technology used for EAI and for building distributed applications. All these technologies have been rathersuccessful
in addressing several of the problems created by application integration. The
success, however, has been restricted to certain settings (e.g., LAN-based systems, homogeneous middleware platforms, etc.). True application integration
requires tools that go one step beyond what conventional middleware and
EAI platforms have achieved. Web services and the associated technology are
beingleveraged to take such a step.
To establish the context for the rest of the book, we provide in this chapter
an introduction to Web services. We look at Web services as a way to expose
the functionality of an information system and make it available through standard Web technologies. The use of standard technologies reduces heterogeneity, and is therefore key to facilitating applicationintegration. Furthermore,
we show that Web services naturally enable new computing paradigms and
architectures, and are specifically geared toward service-oriented computing,
a paradigm often touted in the past but never quite realized.
This chapter is structured along a continuum that goes from the needs
that motivate the introduction of Web services to the solutions that Web services provide. Webegin by showing the limitations of conventional technology
in tackling some of the application integration challenges, thereby raising the
need for a novel technology–a need addressed by Web services. We then describe the essential concepts behind Web services and how they tackle the
application integration problem (Section 5.1). Next, we provide an overview
of Web services middleware, focusingin particular on the functionality that
this middleware must provide to support the development of distributed applications based on Web services (Section 5.2). Finally, we discuss Web services
architectures (Section 5.3).

Web Services - Concepts, Architectures and Applications
Gustavo Alonso, Fabio Casati, Harumi Kuno, Vijay Machiraju
Springer Verlag, ISBN 3-540-44008-9
Copyright SpringerVerlag Berlin Heidelberg 2004

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5 Web Services

5.1 Web Services and their Approach to Distributed
Computing
Before describing the problems Web services try to solve and how they address
them, we define what Web services are.
5.1.1 Defining Web Services
The term Web services is used very often nowadays, although not always with
the same meaning. Nevertheless, the underlyingconcepts and technologies are
to a large extent independent of how they may be interpreted.
Existing definitions range from the very generic and all-inclusive to the
very specific and restrictive. Often, a Web service is seen as an application
accessible to other applications over the Web (see e.g., [72, 133]). This is a
very open definition, under which just about anything that has a URL is a
Webservice. It can, for instance, include a CGI script. It can also refer to a
program accessible over the Web with a stable API, published with additional
descriptive information on some service directory.
A more precise definition is provided by the UDDI consortium, which characterizes Web services as “self-contained, modular business applications that
have open, Internet-oriented, standards-basedinterfaces” [203]. This definition is more detailed, placing the emphasis on the need for being compliant
with Internet standards. In addition, it requires the service to be open, which
essentially means that it has a published interface that can be invoked across
the Internet. In spite of this clarification, the definition is still not precise
enough. For instance, it is not clear what it is...