The Internet Encyclopedia (Volume 3)

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Sahai WL040/Bidgolio-Vol I WL040-Sample.cls July 16, 2003 18:35 Char Count= 0

Web ServicesWeb Services

Akhil Sahai,Hewlett-Packard Laboratories

Sven Graupner,Hewlett-Packard Laboratories

Wooyoung Kim,University of Illinois at Urbana-Champaign

Introduction 754

The Genesis of Web Services 754

Tightly Coupled Distributed Software

Architectures 754

Loosely Coupled Distributed Software

Architectures 755

Client Utility 755

Jini 755

TSpaces 755

Convergence of the Two Independent Trends 755

Web Services Today 755

Web Services Description 756

Web Services Discovery 756

Web Services Orchestration 757

Web Services Platforms 758

Security and Web Services 760

Single Sign-On and Digital Passports 760

Payment Systems for Web Services 762

The Future of Web Services 763

Dynamic Web Services Composition and

Orchestration 764

Personalized Web Services 764

End-to-End Web Service Interactions 764

Future Web Services Infrastructures 765

Conclusion 766

Glossary 766

Cross References 766

References 766

INTRODUCTION

There were two predominant trends in computing over

the past decade—(i) a movement from monolithic soft-

ware to distributed objects and components and (ii) an

increasing focus on software for the Internet. Web ser-

vices (or e-services) are a result of these two trends.

Web services are defined as distributed services that are

identified by Uniform Resource Identifiers (URI’s), whose

interfaces and binding can be defined, described, and dis-

covered by eXtensible Markup Language (XML) artifacts,

and that support direct XML message-based interactions

with other software applications over the Internet. Web

services that perform useful tasks would often exhibit the

following properties:

Discoverable—The foremost requirement for a Web ser-

vice to be useful in commercial scenarios is that it be

discovered by clients (humans or other Web services).

Communicable—Web services adopt a message-driven

operational model where they interact with each other

and perform specified operations by exchanging XML

messages. The operational model is thus referred to

as the Document Object Model (DOM). Some of pre-

eminent communication patterns that are being used

between Web services are synchronous, asynchronous,

and transactional communication.

Conversational—Sending a document or invoking a met-

hod, and getting a reply are the basic communication

primitives in Web services. A sequence of the primi-

tives that are related to each other (thus, conversation)

forms a complex interaction between Web services.

Secure and Manageable—Properties such as security, re-

liability, availability, and fault tolerance are critical for

commercial Web services as well as manageability and

quality of service.

As the Web services gain critical mass in the information

technology (IT) industry as well as academia, a dominant

computing paradigm of that of software as a monolithic

object-oriented application is gradually giving way to soft-

ware as a service accessible via the Internet.

THE GENESIS OF WEB SERVICES

Contrary to general public perception, the development of

Web services followed a rather modest evolutionary path.

The underpinning technologies of Web services borrow

heavily from object-based distributed computing and de-

velopment of the World Wide Web (Berners-Lee, 1996).

In the chapter, we review related technologies that help

shape the notion of Web services.

Tightly Coupled Distributed

Software Architectures

The study of various aspects of distributed computing can

be dated back as early as the invention of time-shared mul-

tiprocessing. Despite the early start, distributed comput-

ing remained impractical until the introduction of Object

Management Group’s (OMG) Common Object Request

Broker Architecture (CORBA) and Microsoft’s Distributed

Component Object Model (DCOM), a distributed ex-

tension to the Component Object Model (COM). Both

CORBA and DCOM create an illusion of a single machine

over a network of (heterogeneous) computers and allow

objects to invoke remote objects as if they were on the

same machine, thereby vastly simplifying object sharing

among applications. They do so by building their abstrac-

tions on more or less OS- and platform-independent mid-

dleware layers. In these software architectures, objects de-

fine a number of interfaces and advertise their services

by registering the interfaces. Objects are assigned identi-

fiers at the time of creation. The identifiers are used for

754