STRUCTURING DESIGN KNOWLEDGE

IN SERVICE-ORIENTED ARCHITECTURE

Dionisis X. Adamopoulos

Department of Technology Education & Digital Systems, University of Piraeus, Greece

Keywords: Web services, Web service engineering, Internet middleware.

Abstract: Web services are emerging technologies that can be considered as the result of the continuous improvement

of Internet services due to the tremendous increase in demand that is being placed on them. They are rapidly

evolving and are expected to change the paradigms of both software development and use, by promoting

software reusability over the Internet, by facilitating the wrapping of underlying computing models with

XML, and by providing diverse and sophisticated functionality fast and flexibly in the form of composite

service offerings. In this paper, the different facets of Web services are identified and a flexible approach to

engineering complex Web services is adopted in the form of a proposed framework for the development of

Web services. After the examination of its main constituent parts, it is argued that its full potential and that

of Web service engineering in general, is realized through the gradual formation of a rich service grid

offering value-added supporting functionality and therefore the main desirable properties of such a service

grid are highlighted. Finally, the paper outlines a validation approach for the proposed framework and

assembles important pointers for future work and concluding remarks.

1 INTRODUCTION

Web-enabled service-oriented computing is becom-

ing the prominent paradigm for distributed comput-

ing and e-commerce, creating significant opportuni-

ties for a variety of providers to develop value-added

services by specifying new Web services or by com-

bining already existing ones. These services are self-

contained, Web influenced programming entities

capable not only of performing business activities on

their own, but also possessing the ability to engage

other Web services in order to complete higher-order

business transactions. Therefore, Web services can

be considered as a special category of telematic ser-

vices, that although they have several unique

characteristics, they remain geographically

distributed entities providing a number of people a

predefined, carefully selected, set of capabilities / fa-

cilities, utilising the resources of telecommunication

networks (Adamopoulos, 2003).

This paper attempts to determine the boundaries

of Web service engineering and, considering the

needs of companies that deploy Web services to sup-

port increasingly sophisticated business processes,

proposes a framework for the development of Web

services and examines its main constituent parts, ad-

dressing important issues for the creation and provi-

sion of a new generation of functionally rich, adapt-

able, web-centric, composite applications. Further-

more, it introduces the concept of service grids as

the necessary infrastructure that will enable Web

services to transform the Web from a collection of

information into a distributed computational entity,

and identifies open matters and current technical

challenges for the Web services community in asso-

ciation with the proposed framework.

2 WEB SERVICE ENGINEERING

A Web service is programmable application logic

accessible using standard Internet protocols, fulfill-

ing a specific task or a set of tasks and representing a

discrete unit of business or system functionality, that

can be combined with other Web services to main-

tain business transactions or workflows (Curbera,

2004)(WSAWG, 2005). By exploiting Web services

an organization is able to provide ("expose") any

business function to any other entity, such as another

business function, an organization, a particular

community, as well as end users.

180

X. Adamopoulos D. (2008).

STRUCTURING DESIGN KNOWLEDGE IN SERVICE-ORIENTED ARCHITECTURE.

In Proceedings of the Third International Conference on Software and Data Technologies - SE/GSDCA/MUSE, pages 180-185

DOI: 10.5220/0001884701800185

 SciTePress

Web services are self-contained, self-describing,

modular software entities that can be published, lo-

cated and invoked across the Web. Each discrete

Web service can be deployed on and accessed from

any node on the Internet, because once a Web ser-

vice is deployed, other applications (and other Web

services) can discover and invoke the deployed ser-

vice. Therefore, multiple Web services can be com-

bined or assembled to form new service configura-

tions and deliver more valuable and sophisticated

functionality supporting diversified business objec-

tives.

As a new domain or scientific discipline at the

boundaries of software engineering and telecommu-

nications, Web service engineering addresses the

technologies and engineering processes required to

define, design, implement, test, verify, validate, de-

ploy, combine, maintain, and manage Web services

that meet user needs in the current or future net-

works. Its main objective is to ensure the introduc-

tion of new and enhanced Web services and their

management, in a fast and efficient manner. It relies

heavily on open distributed object-oriented process-

ing and Internet technology, and ambitiously prom-

ises to significantly facilitate the offering of a wide

variety of highly sophisticated and personalised ser-

vices over the widest possible coverage area.

Finally, it has to be stressed that Web services

represent the convergence between Service-Oriented

Architectures (SOAs) and the Web. SOAs (as the

one proposed by the Telecommunications Informa-

tion Networking Architecture-Consortium, TINA-C)

have evolved over the last 10 years to support high

performance, scalability, reliability and availability

(Adamopoulos, 2003). To achieve these properties,

applications are designed as services, that can be

accessed through a programmable interface and run

on a cluster of centralized application servers. In the

past, clients accessed these services using a tightly

coupled, distributed object protocol, such as Micro-

soft’s DCOM, OMG’s CORBA or Sun’s Java RMI.

While these protocols are very effective for building

a specific application, they limit the flexibility of the

system. Furthermore, each of the protocols is

constrained by dependencies on vendor

implementations, platforms, languages or data en-

coding schemes that severely limit interoperability

and none of them operates effectively over the Web

(Chung, 2003). Web services inherit all the best

features of the SOAs and all the best aspects of

component-based development in general and com-

bine them with the Web. Like components, Web ser-

vices represent functionality that can be easily

reused without knowing how the service is imple-

mented. However, the Web supports universal com-

munication using loosely coupled connections and

Web protocols are completely vendor-, platform-,

and language- independent.

3 A FRAMEWORK FOR THE

DEVELOPMENT OF WEB

SERVICES

Because of the inherent complexity of Web

technologies and the recent diversification of the

telecommunications environment, Web service engi-

neering activities should satisfy a number of re-

quirements, in order to maximise their usefulness,

fulfil the emerging increased expectations regarding

their value and impact, and lead eventually to a Web

populated by a variety of service objects. Current

Web service technology scores rather low compared

to these requirements. Therefore, in an attempt to

revitalize Web service engineering and enable it for

the crucial role that is anticipated to have in the new

emerging telecommunications environment, a Web

service engineering framework is proposed with the

objective to provide a rich conceptual model for the

development and the description of Web services

bringing this technology to its full potential.

The proposed framework is placed inside a

composite organisational context (a “business

ecology”), in order to signify that Web service

engineering activities are normally performed by a

variety of entities / business formations. Although in

practice many of the companies operating in this

sector / area blend various functions into a

composite offering and adopt many different roles,

the major players in this new always-on Web

services landscape are Application Service Providers

(ASPs), Managed Hosting Providers (MHPs), Inter-

net Service Providers (ISPs), network operators, In-

dependent Software Vendors (ISVs) and Business

Service Providers (BSPs) (Wainewright, 2006).

Therefore, the proposed framework is influenced by

their business objectives, their general

telecommunications and IT strategic orientation,

their knowledge, their problem solving attitude and

their experience.

The main constituent parts of the proposed Web

service engineering framework are:

• A Web service development methodology: It is a

methodology that guides service developers

during the entire process of Web service creation.

• A Web service support environment: It is an envi-

ronment aiming to facilitate, both the development

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181

of Web services (in cooperation with the Web ser-

vice development methodology) and their execu-

tion under real conditions. It consists mainly of:

− Web service engineering principles: These

are concepts, guidelines, design patterns,

practices and (in general) mental constructs

that are applicable to Web service

engineering activities.

− A Web service architecture: It contains in a

structured manner all necessary details for the

information and computational modelling of

Web services.

− A Web service execution environment: It

encompasses the necessary computing and

network infrastructure and the appropriate an-

cillary software (e.g. operating systems, data-

base management systems, etc.), which is

needed for and during the execution of a Web

service. Its most important part is the Web

platform, which abstracts over all the other

parts and reduces greatly the effort needed for

the implementation of a Web service.

Furthermore, the Web platform is

accompanied by a collection of software tools

(together with a reuse infrastructure) that are

used according to the Web service

development methodology with the aim to

assist the service developer(s) when applying

the methodology.

A Web services environment conforms to the

conceptual roles and operations that characterize

every SOA. The three basic roles are the service

provider, the service consumer and the service

broker. A service provider offers the service and

publishes the contract that describes its interface. It

then registers the service with a service broker. A

service consumer queries the service broker

according to its specific needs and finds a

compatible service. Then, the service broker informs

the service consumer on where to find the service

and its service contract. Finally, the service

consumer uses the contract to bind the client to the

service. In order for the three conceptual roles to

accomplish the related conceptual operations, a SOA

system must supply / specify three core functional

architecture components; namely transport, de-

scription, and discovery (WSAWG, 2005).

The distributed object platforms (middleware)

that form the basis of SOAs (DCOM, CORBA or

Java RMI) define their own vertical set of formats

and protocols to implement the core SOA functions.

This approach ensures consistency among

applications that share the same middleware, but

prevents interoperability with applications that use

different middleware. It also requires that every

service producer and service consumer that engages

in a conversation must have the appropriate

middleware installed and loaded on its computing

infrastructure.

On the other hand Web services (Internet middle-

ware), unlike traditional SOA systems, do not re-

quire an entirely new set of protocols. The most ba-

sic Web services protocol is XML (an industry stan-

dard), which is used as the message data format and

is also used as the foundation of all other Web ser-

vices protocols. Web services use XML to describe

their interfaces and to encode their messages. How-

ever, XML by itself does not ensure effortless com-

munication. The applications need standard formats

and protocols that allow them to properly interpret

the XML. Hence, the following XML-based tech-

nologies have emerged as the de facto standards for

Web services: Simple Object Access Protocol

(SOAP), Web Services Description Language

(WSDL) and Universal Description, Discovery and

Integration (UDDI).

As most Web service configurations suggest, the

three core functional architecture components

(transport, description, and discovery) are imple-

mented using SOAP, WSDL, and UDDI, respec-

tively, forming the Web Services Architecture

(WSA). A UDDI registry has the role of a service

broker. The register and find operations are

implemented using the UDDI Inquiry and UDDI

Publish APIs. A WSDL document describes the

service contract and is used to bind the client to the

service. All transport functions are performed using

SOAP.

The Web Services Architecture (WSA) provides

the necessary means to create Web services for the

coverage of an infinite variety of needs and to dy-

namically combine them to satisfy more specialized

business requirements at any point in time, by knit-

ting together micro-services (individual process

components) into a broader application entity offer-

ing enriched functionality. However, such Web ser-

vice creation activities can be extremely risky and

difficult as Web services can be relatively simple,

like the delivery of a currency converter or stock

quotes to a cell phone, but also very complex, like a

payment processing service where millions of euros

are being transferred in individual transactions from

one account to another.

Furthermore, all Web services are currently com-

posed in a rather ad hoc and opportunistic manner by

simply combining their operations and input and

output messages. If the requirements change or need

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to be adjusted, then the composition will have to be

respecified and recreated by possible interlinking

additional or modified service interfaces. This ap-

proach leads to a proliferation of badly specified ser-

vice operations and results in unmanageable and

cluttered solutions. In this case, the needs of service

developers that want to reuse the design and imple-

mentation of existing Web services only by exten-

sion or restriction, without developing them from

scratch, cannot be satisfied.

An equally important problematic situation arises

also from the fact that unlike a traditional telecom-

munications enterprise network, many different pro-

viders share the multi-layered network and software

infrastructure of Web services. Therefore, creating

an integrated, end-to-end application delivery infra-

structure incorporated into a Web service requires

close cooperation between all the interconnected,

autonomous participants (providers and enterprises).

It is evident that as Web services become more

sophisticated and more global in reach and capacity,

it becomes increasingly important to provide

additional assistance to service developers in order

to ensure the effective encounter of the above

mentioned problems and the efficient support of

commercial-grade application functionality by Web

services in an incremental manner, with little risk

and at low cost. Recognising these needs, a Web

service development methodology is proposed. This

methodology “covers” in a systematic and structured

manner the entire Web service creation process

through a requirements capture and analysis phase, a

Web service analysis phase, a Web service design

phase, a Web service implementation phase, and a

Web service validation and testing phase. It recog-

nises the inefficiency of current general-purpose

software engineering methodologies to address suc-

cessfully Web service engineering matters and pro-

poses a novel Web service creation process based on

fundamental object-oriented analysis and design

concepts and on important results of service creation

research regarding the development of telematic ser-

vices upon distributed object platforms utilising

SOAs. The novel character of the proposed method-

ology is reinforced by the adoption of an

incremental and iterative use case driven approach,

by the consideration of the special needs imposed by

the Web Services Architecture, by the careful

incorporation of the Unified Modelling Language

(UML) notation and the XML technology

throughout the service creation process, by the

exploitation of specially constructed design patterns,

and by the promotion of reusability and dynamic

Web service compositions.

Unlike other SOA systems, Internet middleware

does not define a specific invocation mechanism. It

simply defines the communication protocols (XML,

SOAP, etc). The specifics of how Web services in-

teract with SOAP and WSDL have been left as an

exercise to the service developer’s community.

Since the WSA is based on standard XML, Web

services can be implemented by using the pervasive

XML processing techniques that are supported by a

variety of software tools, together with ad hoc

invocation implementation patterns. However,

efficiency can be greatly improved by using spe-

cialized Web services platforms, which provide a

ready-made foundation for building and deploying

Web services, based on a set of carefully selected

invocation mechanisms. The advantage of using a

Web services platform is that developers don’t need

to be concerned with constructing or interpreting

SOAP messages.

The two most prominent Web services platforms

currently are Microsoft’s .NET and Sun’s J2EE.

More specifically, Microsoft has defined a set of

standard programming interfaces and class libraries

for the Visual Studio .NET languages within the

.NET framework. On the other hand, the Java Com-

munity Process’ (JCP) has recently defined a set of

standard programming interfaces for Java Web ser-

vices, as part of the J2EE specification. It is evident

that due to the increased capabilities of these plat-

forms and their continual improvement the selection

process is a challenging task (Williams, 2003).

4 THE IMPORTANCE OF

SERVICE GRIDS

A distributed Web services infrastructure will be re-

quired before Web services technology can be

broadly deployed to support mission critical applica-

tions within and across enterprises. The difference

between traditional Web content and Web services

originates from the addition of process – the se-

quence of events that need to happen in order to pro-

duce a result. The fact that the users / participants of

a Web service are distributed and need to complete

certain processes adds important new operating re-

quirements, such as consistency, authenticity, timeli-

ness, integrity, and persistence (

Wainewright, 2006).

The distributed Web services infrastructure must

support these requirements. Service grids constitute

a key component of this infrastructure, especially as

its scope expands beyond the boundaries of the

enterprise to encompass a broad range of business

partners. Service grids provide a set of enabling

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183

utilities and ancillary services to support more robust

connections between providers and users of Web

services. This enabling functionality offered by

service grids is distinct from application

functionality that is directly useful to end-users. It

focuses on supporting the application logic with

functionality like security, routing of messages

across Web services or data transformation so that

one Web service can access data from another Web

service. It can be considered as the equivalent of the

supporting functionality provided by object-oriented

middleware in SOAs, with the difference that in this

case it is delivered as a set of managed services,

rather than installed in the computing infrastructure

communicating at either end of the connection.

The full value of the proposed framework for the

development of Web services is realized when it is

used in combination with a carefully created service

grid in an integrated manner. This will also require

the enhancement of the proposed methodology in

order to take into account the availability of the

managed services offered by the service grid, espe-

cially during the specification of non-functional re-

quirements.

Although service grids are still at a very early

stage of development, there is no doubt that the

adoption of Web services technology will be signifi-

cantly affected by the pace and scope of service grid

deployments. Nevertheless, significant initiatives are

already taking place leading to the emergence of

early generations of service grids and specialized

service grid utilities.

5 CONCLUSIONS AND FUTURE

WORK

Web service engineering provide a sound basis for

developing and deploying interoperable Web ser-

vices, allowing the gradual transformation of the

Internet to a global common information networking

platform where organizations and individuals com-

municate with each other to carry out various com-

mercial activities and to provide value-added func-

tionality. With the emergence of Web services the

Internet has become a computing execution network,

processing commercial transactions and business

applications.

However, many of the standards required for

Web services are not yet fully defined. The SOAP,

WSDL and UDDI specifications that underpin cur-

rent Web services technology form a de facto stan-

dard infrastructure with little endorsement by offi-

cial standards organizations. For this reason, the ex-

isting specifications contain a number of ambiguities

and inconsistencies, and address only basic Web

services communications. Two standards groups are

currently working on the definition of official Web

services standards: The World Wide Web Consor-

tium (W3C) and the Organisation for the Advance-

ment of Structured Information Standards (OASIS).

W3C focuses on core infrastructure specifications

and OASIS focuses on higher-level functionality.

In general, Web services computing poses

significant theoretical and engineering challenges as

developers determine how to leverage emerging

technologies to automate semantically rich

application domains and to create software entities

with an open interoperable character, based on

cross-organisational, heterogeneous software

components. The proposed framework for the

development of Web services aims to address this

movement towards Web-enabled service-oriented

computing, where application logic is offered as a

set of services both within and across enterprises.

Regarding this framework, it is currently attempted

to examine it and specify it in greater detail,

focusing especially on the Web service development

methodology, as it presents increased research inter-

est and practical value. Furthermore, the validation

and evaluation of the proposed methodology is

considered by applying it to the development (from

requirements elicitation up to actual implementation)

of a complex representative Web service (eXtended

e-Learning Interactive eXperience using Internet

Services, eXeLIXIS). This Web service enables

students to specify their learning objectives and

learning preferences, attend specially designed e-

classes that satisfy their needs, participate in training

multiplayer games that enhance their understanding

and test their knowledge, getting feedback in order

to amend or enrich their learning objectives. Two

alternative implementations of this Web service are

being constructed (using Microsoft’s .NET and

Sun’s J2EE) and special emphasis is placed on Web

service composition matters, as the training material

can originate by a variety of providers, and on the

incorporation of session tracking, as the maintenance

of state information for the students by the Web

service improves the overall performance, simplifies

program development and provides for a more

intuitive user interface. This validation attempt aims

to provide tangible evidence about the correctness,

the efficiency and the true practical value of the

proposed methodology. All these efforts treat the

proposed framework as a conceptual “umbrella” for

Web service engineering activities and gradually

ICSOFT 2008 - International Conference on Software and Data Technologies

184

transform it to a more precise and concrete

construct.

Web services constitute undoubtfully a

promising technology that will increasingly assist

the integration of heterogeneous islands of

application logic (objects on the Web) to

homogeneous component-based solutions (a web of

objects), especially when supported by robust

service grids. However, developers should keep in

mind that Web services are still a fast moving target

and an immature technology. Existing object-

oriented middleware such as COM+/.NET, CORBA,

and EJB/RMI may be still necessary to implement

sophisticated back-end services, but Web services

claim a prominent role when these functionality

islands must be connected to fully operational

networked systems.

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