as the architecture of a Web application matures, more and more knowledge about the ... notion of services as model entities for Web application development.
An Evolution-oriented Architecture for Web Applications Guntram Graef and Martin Gaedke
Telecooperation Office (TecO), University of Karlsruhe, Vincenz-Prießnitz Str. 1, 76131 Karlsruhe, Germany, email: {graef, gaedke}@teco.edu Abstract: The Web has become an efficient environment for application delivery. The originally intended idea, as a distributed system for knowledge-interchange, has given way to organizations offering their products and services using the Web as a global point of sale. Although the arising possibilities look promising, the development process remains ad-hoc in real-life Web development. The understanding of Web application development mostly neglects architectural approaches, resulting in Web sites that fail in achieving typical goals like evolvable and maintainable structures of the information space. Beyond that, as the architecture of a Web application matures, more and more knowledge about the domain becomes embodied into code and therefore burdens maintenance and reuse of parts of the application. In this paper, we will propose an architecture and a framework using the notion of services as model entities for Web application development. The object-oriented WebComposition Markup Language, which is an application of the XML, will be presented as basis for a generic evolvable framework for services. Finally, the results of its usage will be described in detail by giving an example of a large-scale transnational Intranet, where the framework is in use. Keywords: Web Engineering, XML, WebComposition, Framework, Service.
1
Introduction
The original purpose of the World Wide Web [B-L90] was merely presenting information. Many of today's Web applications have moved a long way from the basic hypermedia systems of the first days. They have grown into complex distributed applications. Nevertheless, the development process in real life remains rather ad-hoc. This has largely been recognized to be due to the gap between the underlying design models and the implementation model of the Web. Web developers suffered from this when they tried to apply disciplined hypertext development methodologies such as OOHDM [SRB96] and RMM [ISB95] or general models like UML [Lar98] because many design concepts where lost during the implementation. This problem has been addressed with systems like JESSICA [BaS98], W3Objects [ICL96] and WebComposition [GWG97].
The WebComposition approach allows a developer to work within a semantically richer component model and automatically map the results to the Web implementation model. The mapping can be to any target language and is independent of the Web-server platform used. The WebComposition model enables and encourages reuse of code on the component level. In contrast to reusing Web resources, compositional reuse is not limited to a certain level of granularity imposed by the Web implementation model. Furthermore, it is possible to reuse abstractions that have been defined based on concepts such as generalization and specialization. A major problem that remains is how to efficiently reuse larger structures when building Web applications from components. Pressure to do so is especially strong in the Web environment due to much shorter development and
2
maintenance cycles than in most common software application domains. Questions like the following must be considered: What abstractions exist above the component level that are suitable for reuse? In what kind of development artifacts can these abstractions be captured? What does a mechanism look like that enables their reuse? In which way can the reuse process be optimized for productivity? How can it be implemented? What are the real benefits that can be achieved? In the next section, we will describe how services can be used as higher-level abstractions and coherent building blocks for Web applications. We will also discuss frameworks for building Web applications. In the third section, we will introduce our own approach. We suggest a Service-oriented Framework that is based on the WebComposition approach. Further, we introduce the concept of a Service Factory to enable a disciplined evolution of the application. We will describe our concrete implementation in section four and give an evaluation based on theoretical aspects, followed by an example of a commercial application in section five. In the last section, we will conclude with a short summary and outlook for further research.
2
Architectural abstractions for the Web
At first, we introduce an abstraction for functionality provided by a set of components for one or more applications. Then we will discuss frameworks as an abstraction on the domain level of applications. 2.1
Services
One aspect that distinguishes Webapplications from most common desktop applications is that there is no "application in the box" distribution model. What the user receives and pays for is a set of services that might more or less loosely be integrated or federated into an application such as described in [GaT99] or [Zhao97]. Well-known examples are portal sites such as Netscape's Netcenter [NNC99]. On the Web, this grouping can change quickly and the same services
might appear in numerous applications. In this sense, the application might even be built by users according to their own preferences or it might be dynamically generated to address a specific target group [FrK97]. In this context, a service presents a sub-system that enables a user to perform a single task such as placing an order or retrieving a news summary. In a business environment, a service may represent a business procedure. Services can be classified according to their functionality. There might be services for retrieving information, services for ordering products or services for expressing opinion such as online surveys just to name a few examples. The evolution of service classes tends to be subject to other forces of change than both the application and lower level building blocks. The shape of the application is directly driven by the current needs of its users that are closely related to sociological forces such as fashion. Lower level elements, such as components, that serve as building blocks for services are more directly affected by technological change. A class of services on the other hand might be mainly affected by the evolution of general business behavior. This orthogonality as well as the inner coherence of services and classes of services makes them powerful abstractions for building Webapplications. They appear to be suitable entities for reuse. 2.2
Frameworks
Very flexible abstractions for reuse on the level of an application domain are frameworks. As stated in [BMA97] a framework is an integrated set of software components that can be reused to create applications. The construction of applications is possible either by extension of framework components [Jon92] or by customization through parameterization of functional specifications that are used to automatically build applications from framework components [Pre94]. These two approaches to framework reuse are called "white-box" and "black-box" reuse [RoJ96].
3
There exist frameworks for the Web such as for shopping- or e-commerce. Common Web frameworks typically use a black-box core consisting of platform dependent components residing on the Web-server and a white-box part made up of Web-resources. This reflects the notion of [JoF88] that both modes of framework reuse are simultaneously present in most frameworks. The problem is that the white-box part here suffers from the limitations of the Web-implementation model as described in [GGS99]. Furthermore, a very common limitation in today's frameworks for the Web is that code from the white-box part can't be migrated into the black-box part of the framework because of disparities in the implementation models. Well known examples for this kind of frameworks are [Int99] or [LDM99]. A more advanced application development framework for the Web should offer an integrated implementation model for the black-box core as well as for new components developed with the framework. This is needed to facilitate the migration of new components into the black-box during the evolution of the framework. An integrated implementation model is also an important prerequisite for enabling improved reuse and maintenance of domain components.
3
Service Development Framework
In this section, we will introduce a Framework for developing Web services that we built based on the WebComposition approach. 3.1
Basic Architecture
The WebComposition approach presents a generic intermediate model to bridge the gap between fine-grained design and the coarsegrained implementation of Web applications by automatically mapping components to Web resources. WebComposition allows the definition of aggregation and inheritance relationships between components based on a prototype-instance object model such as in SELF [UnS87]. Components are defined using the WebComposition Markup Language (WCML). They are
stored in a Component Store from where they can automatically be mapped to Web resources using our WCML compiler. It is possible to generate code in any target language. Due to the generic approach, the mechanism is completely independent of the Web server platform. Service Development Framework Service Definition
Service Factory
Component Store Framework Components
Service
WCML Compiler
Web Resources
Figure 1 - Architecture of Service Framework All components are described in the same way, including those in the black-box part of the framework and components developed by the framework user. This means there is an integrated implementation model enabling easy migration of components from the white-box part of the framework into the black-box core, facilitating framework evolution. Services can be built by instantiating framework components. A service consists of components for content, layout, navigation, user interaction and processing. During run-time, the code produced by these components is called from the surrounding framework as needed. This general framework behavior is usually described as the Hollywood Principle "Don't call us, we'll call you." [Lar98].
4 User Interaction
Layout
*
Navigation
Content
* Processing
*
*
1
1
*
1
There is at least one Factory Method for each service class capturing its generic architecture. Several sets of Factory Methods can be supplied alternatively as described by the Abstract Factory design pattern.
Service 1
1
Order Content
*
1
Order Service
1
*
Factory Control Function
Order Service Factory Method
Order Processing
Figure 2 - Generic architecture of order services 3.2
Factory-oriented Evolution
A central part of our framework is the concept of a Service Factory, which is based on creational design patterns as described in [GHJ94]. It is intended to facilitate the development of common types of services. The Service Factory enables the reuse of generic architectures of classes of services. As an example, figure 2 shows a simplified description of the generic architecture of order services. An order service component is based on the service component, which it extends. Order services contain their own variant of content and processing components. To create a specific service a service definition must be supplied consisting of various information items. It must define how many components of each required type such as processing components the service should contain. It also describes attribute values for all the components belonging to the service. Thus, it defines all aspects of the service that distinguishes it from other services of the same service class. The Service Factory parses the service definition and uses its knowledge about the generic architecture of the corresponding service class to automatically create the service. The Service Factory itself consists of a Factory Control Function that dispatches the service definitions to the appropriate Factory Methods (Figure 3). The Factory Methods implement the Factory Method creational design pattern.
Information Service Factory Method
…
Figure 3 - Service Factory Architecture
4
Implementation
The implementation of the service development framework as well as that of the underlying WebComposition system rely on the eXtensible Markup Language (XML) [W3C99] and related tools. XML is a subset of SGML [ISO86] and presents a standard mechanism for defining markup languages. It provides basic syntax conventions that are familiar to a large number of users, especially in the Web environment. It enables formal syntax definitions that are used for automatic document checking. There are numerous tools available such as parsers and editors that reduce the development effort usually associated with the introduction of a new language. The WCML is an XML application and enables the definition of components. This facilitated the development of the WCML compiler and other tools since they can use standard XML parser components for document parsing. When implementing the WCML compiler and tools, such as a Component Repository we introduced in [GRG99] and a visual editor for service definitions, we explored two alternatives. We successfully used the XJParse XML parser [Dat99] for developing tools in Java as
5
well as a COM [MSC99] based environment. Both implementation strategies worked well to produce tools that interoperate seamlessly.
5
Evaluation
In this section, we will first discuss general aspects of the Service Development Framework before we will briefly describe its use in a large-scale e-commerce application at Hewlett-Packard. 5.1
General reflection
Our approach offers several levels of reuse. In addition to individual components, larger structures, namely service class architectures and knowledge captured in the framework's component architecture, can be reused. We separate code needed for describing individual services, code common to classes of services and framework code, which is common to all services. This helps to reduce the complexity of the code that has to be maintained by avoiding some unnecessary redundancy. Furthermore, specialized programming and teamwork are encouraged due to the taskoriented closure of services. On the one hand, this can make the development of specialized tools, such as a visual editor for a special but very common type of service, feasible. Specialized tools in contrast to general tools only need to cover a limited problem domain and thus are less complex. On the other hand, specialized programming can lead to a flatter learning curve and lower cognitive load for programmers. This can reduce technical skill requirements enabling a stronger focus on domain expertise. It also can lead to higher productivity, better quality and faster deployment. Another feature of the framework is automatic code generation by the Service Factory and the WCML compiler. In practice, this greatly increases productivity. For example, a service definition describing a service for ordering office chairs can be made up of about 50 tags. The code produced by the framework can easily reach more than 3000 lines of Web code mostly consisting of complex scripting statements. We want to stress that in contrast to many code generator tools for the Web this
code never needs to be modified directly and can always be regenerated automatically Reuse, automation and reduced complexity contribute to enhanced maintainability and extensibility of services, applications and the framework itself. Maintenance also benefits from the central location of code common to classes of services or all services defined within the framework. Changes of corporate identity, process logic or technical infrastructure only need to be implemented once and needn't be repeated in several locations or resources. Migration of existing Web applications and services to the framework is supported by the fact that existing code can be wrapped in WCML components and needn't be reprogrammed. It is also not necessary to migrate applications all at once. It can be done in an incremental approach because framework generated Web resources seamlessly integrate on the Web server with resources of other origin. In our approach, we also gain a high degree of flexibility for migrating to new implementation technologies or for concurrent support of different implementation models. This can be achieved by encapsulating implementation specific code in special WCML components that can be dynamically replaced when generating code for different implementation platforms. The service development framework itself and WCML are entirely independent of the deployment platform. 5.2
Eurovictor II
The Service Development Framework has already been used to fully implement a complete service-oriented e-commerce application. Eurovictor II has been developed at the Telecooperation Office at the University of Karlsruhe in a joint project with HewlettPackard (HP). The system has recently become productive at HP in Europe with more than 10000 regular users. The main goal of Eurovictor II was to enhance usability, evolution, maintenance, creation and management of Web-based services and federation of different intranet services within the
6
company. The system already offers several hundred services mainly in the areas hardware orders, software orders, telecommunication, virtual office and news services. Services have been developed in a decentralized manner by people with moderate technical skills and have automatically been integrated into the system by use of an installation-service.
Service Factory, we have been able to demonstrate how the generic architecture of a class of services can be reused. In an evaluation we pointed out the various advantages of our approach that lead to improved productivity, better quality and cost reduction during development, maintenance and evolution of web applications. We could verify our concept building a largescale e-commerce application at Hewlett Packard in Europe. In the near future, we plan to further improve tool support. Our research will include automatic mining and classification of code structures on the web that can be used as low-level building blocks for services. We are also interested in exploring the dynamic assembly of web applications based on user profiles.
Acknowledgements The authors would like to thank U. Stegemüller of the Hewlett-Packard Corporation, Germany for his cooperation and thoughtful feedback for some of the ideas developed in the approach presented here. Figure 4 Screenshot of a temporarily available service of the Eurovictor II system The quick and easy way in which new services can be developed has not only improved productivity but has also given way to completely new applications not possible before. An interesting example of this has been a service that has been set up in less than one hour for the only purpose of enabling the ordering of pizzas by employees in one HP location during a single day to celebrate a special event at the company. We consider this to be an example of how an organization and their members can quickly react upon a new situation in a flexible way.
6
Conclusions and future work
In this paper, we introduced a generic component based framework supporting the development of web services. We have shown how services can be used as building blocks for applications. By means of a
References [BaS98] R.A. Barta, M.W. Schranz (1998). JESSICA: an object-oriented hypermedia publishing processor. In Computer Networks and ISDN Systems 30 (1998), Special Issue on the 7th Intl. World-Wide Web Conference, Brisbane, Australia, April 1998, pp. 239-249. [B-L90] Tim Berners-Lee (1990). World Wide Web: Proposal for a HyperText Project, http://www.w3.org/Proposal.html [BMA97]D. Brugali, G. Menga, A. Aarsten (1997): The Framework Life Span. In: Communications of the ACM, Vol. 40, No. 10, Oct. 1997, pp. 6568
7
[Cle96] P. Clements (1996). A Survey of Architecture Description Languages. In Proceedings of 8th International Workshop on Software Specification and Design (IWSSD), Paderborn, Germany [Dat99] Datachannel Inc. (1999). XJParser. http://xdev.datachannel.com/downlo ads/xjparser/ [FrK97] L. Frelechoux, T. Kamba (1997). An architecture to support personalized Web applications. In: Computer Networks and ISDN Systems 29, Special Issue on the 6th Intl. WorldWide Web Conference, Santa Clara, CA, USA. [GaT99] M. Gaedke, K. Turowski (1999). Generic Web-Based Federation of Business Application Systems for ECommerce Applications, 2nd Intl. Workshop on Engineering Federated Information Systems (EFIS' 99), Kuehlungsborn, Germany [GGS99] M. Gaedke, H.-W. Gellersen, A. Schmidt, U. Stegemüller, W. Kurr (1999). Object-oriented Web Engineering for Large-scale Web Service Management. In: R. H. Sprague (Ed.) Proceedings of the 32nd Annual Hawaii International Conference On System Sciences, Maui, Hawaii, (CD-ROM).
[GWG97]H.-W. Gellersen, R. Wicke, M. Gaedke (1997). WebCompostion: an object-oriented support system for the Web engineering lifecycle. In: Computer Networks and ISDN Systems 29, Special Issue on the 6th Intl. World-Wide Web Conference, Santa Clara, CA, USA, pp. 14291437. [ICL96] D.B. Ingham, M.C. Little, S.J. Caughey, S.K. Shrivastava (1996). W3Objects: Bringing ObjectOriented Technology To The Web, The Web Journal, 1(1), Proceedings of the 4th International World Wide Web Conference, Boston, USA, pp. 89-105 [Int99]
Intershop Communications (1999). Intershop 3.0 http://www.intershop.com
Inc.
[ISB95] T. Isakowitz, E.A. Stohr, P. Balasubramaninan (1995). RMM: A Methodology for Structured Hypermedia Design, Communications of the ACM, Vol. 38, No. 8, August 1995, pp. 34-44. [ISO86] International Organization for Standardization (1986). Information Processing - Text and Office Systems - Standard Generalized Markup Language (SGML). Ref. No. ISO 8879:1986.
[GHJ94] E. Gamma; R. Helm; R. Johnson; J. Vlissides (1994). Design Patterns: Elements of Reusable ObjectOriented Software. Reading.
[JoF88]
R. Johnson, B. Foote (1988). Designing reusable classes. Journal on Object Oriented Programming 1, 5, June 1988, pp. 22-35
[GRG99] M. Gaedke, J. Rehse, G. Graef (1999). A Repository to facilitate Reuse in Component-Based Web Engineering. International Workshop on Web Engineering at the 8th International World-Wide Web Conference (WWW8), Toronto, Canada. http://budhi.uow.edu.au/ web-engineering99/accepted_papers/ gaedke.html , June 1999
[Jon92]
R. Johnson (1992). Documenting frameworks using patterns. In: Proceedings of OOPSLA 92, Vancouver, Canada, pp. 63-76
[Lar98]
C. Larman (1998). Applying UML and Patterns. Prentice Hall, pp. 455486
[LDM99] IBM Inc. (1999). Domino.Merchant. http://www.lotus.com
Lotus
8
[MSC99] Microsoft Corporation (1999). Component Object Model (COM). http://www.microsoft.com/com [NNC99] Netscape Communications (1999). Netcenter. http://www.netcenter.com [Pre94]
Inc.
W. Pree (1994). Design Patterns for Object-Oriented Software Development. Addison-Wesley
[RoJ96] D. Roberts, R. Johnson (1996). Evolving Frameworks - A Pattern Language For Developing ObjectOriented Frameworks. Third Conference on Pattern Languages of Programs (PLoP '96), Monticello, Illinois, USA [SRB96] D. Schwabe, G. Rossi, S. Barbosa (1996). Systematic Hypermedia De-
sign with OOHDM. In Proceedings of the ACM International Conference on Hypertext, Hypertext '96, Washington. [UnS87] D. Ungar, R. B. Smith (1987). Self: The power of Simplicity, In: OOPSLA'87 Proceedings, pp. 227242. [W3C99] World-Wide Web Consortium (1999). XML: eXtensible Markup Language. http://www.w3.org/XML [Zhao97] Y. Zhao (1997). A Single Login Web Service Integrator - WebEntrance. In: Computer Networks and ISDN Systems 29, Special Issue on the 6th Intl. World-Wide Web Conference, Santa Clara, CA, USA.
About the Authors Guntram Graef obtained a Master degree in computer science from the University of Karlsruhe in 1998. In 1999 he joined TecO as a research assistant and works in collaborative Web engineering projects. Former to that he was an independent consultant designing distributed applications for major companies in Europe and Asia. His current research interest is in methodologies and tools for large-scale Web development, e-commerce system architectures, user profiling and applications of XML. Martin Gaedke is research assistant at the Telecooperation Office (TecO) of the University of Karlsruhe and technical lead in collaborative Web engineering projects. His research interests is in application of software engineering practice to applications in the WWW, and specifically in component-based software engineering, reuse, frameworks, reflection, software architectures, patterns and software evolution for Web applications. He obtained a Master degree in computer science from the University of Karlsruhe in 1997.
