The World Wide Web has succeeded in large part because its software architecture has been designed to meet the needs of an Internet-scale distributed hypermedia system. The Web has been iteratively developed over the past ten years through a series of modifications to the standards that define its architecture. In order to identify those aspects of the Web that needed improvement and avoid undesirable modifications, a model for the modern Web architecture was needed to guide its design, definition, and deployment. Software architecture research investigates methods for determining how best to partition a system, how components identify and communicate with each other, how information is communicated, how elements of a system can evolve independently, and how all of the above can be described using formal and informal notations. My work is motivated by the desire to understand and evaluate the architectural design of network-based application software through principled use of architectural constraints, thereby obtaining the functional, performance, and social properties desired of an architecture. An architectural style is a named, coordinated set of architectural constraints. This dissertation defines a framework for understanding software architecture via architectural styles and demonstrates how styles can be used to guide the architectural design of network-based application software. A survey of architectural styles for network-based applications is used to classify styles according to the architectural properties they induce on an architecture for distributed hypermedia. I then introduce the Representational State Transfer (REST) architectural style and describe how REST has been used to guide the design and development of the architecture for the modern Web. REST emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. I describe the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, contrasting them to the constraints of other architectural styles. Finally, I describe the lessons learned from applying REST to the design of the Hypertext Transfer Protocol and Uniform Resource Identifier standards, and from their subsequent deployment in Web client and server software.

We are interested in designing systems that support communication and collaboration among large groups of people over computer networks. We begin by asking what properties of the physical world support graceful human-human communication in face-to-face situations, and argue that it is possible to design digital systems that support coherent behavior by making participants and their activities visible to one another. We call such systems “socially translucent systems ” and suggest that they have three characteristics—visibility, awareness, and accountability—which enable people to draw upon their social experience and expertise to structure their interactions with one another. To motivate and focus our ideas we develop a vision of knowledge communities, conversationally based systems that support the creation, management and reuse of knowledge in a social context. We describe our experience in designing and deploying one layer of functionality for knowledge communities, embodied in a working system called “Babble, ” and discuss research issues raised by a socially translucent approach to design. Categories and Subject Descriptors: H.1.2 [Models and Principles]: User/Machine Systems—Human factors; Human information processing; H.5.2 [Information Interfaces and

Modeling a city poses a number of problems to computer graphics. Every urban area has a transportation network that follows population and environmental influences, and often a superimposed pattern plan. The buildings appearances follow historical, aesthetic and statutory rules. To create a virtual city, a roadmap has to be designed and a large number of buildings need to be generated. We propose a system using a procedural approach based on L-systems to model cities. From various image maps given as input, such as land-water boundaries and population density, our system generates a system of highways and streets, divides the land into lots, and creates the appropriate geometry for the buildings on the respective allotments. For the creation of a city street map, L-systems have been extended with methods that allow the consideration of global goals and local constraints and reduce the complexity of the production rules. An L-system

Design patterns raise the abstraction level at which people design and communicate design of object-oriented software. But design patterns still leave the mechanics of their implementation to the programmer. This paper describes the architecture and implementation of a tool that automates the implementation of design patterns. The user of the tool supplies application-specific information for a given pattern, from which the tool generates all the pattern-prescribed code automatically. The tool has a distributed architecture that lends itself to implementation with off-the-shelf components. 1

In this paper we discuss the use of an object-oriented approach for webbased applications design, based on a method named Object-Oriented Hypermedia Design Method (OOHDM). We first motivate our work discussing the problems encountered while designing large scale, dynamic web-based applications, which combine complex navigation patterns with sophisticated computational behavior. We argue that a method providing systematic guidance to design is needed. Next, we introduce OOHDM, describing its main activities, namely: conceptual design, navigational design, abstract interface design and implementation, and discuss how OOHDM designs can be implemented in the WWW. Finally, related work and future research in this area are further discussed.

In this paper we analyze the process of hypermedia applications design and implementation, focusing in particular on two critical aspects of these applications: the navigational and interface structure. We discuss the way in which we build the navigation and abstract interface models using the Object-Oriented Hypermedia Design Method (OOHDM); we show which concerns must be taken into account for each task by giving examples from a real project we are developing, the Portinari Project. We show which implementation concerns must be considered when defining interface behavior, discussing both a Toolbook and a HTML implementation of the example application. Keywords: Hypermedia Design, Methodology, Modeling, Object Orientation, Navigation, Interfaces 1. Introduction In the past three years there has been growing interest in hypermedia design approaches [Izakowitz 95, Garzotto 93, Lange 94]. There are many different problems the hypermedia designer has to deal with, since the combination...

Software design patterns are the core abstractions from successful recurring problem solutions in software design. Composite design patterns are the core abstractions from successful recurring frameworks. A composite design pattern is a pattern that is best described as the composition of further patterns the integration of which shows a synergy that makes the composition more than just the sum of its parts. This paper presents examples of composite patterns, discusses a role-based analysis and composition technique, and demonstrates that composite patterns extend the pattern idea from single problem solutions to object-oriented frameworks. 1

Design patterns have proven to be very useful for the design of object-oriented systems. The power of design patterns stems from their ability to provide generic solutions to reappearing problems that can be specialised for particular situations. The implementation of design patterns, however, has received only little attention and we have identified four problems associated with the implementation of design patterns using conventional object-oriented languages. First, the traceability of a design pattern in the implementation is often insufficient; often the design pattern is ‘lost’. Second, since several patterns require an object to forward messages to other objects to increase flexibility, the self problem often occurs. Thirdly, since the pattern implementation is mixed with the domain class, the reusability of pattern implementations is often limited. Finally, implementing design patterns may present significant implementation overhead for the software engineer. Often, a, potentially large, number of simple methods has to be implemented with trivial behaviour, e.g. forwarding a message to another object. In this paper, a solution to these problems is presented in the context of the layered object model (LayOM). LayOM provides language support for the explicit representation of design patterns in the programming language. LayOM is an extended object-oriented language in that it contains several components that are not part of the conventional object model, such as states, categories and layers. Layers are used to represent design patterns at the level of the programming language and example layer types for eight design patterns are presented, i.e. Adapter, Bridge, Composite, Facade, State, Observer, Strategy andMediator. Since LayOM is an extensible language, the software engineer may extend the language model with abstractions for other design patterns. 1

A safety case should present a clear, comprehensive and defensible argument that a system is acceptably safe to operate within a particular context. However, many existing safety cases, in their attempt to manage potentially complex arguments, are poorly structured, presented and understood. This creates problems in developing and maintaining safety cases, and in capturing successful safety arguments for use on future projects. This thesis defines and demonstrates a coherent approach to the development, presentation, maintenance and reuse of the safety arguments within a safety case. This approach is based upon a graphical technique -- the Goal Structuring Notation (GSN) -- and has three strands. Firstly, a method for the use of GSN is defined together with an approach to supporting incremental safety case development. Secondly, the thesis presents a systematic process for the maintenance of a GSN-structured safety argument. Thirdly, the concept of `Safety Case Patterns' is defined as a means of supporting and promoting the reuse of successful safety arguments between safety cases. Examples of the approach are provided throughout. Evaluation of the approach is described through tool implementation, case studies, pilot projects and industrial project applications. Through these activities the approach has been shown to be both a valid and capable tool for safety case management.

Software architecture is an area of growing importance to practitioners and researchers in government, industry, and academia. Journals and international workshops are devoted to it. Working groups are formed to study it. Textbooks are emerging about it. The government is investing in the development of software architectures as core products in their own right. Industry is marketing architectural frameworks such as CORBA. Why all the interest and investment? What is software architecture, and why is it perceived as providing a solution to the inherent difficulty in designing and developing large, complex systems? This report will attempt to summarize the concept of software architecture for an intended audience of mid to senior level management. The reader is presumed to have some familiarity with common software engineering terms and concepts, but not to have a deep background in the field. This report is not intended to be overly-scholarly, nor is it intended to provide the techni...