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.

ion and delegation: Agents can be made extensible and composable in ways that common iconic interface objects cannot. Because we can "communicate" with them, they can share our goals, rather than simply process our commands. They can show us how to do things and tell us what went wrong (Miller and Neches 1987). . Flexibility and opportunism: Because they can be instructed at the level of 16 BRADSHAW goals and strategies, agents can find ways to "work around" unforeseen problems and exploit new opportunities as they help solve problems. . Task orientation: Agents can be designed to take the context of the person's tasks and situation into account as they present information and take action. . Adaptivity: Agents can use learning algorithms to continually improve their behavior by noticing recurrent patterns of actions and events. Toward Agent-Enabled System Architectures In the future, assistant agents at the user interface and resource-managing agents behind the scenes will increas...

In this paper, I will discuss the conceptual foundation of agent-oriented software development by relating the fundamental elements of the agent-oriented view to those of other, well established programming paradigms, especially the object-oriented approach. Furthermore, I will motivate the concept of autonomy as the basic property of the agent-oriented school and discuss the development history of programming paradigms that lead to this perspective on software systems. The paper will be concluded by an outlook on how the new paradigm can change the way we think about software systems.

Design patterns are distilled from many real systems to catalog common programming practice. We have analyzed several published design patterns and looked for patterns of working around constraints of the implementation language. Some object-oriented design patterns are distorted or overly complicated because of the lack of supporting language constructs or mechanisms. Welay a groundwork of generalpurpose language constructs and mechanisms that, if provided by a statically typed, object-oriented language, would better support the implementation of design patterns and, thus, benefit the construction of many real systems. In particular, our catalog of language constructs includes subtyping separate from inheritance, lexically scoped closure objects independent of classes, and multimethod dispatch. The proposed constructs and mechanisms are not radically new, but rather are adopted from a varietyof languages and combined in a new, orthogonal manner. We argue that by describing design patterns in terms of the proposed constructs and mechanisms, pattern descriptions become simpler and, therefore, accessible to a larger number of language communities. Constructs and mechanisms lacking in a particular language can be implemented using paradigmatic idioms.

konradsa,chengb¢ In software engineering, design patterns propose solution skeletons for common design problems. The solution skeleton is described in such a way that the design can be used for other projects, where each application tailors the design to specific project constraints. This paper describes research into investigating how a similar approach to reuse can be applied to requirements specifications, which we term requirements patterns. Specifically, we explore how object-oriented modeling notations, such as the Unified Modeling Language (UML), can be used to represent common requirements patterns. Structural and behavioral information are captured as part of a requirements pattern. In order to maximize reuse, we focus on requirements patterns for embedded systems. This paper also describes case studies that illustrate how we have applied these general patterns to multiple embedded systems applications from the automotive industry. 1.

Software architecture plays an important role in determining software quality characteristics, such as maintainability, reliability, reusability, and performance. Performance effects of architectural decisions can be evaluated at an early stage by constructing and analyzing quantitative performance models, which capture the interactions between the main components of the system as well as the performance attributes of the components themselves. The paper proposes a systematic approach to building Layered Queueing Network (LQN) performance models from a UML description of the high-level architecture of a system, and more exactly from the architectural patterns used for the system. The performance model structure retains a clear relationship with the system architecture, which simplifies the task of converting performance analysis results into conclusions and recommendations related to the software architecture. In the second part of the paper, the proposed approach is applied to a telec...

A parallel program archetype is an abstraction that captures the common features of a class of problems with similar computational structure and combines them with a parallelization strategy to produce a pattern of dataflow and communication. Such abstractions are useful in application development, both as a conceptual framework and as a basis for tools and techniques. This paper describes an approach to parallel application development based on archetypes and presents two example archetypes with applications. 1 Introduction This paper proposes a specific method of exploiting computational and dataflow patterns to help in developing reliable parallel programs. A great deal of work has been done on methods of exploiting design patterns in program development. This paper restricts attention to one kind of pattern that is relevant in parallel programming: the pattern of the parallel computation and communication structure. Methods of exploiting design patterns in program develop...

Managing inter-object dependencies in object-oriented systems is a complex task. Changes of one object often require dependent objects to change accordingly. Making every object explicitly inform every dependent object about its state changes intertwines object interfaces and implementations, thereby hampering system evolution and maintenance. These problems can be overcome by introducing the notion of Implicit Invocation to object-oriented systems as a decoupling mechanism between objects. This paper presents the Event Notification pattern, a pattern to smoothly integrate implicit invocation mechanisms with object-oriented designs. State changes of objects, dependencies of other objects on them and the maintenance links between these objects are made explicit as first class objects. The resulting structure is highly flexible and can be used to manage inter-object dependencies in object-oriented systems efficiently. 1

The field of software design patterns has seen an explosion in interest in the last three years. Work to date has been on the recognition, cataloging, and finding of patterns with little attention to the use of patterns especially by students and practitioners not well-versed in object-oriented technologies. This project addresses novice use directly yielding benefits for both the established pattern community and to students and educators new to the field. Although we do not claim that design patterns are Brooks' silver bullet [8], their use can help cope with the accidental complexity of software development and, we argue, their use is essential for a successful adoption of object-oriented techniques in academic computer science programs. This project addresses practical and pedagogical concerns of the computer science and software engineering communities by addressing the teaching and learning of patterns. In this paper we argue that design patterns are an essential programming and ...

patterns are an accepted way to facilitate and support reuse. This paper focuses on patterns in the field of Ontology Engineering and proposes a classification scheme for ontology patterns. The scheme divides ontology patterns into five levels: Application Patterns, Architecture Patterns, Design Patterns, Semantic Patterns, and Syntactic Patterns. Semantic and Syntactic Patterns are quite well-researched but the higher levels of pattern abstraction are so far almost unexplored. To illustrate the possibilities of patterns on these levels some examples are discussed, together with ideas of future work. 1