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 present a domain-independent model of hierarchical software system design and construction that is based on interchangeable software components and largescale reuse. The model unifies the conceptualizations of two independent projects, Genesis and Avoca, that are successful examples of software component/building-block technologies and domain modeling. Building-block technologies exploit large-scale reuse, rely on open architecture software, and elevate the granularity of programming to the subsystem level. Domain modeling formalizes the similarities and differences among systems of a domain. We believe our model is a blue-print for achieving software component technologies in many domains.

This thesis defines a set of program restructuring operations (refactorings) that support the design, evolution and reuse of object-oriented application frameworks. The focus of the thesis is on automating the refactorings in a way that preserves the behavior of a program. The refactorings are defined to be behavior preserving, provided that their preconditions are met. Most of the refactorings are simple to implement and it is almost trivial to show that they are behavior preserving. However, for a few refactorings, one or more of their preconditions are in general undecidable. Fortunately, for some cases it can be determined whether these refactorings can be applied safely. Three of the most complex refactorings are defined in detail: generalizing the inheritance hierarchy, specializing the inheritance hierarchy and using aggregations to model the relationships among classes. These operations are decomposed into more primitive parts, and the power of these operations is discussed from the perspectives of automatability and usefulness in supporting design. Two design constraints needed in refactoring are class invariants and exclusive components. These constraints are needed to ensure that behavior is preserved across some refactorings. This thesis gives some conservative algorithms for determining whether a program satisfies these constraints, and describes how to use this design information to refactor a program.

We propose a small number of basic concepts that can be used to explain the architecture of fault-tolerant distributed systems and we discuss a list of architectural issues that we find useful to consider when designing or examining such systems. For each issue we present known solutions and design alternatives, we discuss their relative merits and we give examples of systems which adopt one approach or the other. The aim is to introduce some order in the complex discipline of designing and understanding fault-tolerant distributed systems. 1 Introduction Computing systems consist of a multitude of hardware and software components that are bound to fail eventually. In many systems, such component failures can lead to unanticipated, potentially disruptive failure behavior and to service unavailability. Some systems are designed to be fault-tolerant: they either exhibit a well-defined failure behavior when components fail or mask component failures to users, that is, continue to provid...

This paper describes an approach to writing requirements specifications for processcontrol systems, a specification language that supports this approach, and an example application of the approach and the language on an industrial aircraft collision avoidance system (TCAS II). The example specification demonstrates (1) the practicality of writing a formal requirements specification for a complex, process-control system and (2) the feasibility of building a formal model of a system using a specification language that is readable and reviewable by applications experts who are not computer scientists or mathematicians. Some lessons learned in the process of this work, which are applicable both to forward and reverse engineering, are also presented.

Although software documentation standards often go into great detail about the format of documents, describing such details as paragraph numbering and section headings, they fail to give precise descriptions of the information to be contained in the documents. This paper does the opposite; it defines the contents of documents without specifying their format or the notation to be used in them. We describe documents such as the "System Requirements Document", the "System Design Document", the "Software Requirements Document", the "Software Behaviour Specification ", the "Module Interface Specification", and the "Module Internal Design Document" as representations of one or more mathematical relations. By describing those relations, we specify what information should be contained in each document. 1 Introduction Engineers are expected to make disciplined use of science, mathematics and technology to build useful products. Those who construct computer systems are clearly Enginee...

A "refinement" is a functionality addition to a software project that can affect multiple dispersed implementation entities (functions, classes, etc.). In this paper, we examine large-scale refinements in terms of a fundamental object-oriented technique called collaboration-based design. We explain how collaborations can be expressed in existing programming languages or be supported with new language constructs (which we have implemented as extensions to the Java language). We present a specific expression of large-scale refinements called mixin layers, and demonstrate how it overcomes the scalability difficulties that plagued prior work. We also show how we used mixin layers as the primary implementation technique for building an extensible Java compiler, JTS.

Maintenance tends to degrade the structure of software, ultimately making maintenance more costly. At times, then, it is worthwhile to manipulate the structure of a system to make changes easier. However, it is shown that manual restructuring is an error-prone and expensive activity. By separating structural manipulations from other maintenance activities, the semantics of a system can be held constant by a tool, assuring that no errors are introduced by restructuring. To allow the maintenance team to focus on the aspects of restructuring and maintenance requiring human judgment, a transformation-based tool can be provided---based on a model that exploits preserving data flow-dependence and control flow-dependence---to automate the repetitive, errorprone, and computationally demanding aspects of re...

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...

attention as an important sub#eld of software engineering. During that time there has been considerable progress in developing the technological and methodological base for treating architectural design as an engineering discipline. However, much remains to be done to achieve that goal. Moreover, the changing face of technology raises anumber of new challenges for software architecture. This paper examines some of the important trends of software architecture in research and practice, and speculates on the important emerging trends, challenges, and aspirations.