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.

This paper proposes a concurrent model that takes into account such important concerns. We insist on concept unifications: the underlying reasons that make object-oriented programming adapted to concurrency. The model characteristics, especially reusability, permit us to define a concurrent object-oriented design method.

a "one-of-a-kind" process, where most software is developed from scratch, even though much of the code is similar to code written for other applications. The cost of these systems can be drastically reduced and the capability of these systems improved by providing a suitable software framework for all R&A sys~ terns, We describe a novel software framework, based on the notion of dynamically reconfigurable software for sensor-based control systems. Tools to support the implementation of this framework have been built into the Chimera 3.0 Real-Time Operating System. The framework provides for the systematic development and predictable execution of flexible R&A applications while maintaining the ability to reuse code f)om previous applications. It combines object-oriented design of software with port-automaton design of digital control systems. A control module is an instance of a class of port-based objects. A task set is formed by integrating objects from a module library to form a specific configuration. An implementation using global state variables for the automatic integration of port-based objects is presented. A control subsystem is a collection of jobs which are executed one at a time, and can be programmed by a user. Multiple control subsystems can execute in parallel, and operate either independently or cooperatively. One of the fundamental concepts of reconfigurable software design is that modules are developed independent of the target hard- ware. Our framework defines classes of reconfigurable device driver objects for proving hardware independence to IJO devices, sensors, actuators, and special purpose processors. Hardware independent real-time communication mechanisms for inter-subsystem eommurtication are also described. A/ong with providing a foundatio...

Software architects use a number of commonly-recognized “styles” to guide their design of system structures. Each of these is appropriate for some classes of problems, but none is suitable for all problems. How, then, does a software designer choose an architecture suitable for the problem at hand? Two kinds of information are required: (1) careful discrimination among the candidate architectures and (2) design guidance on how to make appropriate choices. Here we support careful discrimination with a preliminary classification of styles. We use a two-dimensional classification strategy with control and data issues as the dominant organizing axes. We position the major styles within this space and use finer-grained discriminations to elaborate variations on the styles. This provides a framework for organizing design guidance, which we partially flesh out with rules of thumb.

ix 1 Introduction 1 2 Quality Attribute Characterizations 3 3 ABAS Example 7 3.1 Problem Description 8 3.1.1 Criteria for Choosing this ABAS 8 3.2 Stimulus/Response Attribute Measures 9 3.3 Architectural Style 9 3.4 Analysis 10 3.4.1 Reasoning 11 3.4.2 Analysis and Design Heuristics 12 3.5 Summary 12 4 Using ABASs for Design and Analysis 13 4.1 Model Problem 13 4.2 Applying the Synchronization ABAS 14 4.3 Composing Like-Attribute ABASs 16 4.4 Composing ABASs of Different Attribute Types 20 4.5 Summary 23 5 ABASs in Analysis 25 6 Conclusion 27 Example ABASs 29 Appendix A Synchronization ABAS 31 A.1 Problem Description 31 A.1.1 Criteria for Choosing this ABAS 31 A.2 Stimulus/Response Attribute Measures 31 ii CMU/SEI-99-TR-022 A.3 Architectural Style 32 A.4 Analysis 33 A.4.1 Reasoning 33 A.4.2 Analysis and Design Heuristics 36 Appendix B Data Indirection ABAS 37 B.1 Problem Description 37 B.1.1 Criteria for Choosing this ABAS 37 B.2 Stimulus/Response Attribute Measures 37 B.3 Archi...

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An important goal in software engineering is to describe complex software systems at an architectural level of abstraction. While there are many useful architectural paradigms (pipes, blackboards, etc.) they are typically understood only idiomatically and applied in an ad hoc fashion. We show how a formal model allows us to say precisely what we mean by a software architecture, explore its properties, and systematically describe instances of the architecture. We illustrate the approach using the well-known example of pipe-filter architectures. This research was sponsored by the National Science Foundation under Grants CCR-9109469 and CCR-9112880, and by Siemens Corporate Research, Inc. The views and conclusions contained in this document are those of the author and should not be interpreted as representing the official policies, either expressed or implied, of Siemens or the U.S. Government. Keywords: Dataflow Systems, Pipes and Filters, Software Architecture, Software Engineering, S...

The importance of object state testing is illustrated through a simple example. We show that certain errors in the implementation of object state behavior cannot be readily detected by conventional structural testing, functional testing, and state testing. We describe an object state test model and outline a reverse engineering method for extracting object state behaviors from C++ source code. The object state test model is a hierarchical, concurrent, communicating state machines. It resembles the concepts of inheritance and aggregation in the object-oriented paradigm, rather than the concept of state decomposition as in some existing models. The reverse engineering method is based on symbolic execution to extract the states and effects of the member functions. The symbolic execution results are used to construct the state machines. The usefulness of the model and the method is discussed in the context of object state testing in the detection of a state behavior error. 1 Introduction ...

For my parents Safety critical computing is a relatively young and rapidly developing technology, which nevertheless is being deployed in applications where a single accident may have extremely severe consequences. The safety record of critical systems presently in service is reasonably good, but increasing expectations of functionality and performance are challenging the capabilities of current design and assessment processes. One specific area where limitations of existing methods are becoming obvious is in the analysis techniques that are used to derive safety requirements and to provide evidence that they have been satisfied. There are significant practical problems in using existing analysis techniques to evaluate computer systems, but few viable new computerspecific methods have been developed. This thesis proposes and evaluates a set of principles for the design of effective techniques to address novel computer system safety analysis requirements. The principles are based on an appreciation of the technical concepts underlying successful existing system level analysis techniques, and of the practical qualities necessary to make a method industrially acceptable. The

The Chimera Methodology is a new software engineering paradigm which addresses the problem of developing dynamically reconfigurable and reusable real-time software. The foundation of the Chimera methodology is the port-based object model of a reusable software component. The model is obtained by applying the port-automaton formal computational model to object-based design. Global state variable table real-time communication is used to integrate port-based objects, which eliminates the need for writing and debugging glue code. The Chimera real-time operating system provides tools to support the software models defined by the Chimera methodology, so that real-time software can be executed predictably using common real-time scheduling algorithms. A hypermedia user interface has been designed to allow users to easily assemble the real-time software components that are designed based on the Chimera methodology. Use of the methodology can result in a significant decrease the development time and cost of real-time applications.