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.

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This article explores memory sharing and protection support in Opal, a single-address-space operating system designed for wide-address (64-bit) architectures. Opal threads execute within protection domains in a single shared virtual address space. Sharing is simplified, because addresses are context independent. There is no loss of protection, because addressability and access are independent; the right to access a segment is determined by the protection domain in which a thread executes. This model enables beneficial code- and data-sharing patterns that are currently prohibitive, due in part to the inherent restrictions of multiple address spaces, and in part to Unix programming style. We have designed and implemented an Opal prototype using the Mach 3.0 microkernel as a base. Our implementation demonstrates how a single-address-space structure can be supported alongside of other environments on a modern microkernel operating system, using modern wide-address architectures. This article justifies the opal model and its goals for sharing and protection, presents the system and its abstractions, describes the prototype implementation,

This paper presents Slice, a new storage system architecture for highspeed LANs incorporating network-attached block storage. Slice interposes a request switching filter -- called a /proxy -- along the network path between the client and the network storage system (e.g., in a network adapter or switch). The purpose of the/proxy is to route requests among a server ensemble that implements the file service. We present a prototype that uses this approach to virtualize the standard NFS file protocol to provide scalable, high-bandwidth file service to ordinary NFS clients. The paper presents and justifies the architecture, proposes and evaluates several request routing policies realizable within the architecture, and explores the effects of these policies on service structure

The appearance of 64-bit address space architectures, such as the DEC Alpha, HP PA-RISC, and MIPS R4000, signals a radical shift in the amount of address space available to operating systems and applications. This shift provides the opportunity to reexamine fundamental operating system structure specifically, to change the way that operating systems use address space. This paper

and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made.

Today, CORBA architecture brings the major industrial solution for achieving the interoperability between distributed software components in heterogeneous environments. While the CORBA project attempts to federate distributed mechanisms within a unique architecture, its internal model is not very flexible and seems not to be suitable for future evolutions. In this paper, we present OpenCorba, a reflective open broker, enabling users to adapt dynamically the representation and the execution policies of the software bus. We first expose the reflective foundations underlying the implementation of OpenCorba: i) metaclasses which provide a better separation of concerns in order to improve the class reuse; ii) a protocol which enables the dynamic changing of metaclass in order to allow run-time adaptation of systems. Based on this reflective environment, OpenCorba enables the adaptability of the internal characteristics of the broker in order to change its run-time behavior (e...

A process is an operating system abstraction representing an instance of a running computer program. Process migration is the act of transferring a process between two machines during its execution. Several implementations

ions Mesaac Makpangou Yvon Gourhant Jean-Pierre Le Narzul Marc Shapiro INRIA, B.P. 105, 78153 Rocquencourt C'edex, France tel.: +33 (1) 39-63-52-93, fax: +33 (1) 39 63 53 30 e-mail: mak@sor.inria.fr, telex: 697 033 F October 1, 1991 Keywords: Distributed objects, object oriented programming, distributed abstractions, fragmented objects, connective objects, FOG Abstract Fragmented Objects (FOs) extend the object concept to a distributed environment. The abstract view of a FO is a single, shared object, of which the distribution is hidden to clients. In the concrete view the FO designer controls (if wished) the distribution of data and function and of the communication between fragments. FO programming is supported by the FOG language, an extension of C++, and by a toolbox of predefined FOs. The FOG compiler ensures distributed typesafety of both the external and internal interfaces, verifies the encapsulation of FO instances, and automatically generates whatever coercions are necess...

No single encoding scheme or fault model is optimal for all data. A versatile storage system allows them to be matched to access patterns, reliability requirements, and cost goals on a per-data item basis. Ursa Minor is a cluster-based storage system that allows data-specific selection of, and on-line changes to, encoding schemes and fault models. Thus, different data types can share a scalable storage infrastructure and still enjoy specialized choices, rather than suffering from "one size fits all." Experiments with Ursa Minor show performance benefits of 2--3 when using specialized choices as opposed to a single, more general, configuration. Experiments also show that a single cluster supporting multiple workloads simultaneously is much more efficient when the choices are specialized for each distribution rather than forced to use a "one size fits all" configuration. When using the specialized distributions, aggregate cluster throughput nearly doubled.