This paper presents SIENA, an event notification service that we have designed and implemented to exhibit both expressiveness and scalability. We describe the service's interface to applications, the algorithms used by networks of servers to select and deliver event notifications, and the strategies used Effort sponsored by the Defense Advanced Research Projects Agency, and Air Force Research Laboratory, Air Force Materiel Command,USAF, under agreement numbers F30602-94-C-0253, F3060297 -2-0021, F30602-98-2-0163, F30602-99-C-0174, F30602-00-2-0608, and N66001-00-8945; by the Air Force Office of Scientific Research, Air Force Materiel Command, USAF, under grant number F49620-98-1-0061; and by the National Science Foundation under Grant Number CCR-9701973. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Defense Advanced Research Projects Agency, Air Force Research Laboratory, or the U.S. Government

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 factors out the common denominator underlying these variants: full decoupling of the communicating entities in time, space, and synchronization. We use these three decoupling dimensions to better identify commonalities and divergences with traditional interaction paradigms. The many variations on the theme of publish/subscribe are classified and synthesized. In particular, their respective benefits and shortcomings are discussed both in terms of interfaces and implementations

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 application. The modern Web architecture 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. In this article we introduce the Representational State Transfer (REST) architectural style, developed as an abstract model of the Web architecture and used to guide our redesign and definition of the Hypertext Transfer Protocol and Uniform Resource Identifiers. We 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. We then compare the abstract model to the currently deployed Web architecture in order to elicit mismatches between the existing protocols and the applications they are intended to support.

This paper presents an algorithm for content-based forwarding, an essential function in content-based networking. Unlike in traditional address-based unicast or multicast networks, where messages are given explicit destination addresses, the movement of messages through a content-based network is driven by predicates applied to the content of the messages. Forwarding in such a network amounts to evaluating the predicates stored in a router's forwarding table in order to decide to which neighbor routers the message should be sent. We are interested in finding a forwarding algorithm that can make this decision as quickly as possible in situations where there are large numbers of predicates and high volumes of messages. We present such an algorithm and give the results of studies evaluating its performance.

The development of complex distributed systems demands for the creation of suitable architectural styles (or paradigms) and related run-time infrastructures. An emerging style that is receiving increasing attention is based on the notion of event. In an event-based architecture, distributed software components interact by generating and consuming events. The occurrence of an event in a component (called source) is asynchronously notified to any other component (called recipient) that has declared some interest in it. This paradigm holds the promise of supporting a flexible and effective interaction among highly reconfigurable distributed software components. We have developed an object-oriented infrastructure, called JEDI (Java Event-based Distributed Infrastructure), to support the development and operation of event-based systems. During the past year, JEDI has been used to implement a significant example of distributed system, namely, the OPSS workflow management system. The paper ...

We argue that the needs of many classes of modern applications, especially those targeted at mobile or wireless computing, demand the services of content-based publish/subscribe middleware, and that this middleware in turn demands a new kind of communication infrastructure for its proper implementation. We refer to this new communication infrastructure as content-based networking. The service model of this network must directly support the interface of an advanced content-based publish/subscribe middleware service. At the same time, the implementation must be architected as a true distributed network, providing appropriate guarantees of reliability, security, and performance. We do not propose content-based networking as a replacement for IP, nor do we advocate an implementation of a publish/subscribe middleware at the network level (i.e., within routers). Instead, we argue that contentbased networking must be designed according to established networking principles and techniques. To this end, in this paper, we formulate the foundational concepts of content-based networking, and relate them to the corresponding concepts in traditional networking. We also briefly review our experience with content-based publish/subscribe middleware and suggest some open research problems in the area of content-based networking.

Few tools exist to address the post-development activities of con guring, releasing, installing, updating, reconfiguring, and even de-installing a software system. Certainly there is no unified approach for all of these activities, and none that can take full advantage of a wide-area network. The Software Dock represents an architecture for supporting post-development activities in such a setting. It is designed as a system of loosely-coupled, cooperating, distributed components that are bound together by a wide-area messaging and event system. In this paper we describe the Software Dock architecture and discuss the use of a prototype in deploying a complex system.

Publish-subscribe is a communication paradigm that supports dynamic, many-to-many communications in a distributed environment. Content-based pub-sub systems are often implemented on a peer-to-peer infrastructure that enables information dissemination from information producers (publishers) to consumers (subscribers) through a subscription mechanism. In a wide-area pubsub network, the pub-sub service must handle information dissemination across distinct authoritative domains, heterogeneous platforms and a large, dynamic population of publishers and subscribers. Such an environment raises serious security concerns. In this paper, we investigate the security issues and requirements that arise in an internet-scale content-based pub-sub system. We distinguish among those requirements that can be

Empirical evaluation of software systems in actual usage situations is critical in software engineering. Prototyping, beta testing, and usability testing are widely used to refine system requirements, detect anomalous or unexpected system and user behavior, and to evaluate software usefulness and usability. The World Wide Web enables cheap, rapid, and large-scale distribution of software for evaluation purposes. However, current techniques for collecting usage data have not kept pace with the opportunities presented by Web-based deployment. This paper presents an approach and prototype system that makes large-scale collection of usage data over the Internet a practical possibility. A general framework for comparing software monitoring systems is presented and used to compare the proposed approach to existing techniques.