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.

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.

As the Web continues to evolve, the sophistication of the programs that are employed in interacting with it will also increase in sophistication. Web agents, programs acting autonomously on some task, are already present in the form of spiders. Agents offer substantial benefits and hazards, and because of this, their development must involve not only attention to technical details, but also the ethical concerns relating to their resulting impact. These ethical concerns will differ for agents employed in the creation of a service and agents acting on behalf of a specific individual. An ethic is proposed that addresses both of these perspectives. The proposal is predicated on the assumption that agents are a reality on the Web, and that there are no reasonable means of preventing their proliferation. 1 -- Introduction The ease of construction and potential Internet-wide impact of autonomous software agents on the World Wide Web [1] has spawned a great deal of discussion and occasional c...

In this paper we discuss some of the problems of the current Web and show how the introduction of object-orientation provides flexible and extensible solutions. Web resources become encapsulated as objects, with well-defined interfaces through which all interactions occur. The interfaces and their implementations can be inherited by builders of objects, and methods (operations) can be redefined to better suit the object. New characteristics, such as concurrency control and persistence, can be obtained by inheriting from suitable base classes, without necessarily requiring any changes to users of these resources. We describe the W3Object model which we have developed based upon these ideas, and show, through a prototype implementation, how we have used the model to address the problems of referential integrity and transparent object (resource) migration. We also give indications of future work.

. Web design is an inherently error-prone process. To help with the detection of errors in the structure and connectivity of Web pages, we propose to apply model-checking techniques to the analysis of the World Wide Web. Model checking the Web is different in many respects from ordinary model checking of system models, since the Kripke structure of the Web is not known in advance, but can only be explored in a gradual fashion. In particular, the model-checking algorithms cannot be phrased in ordinary -calculus, since some operations, such as the computation of sets of predecessor Web pages and the computations of greatest fixpoints, are not possible on the Web. We introduce constructive -calculus, a fixpoint calculus similar to -calculus, but whose formulas can be effectively evaluated over the Web; and we show that its expressive power is very close to that of ordinary -calculus. Constructive -calculus can be used not only for phrasing Web model-checking algorithms, but also for the analysis of systems having a large, irregular state space that can be only gradually explored, such as software systems. On the basis of these ideas, we have implemented the Web model checker MCWEB, and we describe some of the issues that arose in its implementation, as well as the type of errors that it was able to find. 1

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A lexical signature (LS) consisting of several key words from a Web document is often sufficient information for finding the document later, even if its URL has changed. We conduct a large-scale empirical study of nine methods for generating lexical signatures, including Phelps and Wilensky’s original proposal (PW), seven of our own static variations, and one new dynamic method. We examine their performance on the Web over a 10-month period, and on a TREC data set, evaluating their ability to both (1) uniquely identify the original (possibly modified) document, and (2) locate other relevant documents if the original is lost. Lexical signatures chosen to minimize document frequency (DF) are good at unique identification but poor at finding relevant documents. PW works well on the relatively small TREC data set, but acts almost identically to DF on the Web, which contains billions of documents. Term-frequency-based lexical signatures (TF) are very easy to compute and often perform well, but are highly dependent on the ranking system of the search engine used. The term-frequency inverse-document-frequency- (TFIDF-) based method and hybrid methods (which combine DF with TF or TFIDF) seem to be the most promising candidates among static methods for generating effective lexical signatures. We propose a dynamic LS generator

A lexical signature of a web page is often sufficient for finding the page, even if its URL has changed. We conduct a largescale empirical study of eight methods for generating lexi- cal signatures, including Phelps and Wilensky's [14] original proposal (PW) and seven of our own variations. We exmnine their performance on the web and on a TREC data set, evaluating their ability both to uniquely identify the origi- nal document and to locate other relevant documents if the original is lost. Lexical signatures chosen to minimize document frequency (DF) are good at unique identification but poor at finding relevant documents. PW works well on the relatively small TREC data set, but acts almost identically to DF on the web, which contains billions of documents. Term-frequency-based lexical signatures (TF) are very easy to compute and often perform well, but are highly dependent on the ranking system of the search engine used. In general, TFIDF-based method and hybrid methods (which combine DF with TF or TFIDF) seem to be the most promising candidates for generating effective lexical signatures.

The difference between displaying networks with 100–1,000 nodes and displaying ones with 10,000–100,000 nodes is not merely quantitative, it is qualitative. Layout algorithms suitable for the former are too slow for the latter, requiring new algorithms or modified (often relaxed) versions of existing algorithms to be invented. The density of nodes and edges displayed per inch of screen real estate requires special visual techniques to filter the graphs and focus attention. Compounding the problem is that large real-life networks are often weighted graphs and usually have additional data associated with the nodes and edges. A system for investigating and exploring such large, complex datasets needs to be able to display both graph structure and node and edge attributes so that patterns and information hidden in the data can be seen. In this article we describe a tool that addresses these needs, the NicheWorks tool. We describe and comment on the available layout algorithms and the linked views interaction system, and detail two examples of the use of NicheWorks for analyzing Web sites and detecting international telephone fraud.

Maintenance will be critical to digital libraries, especially those that promote broad access to diverse, informal materials. If ignored, maintenance issues within the digital library -- especially those relating to its materials -- will threaten its usefulness and even its long-term viability. We perceive the maintenance problem to be both technical and institutional, and this paper considers the maintenance of the digital library as both institution and technology. The paper examines collection maintenance from several vantage points, including software architecture and the type of collection, arguing that digital libraries that contain informal and dynamic material will have substantially greater maintenance problems. The paper ends with an examination of potential technical solutions. KEYWORDS: digital library, collection, maintenance, World Wide Web, organizational memory, usability INTRODUCTION As with any new technology-based idea, there has been considerable controversy ove...