Finally, we perform a series of experiments employing several replication schemes and compare them with a non-redundant single service. Through the experiments, we evaluate both the reliability of the Web service paradigm and the correctness of the Web service composition algorithm. / In the paradigm, N-version programming technique is applied to increase the diversity of the system. As different versions of Web services or even different versions of their components are abundantly available in the Internet, the combination of different versions of the Web service or their components is thus becoming critical for enabling different versions in a server application using the N-version approach. We propose a dynamic Web service composition algorithm and evaluate with Petri-Net for verification purposes. / Moreover, we model the Web services with Markov chains and Petri-Nets to demonstrate the performance and reliability of the constructed Web services. Also, we develop the mathematical models to analyze the reliability of the Web services. / One of the latest achievements of the Internet usage is the availability of Web services technology and its dependability is becoming one of the most critical goals in Web related research. In this thesis, we propose a design paradigm for reliable Web services and a Web service composition algorithm. We describe the methods of dependability enhancement by redundancy in space and redundancy in time, using Round-robin scheduling technique, N-version programming and recovery block. The Web services are coordinated by a replication manager. It provides a Round-robin algorithm for scheduling the workload of the Web services and keeps updating the availability of each Web service. The replication algorithm and the detailed system configuration are described. / Chan, Pik Wah. / "March 2008." / Adviser: Micheal R. Lyn. / Source: Dissertation Abstracts International, Volume: 70-03, Section: A, page: 0722. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references (p. 114-124). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Web services transactions have some unique characteristics. A Web transaction may be composed of a number of individual Web services, executed across multiple loosely coupled autonomous systems. Each Web service may be executed on an independent system belonging to an independent provider. There raises the question whether Web transactions can and should be maintained as a single business unit in a similar way to how transactions are maintained in classical database systems. In classical database systems, the transaction management protocol and mechanism are constrained by the primary properties of atomicity, consistency, isolation and durability (ACID). These ACID properties are the cornerstone of maintaining data integrity in transaction management. However, ACID properties were meant for centralised systems and are better suited for short transactions. Unlike short transactions, Web services transactions may be long-running; they can take hours or even days depending on the application. Composing certain actions from loosely coupled distributed business processes across multiple distributed applications is one of the essentials of Web services transactions. The classic ACID model, which is tightly coupled, is therefore seen as too rigid to support all the requirements of the new Web transactions model. The research proposes a system that increases throughput while maintaining the consistency and correctness required by the particular applications that are using the model; the system is known as AuTrA (Adaptable user-defined Transaction relaxed Approach). AuTrA allows relaxation of each ACID property. The model is adaptable to meet different situations with different characteristics. For instance, in some cases it will be appropriate to relax atomicity, whereas in others it may be appropriate to relax isolation and atomicity while maintaining consistency. The research explores how transaction support for Web services can be customised to suit the needs of varying applications and result in improved service. The AuTrA prototype has been implemented. The experimental results show that the AuTrA application is able to support the basic features of Web services transaction management, allowing users to specify their correctness requirements, and it can increase throughput of transactions in models in a flexible and reliable manner. Additional facilities allow users to specify application-specific, non-ACID criteria that can increase throughput. Safeguards have also been implemented to prevent execution of inappropriate user specifications, such as relaxation of properties that may damage data integrity. AuTrA can be used as a tool by software developers who need to compose applications from independent Web services and who wish to build applications which result in improved performance while maintaining application-required consistency.