Several technologies and languages exist for the development and implementation of distributed systems. Furthermore, several models for teaching computer programming and teaching programming in a distance-based educational environment exist. Limited literature, however, is available on models for teaching distributed computing in a distance-based educational environment. The focus of this study is to examine how distributed computing should be taught in a distance-based educational environment so as to ensure effective and quality learning for students. The required effectiveness and quality should be comparable to those for students exposed to laboratories, as commonly found in residential universities. This leads to an investigation of the factors that contribute to the success of teaching distributed computing and how these factors can be integrated into a distance-based teaching model. The study consisted of a literature study, followed by a comparative study of available tools to aid in the learning and teaching of distributed computing in a distance-based educational environment. A model to accomplish this teaching and learning is then proposed and implemented. The findings of the study highlight the requirements and challenges that a student of distributed computing in a distance-based educational environment faces and emphasises how the proposed model can address these challenges. This study employed qualitative research, as opposed to quantitative research, as qualitative research methods are designed to help researchers to understand people and the social and cultural contexts within which they live. The research methods employed are design research, since an artefact is created, and a case study, since “how” and “why” questions need to be answered. Data collection was done through a survey. Each method was evaluated via its own well-established evaluation methods, since evaluation is a crucial component of the research process. / Computing / M. Sc. (Computer Science)

Made available in DSpace on 2014-06-11T19:22:35Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-06-06Bitstream added on 2014-06-13T20:09:51Z : No. of bitstreams: 1 sanchez_frl_me_ilha.pdf: 660422 bytes, checksum: f8ab299d7cef18ca3a218acf27a94f43 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho foram implementados diversos agentes heuristicos construtivos, baseados no modelo híbrido linear, que fazem parte de um time assíncrono que tem como objetivo gerar configurações de boa qualidade para inicializar as metaheuríticas que resolvem o problema do planejamento da expansão da transmissão dos sistemas de energia elétrica. A teoria de times assíncronos foi aplicada para reunir as qualidades individuais dos métodos heurísticos, de uma maneira que, partindo de uma configuração base (sem adições) e utilizando um fluxo de dados cíclico, os agentes construtivos adicionassem circuitos a esta configuração de maneira sistemática e aleatória até que esta atenda as demandas de carga solicitadas pelo sistema elétrico em um horizonte futuro. Estas configurações foram então utilizadas por um algoritmo genético no intuito de validar a qualidade das mesmas. Os algoritmos foram implementados em Fortran, utilizando as rotinas de trocas de mensagens do LAM-MPI e simulados para sistemas teste de pequeno, médio e grande porte em ambiente de processamento distribuido. Os resultados comprovam que os times ass´ıncronos de vários metodos heurísticos são mais eficazes comparados com uma única heurística. / In this study, it was implemented several constructive heuristic algorithms, based on hybrid linear model, which are part of a asynchronous team that aims to generate initial solutions with good quality for meta-heuristics that solve the transmission expansion planning problem of electric power systems. The theory of asynchronous team was applied to meet the individual qualities of each heuristic method, in a way that, starting from a base network configuration and using a cyclical flow of data, heuristic agents add circuits to is configuration in a systematic and random way until they meet the load demands requested by the electrical system on a future horizon. Then these configurations are utilized by a genetic algorithm in order to validate the quality of them. The algorithms were implemented in Fortran, using exchanging messages routines from LAM-MPI and simulated for small, medium and large size test-systems in distributed processing environment. The results show that the solutions obtained with asynchronous teams of several heuristic methods are more effective than the solutions with a single heuristic algorithm.

A discrete event system is a system whose state space is given by a discrete set and whose state transition mechanism is event-driven i.e. its state evolution depends only on the occurrence of discrete events over the time. These systems are used in many fields of application (telecommunication networks, aeronautics, aerospace,). The validity of these systems is then an important issue and to ensure it we can use supervisory control methods. These methods consist in imposing a given specification on a system by means of a controller which runs in parallel with the original system and which restricts its behavior. In this thesis, we develop supervisory control methods where the system can have an infinite state space and the controller has a partial observation of the system (this implies that the controller must define its control policy from an imperfect knowledge of the system). Unfortunately, this problem is generally undecidable. To overcome this negative result, we use abstract interpretation techniques which ensure the termination of our algorithms by overapproximating, however, some computations. The aim of this thesis is to provide the most complete contribution it is possible to bring to this topic. Hence, we consider more and more realistic problems. More precisely, we start our work by considering a centralized framework (i.e. the system is controlled by a single controller) and by synthesizing memoryless controllers (i.e. controllers that define their control policy from the current observation received from the system). Next, to obtain better solutions, we consider the synthesis of controllers that record a part or the whole of the execution of the system and use this information to define the control policy. Unfortunately, these methods cannot be used to control an interesting class of systems: the distributed systems. We have then defined methods that allow to control distributed systems with synchronous communications (decentralized and modular methods) and with asynchronous communications (distributed method). Moreover, we have implemented some of our algorithms to experimentally evaluate the quality of the synthesized controllers. / <p><p>Un système à événements discrets est un système dont l'espace d'états est un ensemble discret et dont l'évolution de l'état courant dépend de l'occurrence d'événements discrets à travers le temps. Ces systèmes sont présents dans de nombreux domaines critiques tels les réseaux de communications, l'aéronautique, l'aérospatiale. La validité de ces systèmes est dès lors une question importante et une manière de l'assurer est d'utiliser des méthodes de contrôle supervisé. Ces méthodes associent au système un dispositif, appelé contrôleur, qui s'exécute en parrallèle et qui restreint le comportement du système de manière à empêcher qu'un comportement erroné ne se produise. Dans cette thèse, on s'intéresse au développement de méthodes de contrôle supervisé où le système peut avoir un espace d'états infini et où les contrôleurs ne sont pas toujours capables d'observer parfaitement le système; ce qui implique qu'ils doivent définir leur politique de contrôle à partir d'une connaissance imparfaite du système. Malheureusement, ce problème est généralement indécidable. Pour surmonter cette difficulté, nous utilisons alors des techniques d'interprétation abstraite qui assurent la terminaison de nos algorithmes au prix de certaines sur-approximations dans les calculs. Le but de notre thèse est de fournir la contribution la plus complète possible dans ce domaine et nous considèrons pour cela des problèmes de plus en plus réalistes. Plus précisement, nous avons commencé notre travail en définissant une méthode centralisée où le système est contrôlé par un seul contrôleur qui définit sa politique de contrôle à partir de la dernière information reçue du système. Ensuite, pour obtenir de meilleures solutions, nous avons défini des contrôleurs qui retiennent une partie ou la totalité de l'exécution du système et qui définissent leur politique de contrôle à partir de cette information. Malheureusement, ces méthodes ne peuvent pas être utilisées pour contrôler une classe intéressante de systèmes: les sytèmes distribués. Nous avons alors défini des méthodes permettant de contrôler des systèmes distribués dont les communications sont synchrones (méthodes décentralisées et modulaires) et asynchrones (méthodes distribuées). De plus, nous avons implémenté certains de nos algorithmes pour évaluer expérimentalement la qualité des contrôleurs qu'ils synthétisent. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

A reactive control system is a computer system reacting to certain stimuli emitted by its environment in order to maintain it in a desired state. Distributed reactive control systems are generally composed of several processes, running in parallel on one or more computers, communicating with one another to perform the required control task. By their very nature, distributed reactive control systems are hard to design. Their distributed nature and/or the communication scheme used can introduce subtle unforeseen behaviours. When dealing with critical applications, such as plane control systems, or traffic light control systems, those unintended behaviours can have disastrous consequences. It is therefore essential, for the designer, to ensure that this does not happen. For that purpose, rigorous and systematic techniques can (and should) be applied as early as possible in the development process. In that spirit, this work aims at providing the designer with the necessary tools in order to facilitate the development and validation of such distributed reactive control systems. In particular, we show how using a dedicated language called dSL (Distributed Supervision language) can be used to ease the development process. We also study how validations techniques such as model-checking and testing can be applied in this context. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

<P align="justify">Model-checking is an automated technique which aims at verifying properties of computer systems. A model-checker is fed with a model of the system (which capture all its possible behaviors) and a property to verify on this model. Both are given by a convenient mathematical formalism like, for instance, a transition system for the model and a temporal logic formula for the property.</P><p><p><P align="justify">For several reasons (the model-checking is undecidable for this class of model or the model-checking needs too much resources for this model) model-checking may not be applicable. For safety properties (which basically says "nothing bad happen"), a solution to this problem uses a simpler model for which model-checkers might terminate without too much resources. This simpler model, called the abstract model, over-approximates the behaviors of the concrete model. However the abstract model might be too imprecise. In fact, if the property is true on the abstract model, the same holds on the concrete. On the contrary, when the abstract model violates the property, either the violation is reproducible on the concrete model and so we found an error; or it is not reproducible and so the model-checker is said to be inconclusive. Inconclusiveness stems from the over-approximation of the concrete model by the abstract model. So a precise model yields the model-checker to conclude, but precision comes generally with an increased computational cost.</P><p><p><P align="justify">Recently, a lot of work has been done to define abstraction refinement algorithms. Those algorithms compute automatically abstract models which are refined as long as the model-checker is inconclusive. In the thesis, we give a new abstraction refinement algorithm which applies for safety properties. We compare our algorithm with previous attempts to build abstract models automatically and show, using formal proofs that our approach has several advantages. We also give several extensions of our algorithm which allow to integrate existing techniques used in model-checking such as acceleration techniques.</P><p><p><P align="justify">Following a rigorous methodology we then instantiate our algorithm for a variety of models ranging from finite state transition systems to infinite state transition systems. For each of those models we prove the instantiated algorithm terminates and provide encouraging preliminary experimental results.</P><p><br><p><br><p><P align="justify">Le model-checking est une technique automatisée qui vise à vérifier des propriétés sur des systèmes informatiques. Les données passées au model-checker sont le modèle du système (qui en capture tous les comportements possibles) et la propriété à vérifier. Les deux sont donnés dans un formalisme mathématique adéquat tel qu'un système de transition pour le modèle et une formule de logique temporelle pour la propriété.</P><p><p><P align="justify">Pour diverses raisons (le model-checking est indécidable pour cette classe de modèle ou le model-checking nécessite trop de ressources pour ce modèle) le model-checking peut être inapplicable. Pour des propriétés de sûreté (qui disent dans l'ensemble "il ne se produit rien d'incorrect"), une solution à ce problème recourt à un modèle simplifié pour lequel le model-checker peut terminer sans trop de ressources. Ce modèle simplifié, appelé modèle abstrait, surapproxime les comportements du modèle concret. Le modèle abstrait peut cependant être trop imprécis. En effet, si la propriété est vraie sur le modèle abstrait alors elle l'est aussi sur le modèle concret. En revanche, lorsque le modèle abstrait enfreint la propriété :soit l'infraction peut être reproduite sur le modèle concret et alors nous avons trouvé une erreur ;soit l'infraction ne peut être reproduite et dans ce cas le model-checker est dit non conclusif. Ceci provient de la surapproximation du modèle concret faite par le modèle abstrait. Un modèle précis aboutit donc à un model-checking conclusif mais son coût augmente avec sa précision.</P><p><P align="justify">Récemment, différents algorithmes d'abstraction raffinement ont été proposés. Ces algorithmes calculent automatiquement des modèles abstraits qui sont progressivement raffinés jusqu'à ce que leur model-checking soit conclusif. Dans la thèse, nous définissons un nouvel algorithme d'abstraction raffinement pour les propriétés de sûreté. Nous comparons notre algorithme avec les algorithmes d'abstraction raffinement antérieurs. A l'aide de preuves formelles, nous montrons les avantages de notre approche. Par ailleurs, nous définissons des extensions de l'algorithme qui intègrent d'autres techniques utilisées en model-checking comme les techniques d'accélérations.</P><p><P align="justify">Suivant une méthodologie rigoureuse, nous instancions ensuite notre algorithme pour une variété de modèles allant des systèmes de transitions finis aux systèmes de transitions infinis. Pour chacun des modèles nous établissons la terminaison de l'algorithme instancié et donnons des résultats expérimentaux préliminaires encourageants.</P><p><p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Modern science relies on workflow technology to capture, process, and analyse data obtained from scientific instruments. Scientific workflows are precise descriptions of experiments in which multiple computational tasks are coordinated based on the dataflows between them. Orchestrating scientific workflows presents a significant research challenge: they are typically executed in a manner such that all data pass through a centralised computer server known as the engine, which causes unnecessary network traffic that leads to a performance bottleneck. These workflows are commonly composed of services that perform computation over geographically distributed resources, and involve the management of dataflows between them. Centralised orchestration is clearly not a scalable approach for coordinating services dispersed across distant geographical locations. This thesis presents a scalable decentralised service-oriented orchestration system that relies on a high-level data coordination language for the specification and execution of workflows. This system's architecture consists of distributed engines, each of which is responsible for executing part of the overall workflow. It exploits parallelism in the workflow by decomposing it into smaller sub-workflows, and determines the most appropriate engines to execute them using computation placement analysis. This permits the workflow logic to be distributed closer to the services providing the data for execution, which reduces the overall data transfer in the workflow and improves its execution time. This thesis provides an evaluation of the presented system which concludes that decentralised orchestration provides scalability benefits over centralised orchestration, and improves the overall performance of executing a service-oriented workflow.

This thesis addresses software design within the field of Information and Communications Technology for Development (ICTD). Specifically, it makes a case for the design and development of software which is custom-made for the context of marginalised rural areas (MRAs). One of the main aims of any ICTD project is sustainability and such sustainability is particularly difficult in MRAs because of the high costs of projects located there. Most literature on ICTD projects focuses on other factors, such as management, regulations, social and community issues when discussing this issue. Technical matters are often down-played or ignored entirely. This thesis argues that MRAs exhibit unique technical characteristics and that by understanding these characteristics, one can possibly design more cost-effective software. One specific characteristic is described and addressed in this thesis – a characteristic we describe here for the first time and call a network island. Further analysis of the literature generates a picture of a distributed network of access nodes (DANs) within such network islands, which are connected by high speed networks and are able to share resources and stimulate usage of technology by offering a wide range of services. This thesis attempts to design a fitting middleware platform for such a context, which would achieve the following aims: i) allow software developers to create solutions for the context more efficiently (correctly, rapidly); ii) stimulate product managers and business owners to create innovative software products more easily (cost-effectively). A given in the context of this thesis is that the software should use free/libre open source software (FLOSS) – good arguments do also exist for the use of FLOSS. A review of useful FLOSS frameworks is undertaken and several of these are examined in an applied part of the thesis, to see how useful they may be. They form the basis for a walking skeleton implementation of the proposed middleware. The Spring framework is the basis for experiments, along with Spring-Webservices, JMX and PHP 5’s web service capabilities. This thesis builds on three years of work at the Siyakhula Living Lab (SLL), an experimental testbed in a MRA in the Mbashe district of the Eastern Cape of South Africa. Several existing products are deployed at the SLL in the fields of eCommerce, eGovernment and eLearning. Requirements specifications are engineered from a variety of sources, including interviews, mailing lists, the author’s experience as a supervisor at the SLL, and a review of the existing SLL products. Future products are also investigated, as the thesis considers current trends in ICTD. Use cases are also derived and listed. Most of the use cases are concerned with management functions of DANs that can be automated, so that operators of DANs can focus on their core business and not on technology. Using the UML Components methodology, the thesis then proceeds to design a middleware component architecture that is derived from the requirements specification. The process proceeds step-by-step, so that the reader can follow how business rules, operations and interfaces are derived from the use cases. Ultimately, the business rules, interfaces and operations are related to business logic, system interfaces and operations that are situated in specific components. The components in turn are derived from the business information model, that is derived from the business concepts that were initially used to describe the context for the requirements engineering. In this way, a logical method for software design is applied to the problem domain to methodically derive a software design for a middleware solution. The thesis tests the design by considering possible weaknesses in the design. The network aspect is tested by interpolating from formal assumptions about the nature of the context. The data access layer is also identified as a possible bottleneck. We suggest the use of fast indexing methods instead of relational databases to maintain flexibility and efficiency of the data layer. Lessons learned from the exercise are discussed, within the context of the author’s experience in software development teams, as well as in ICTD projects. This synthesis of information leads to warnings about the psychology of middleware development. We note that the ICTD domain is a particularly difficult one with regards to software development as business requirements are not usually clearly formulated and developers do not have the requisite domain knowledge. In conclusion, the core arguments of the thesis are recounted in a bullet form, to lay bare the reasoning behind this work. Novel aspects of the work are also highlighted. They include the description of a network island, and aspects of the DAN middleware requirements engineering and design. Future steps for work based on this thesis are mapped out and open problems relating to this research are touched upon.

No description available.

No description available.

The purpose of this project is to investigate the use of the emerging XML technologies to improve online Business to Business (B2B) supply chain processes.