RamboNodes for the Metropolitan Ad Hoc Network

We present an algorithm to store data robustly in a large, geographically distributed network by means of localized regions of data storage that move in response to changing conditions. For example, data might migrate away from failures or toward regions of high demand. The PersistentNode algorithm provides this service robustly, but with limited safety guarantees. We use the RAMBO framework to transform PersistentNode into RamboNode, an algorithm that guarantees atomic consistency in exchange for increased cost and decreased liveness. In addition, a half-life analysis of RamboNode shows that it is robust against continuous low-rate failures. Finally, we provide experimental simulations for the algorithm on 2000 nodes, demonstrating how it services requests and examining how it responds to failures

Permission-based fault tolerant mutual exclusion algorithm for mobile Ad Hoc networks

This study focuses on resolving the problem of mutual exclusion in mobile ad hoc networks. A Mobile Ad Hoc Network (MANET) is a wireless network without fixed infrastructure. Nodes are mobile and topology of MANET changes very frequently and unpredictably. Due to these limitations, conventional mutual exclusion algorithms presented for distributed systems (DS) are not applicable for MANETs unless they attach to a mechanism for dynamic changes in their topology. Algorithms for mutual exclusion in DS are categorized into two main classes including token-based and permission-based algorithms. Token-based algorithms depend on circulation of a specific message known as token. The owner of the token has priority for entering the critical section. Token may lose during communications, because of link failure or failure of token host. However, the processes for token-loss detection and token regeneration are very complicated and time-consuming. Token-based algorithms are generally non-fault-tolerant (although some mechanisms are utilized to increase their level of fault-tolerance) because of common problem of single token as a single point of failure. On the contrary, permission-based algorithms utilize the permission of multiple nodes to guarantee mutual exclusion. It yields to high traffic when number of nodes is high. Moreover, the number of message transmissions and energy consumption increase in MANET by increasing the number of mobile nodes accompanied in every decision making cycle. The purpose of this study is to introduce a method of managing the critical section,named as Ancestral, having higher fault-tolerance than token-based and fewer message transmissions and traffic rather that permission-based algorithms. This method makes a tradeoff between token-based and permission-based. It does not utilize any token, that is similar to permission-based, and the latest node having the critical section influences the entrance of the next node to the critical section, that is similar to token-based algorithms. The algorithm based on ancestral is named as DAD algorithms and increases the availability of fully connected network between 2.86 to 59.83% and decreases the number of message transmissions from 4j-2 to 3j messages (j as number of nodes in partition). This method is then utilized as the basis of dynamic ancestral mutual exclusion algorithm for MANET which is named as MDA. This algorithm is presented and evaluated for different scenarios of mobility of nodes, failure, load and number of nodes. The results of study show that MDA algorithm guarantees mutual exclusion,dead lock freedom and starvation freedom. It improves the availability of CS to minimum 154.94% and 113.36% for low load and high load of CS requests respectively compared to other permission-based lgorithm.Furthermore, it improves response time up to 90.69% for high load and 75.21% for low load of CS requests. It degrades the number of messages from n to 2 messages in the best case and from 3n/2 to n in the worst case. MDA algorithm is resilient to transient partitioning of network that is normally occurs due to failure of nodes or links

Prinzipien der Replikationskontrolle in verteilten Datenbanksystemen?

Durch Datenreplikation können prinzipiell schnellere Zugriffszeiten und eine beliebig hohe Fehlertoleranz in verteilten Datenbanksystemen erreicht werden. Anderseits erhöht Replikation die Gefahr von Inkonsistenzen und den Aufwand von Änderungsoperationen. Zur Lösung dieses Zielkonflikts wurden in der Literatur viele unterschiedliche Replikationsverfahren vorgeschlagen. Dieser Überblicksartikel beschreibt die den einzelnen Verfahren zugrundeliegenden Prinzipien zur Replikationskontrolle. Dazu werden die durch den Kopieneinsatz resultierenden Probleme erläutert, daraus Kriterien zur anschließenden Klassifikation abgeleitet und danach ausgewählte Replikationsverfahren näher vorgestellt

Система підтримки прийняття консенсусу між серверами у розподілених системах

Пояснювальна записка дипломного проекту складається з чотирьох розділів, містить 23 таблиці, 2 додатки та 46 джерел – загалом 116 сторінок. Об`єкт дослідження: алгоритми прийняття консенсусу. Мета дипломного проекту: розробити систему, що дозволяє адмініструвати кластер з кількох серверів, у якому працює покращений алгоритм прийняття консенсусу. У першому розділі було проведено загальний огляд задачі консенсусу та розглянуто особливості розподілених систем. У другому розділі проведено аналіз наявних методів рішення задачі консенсусу, зроблено вибір методу для покращення, введено оптимізацію алгоритму. У третьому розділі наведено ряд функцінальних та нефункціональних вимог до системи, обрано програмні засоби для реалізації системи, наведено інструкцію користувача. У четвертому розділі було розроблено стартап-проект, для якого проведено детальний аналіз ринку, сильних та слабких сторін. Розроблено маркетингову стратегію.The explanatory note of the diploma project consists of four sections, contains 23 tables, 2 applications and 46 sources - a total of 116 pages. Object of research: algorithms of acceptance of consensus. The purpose: to develop a system that allows you to administer a cluster of multiple servers, which has an improved algorithm for consensus acceptance. The first section provides an overview of the consensus problem and considers the features of distributed systems. In the second section the analysis of available methods of the decision of a problem of consensus is carried out, the choice of a method for improvement is made, optimization of algorithm is entered. The third section lists a number of functional and non-functional requirements for the system, selected software for the system, provides user instructions. In the fourth section, a startup project was developed, for which a detailed analysis of the market, strengths and weaknesses was conducted. Developed marketing strategy

Design and evaluation of a voting tool in a collaborative environment

This dissertation researchdesigned, implemented, and evaluated a Web-based Dynamic Voting Toolfor small group decision-making in a collaborative environment. In this dissertation, theliterature on voting tools in current GDSS research is presented. Variousvoting theories and methods are analyzed, and the advantages and weaknessesare compared, so as to gain a better understanding of how to apply thesedifferent voting methods to diverse decision-making situations. A briefoverview of scaling theories is also given, with an emphasis on Thurstone\u27sLaw. The basic features of someweb-based voting tool implementations are reviewed along with a discussionof the pros and cons of Intemet voting. A discussion of Human DynamicVoting (HDV) follows; HDV allows multiple voting and continuous feedbackin a group process. The Dynamic Voting Tool designed and developed bythe author (i.e., Zheng Li) integrated multiple scaling and voting methods,and supported dynamic voting. Its features, user feedback, and futureimprovements are further discussed. A controlled experiment wasconducted to evaluate the effects of the Dynamic Voting Tool (alongwith the List Gathering Tool by Yuanqiong Wang) interacting with smallgroup process. The design and procedures of the experiment, and thedata analysis results extracted from 187 student subjects from New JerseyInstitute of Technology are reported. While the System Survey yieldedvery positive feedback on the voting tool, the hypotheses tested bythe Post-Questionnaire and expert judgments showed no major positivesignificant results. This was probably due to the complexity of thetask and procedures, lack of motivation of the subjects, bad timing,insufficient training, and uneven distribution of subjects, etc. Several field studies usingthe Social Decision Support System (SDSS) Toolkit (List Gathering Tool+ Dynamic Voting Tool) are presented. The SDSS system worked well whenthe subjects were motivated. The field studies show that the toolkitcan be used in course evaluations, or other practical applications. Finally, it is suggestedthat future research can focus on improving the voting tool with truedynamic features, exploring more issues on SDSS systems design and experimentation,and exploring the relationship of voting and GSS

Robust services in dynamic systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (p. 191-202).Our growing reliance on online services accessible on the Internet demands highly- available systems that work correctly without interruption. This thesis extends previous work on Byzantine-fault-tolerant replication to meet the new requirements of current Internet services: scalability and the ability to reconfigure the service automatically in the presence of a changing system membership. Our solution addresses two important problems that appear in dynamic replicated services: First, we present a membership service that provides servers and clients in the system with a sequence of consistent views of the system membership (i.e., the set of currently available servers). The membership service is designed to be scalable, and to handle membership changes mostly automatically. Furthermore, the membership service is itself reconfigurable, and tolerates arbitrary faults of a subset of the servers that are implementing it at any instant. The second part of our solution is a generic methodology for transforming replicated services that assume a fixed membership into services that support a dynamic system membership. The methodology uses the output from the membership service to decide when to reconfigure.(cont.) We built two example services using this methodology: a dynamic Byzantine quorum system that supports read and write operations, and a dynamic Byzantine state machine replication system that supports any deterministic service. The final contribution of this thesis is an analytic study that points out an obstacle to the deployment of replicated services based on a dynamic membership. The basic problem is that maintaining redundancy levels for the service state as servers join and leave the system is costly in terms of network bandwidth. To evaluate how dynamic the system membership can be, we developed a model for the cost of state maintenance in dynamic replicated services, and we use measured values from real-world traces to determine possible values for the parameters of the model. We conclude that certain deployments (like a volunteer-based system) are incompatible with the goals of large- scale reliable services. We implemented the membership service and the two example services. Our performance results show that the membership service is scalable, and our replicated services perform well, even during reconfigurations.by Rodrigo Seromenho Miragaia Rodrigues.Ph.D

Using policy to control data synchronisation in middleware for an ad-hoc mobile network

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