Scalability approaches for causal multicast: a survey

The final publication is available at Springer via http://dx.doi.org/10.1007/s00607-015-0479-0Many distributed services need to be scalable: internet search, electronic commerce, e-government... In order to achieve scalability, high availability and fault tolerance, such applications rely on replicated components. Because of the dynamics of growth and volatility of customer markets, applications need to be hosted by adaptive, highly scalable systems. In particular, the scalability of the reliable multicast mechanisms used for supporting the consistency of replicas is of crucial importance. Reliable multicast might propagate updates in a pre-determined order (e.g., FIFO, total or causal). Since total order needs more communication rounds than causal order, the latter appears to be the preferable candidate for achieving multicast scalability, although the consistency guarantees based on causal order are weaker than those of total order. This paper provides a historical survey of different scalability approaches for reliable causal multicast protocols.This work was supported by European Regional Development Fund (FEDER) and Ministerio de Economia y Competitividad (MINECO) under research Grant TIN2012-37719-C03-01.Juan Marín, RD.; Decker, H.; Armendáriz Íñigo, JE.; Bernabeu Aubán, JM.; Muñoz Escoí, FD. (2016). Scalability approaches for causal multicast: a survey. Computing. 98(9):923-947. https://doi.org/10.1007/s00607-015-0479-0S923947989Adly N, Nagi M (1995) Maintaining causal order in large scale distributed systems using a logical hierarchy. In: IASTED Intnl Conf on Appl Inform, pp 214–219Aguilera MK, Chen W, Toueg S (1997) Heartbeat: a timeout-free failure detector for quiescent reliable communication. In: 11th Intnl Wshop on Distrib Alg (WDAG), Saarbrücken, pp 126–140Almeida JB, Almeida PS, Baquero C (2004) Bounded version vectors. 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Byzantine fault-tolerant vote collection for D-DEMOS, a distributed e-voting system

Τα συστήματα διαχείρισης εκλογών είναι μια δυναμική τεχνολογία που επιτρέπει την βελτίωση της δημοκρατικής διαδικασίας μέσω της μείωσης του κόστους υλοποίησης εκλογών, της αύξησης της συμμετοχής των ψηφοφόρων και της αμεσότητας παραγωγής αποτελεσμάτων. Επίσης, δίνουν την δυνατότητα στους ψηφοφόρους να επιβεβαιώσουν άμεσα την ορθή λειτουργία ολόκληρης της εκλογικής διαδικασίας. Δυστυχώς, τα υπάρχοντα τέτοια συστήματα είναι σχεδιασμένα με κεντρικά συστατικά, τα οποία και αποτελούν μοναδικά σημεία αποτυχίας. Αυτό μπορεί να οδηγήσει στην απώλεια διαθεσιμότητας, εμπιστευτικότητας, καθώς και της ακεραιότητας του εκλογικού αποτελέσματος. Σε αυτή τη διατριβή εξετάζουμε την εισαγωγή ανοχής λαθών στα εκλογικά συστήματα, μέσω της εισαγωγής κατανεμημένων συστατικών. Αυτό είναι περίπλοκο γιατί, εκτός από την ακεραιότητα και διαθεσιμότητα, σε ένα εκλογικό σύστημα είναι σημαντικό να διαφυλαχθεί και η εμπιστευτικότητα, απέναντι σε έναν κακόβουλο αντίπαλο. Εστιάζουμε στην φάση συλλογής ψήφων του εκλογικού συστήματος, η οποία είναι ένα κρίσιμο τμήμα της εκλογικής διαδικασίας. Χρησιμοποιούμε το σύγχρονο αλλά κεντρικοποιημένο σύστημα διαχείρισης εκλογών DEMOS σαν βάση για την μελέτη μας. Αυτό το σύστημα χρησιμοποιεί κωδικούς που αντιστοιχούν στις δυνατές επιλογές των ψηφοφόρων, μια Αρχή Εκλογών η οποία αρχικοποιεί τις εκλογές, συλλέγει τις ψήφους και παράγει το αποτέλεσμα, και έναν Πίνακα Ανακοινώσεων για την διατήρηση των στοιχείων των εκλογών μακροπρόθεσμα. Εξάγουμε τον μηχανισμό συλλογής ψήφων από την κεντρικοποιημένη Αρχή Εκλογών του αρχικού συστήματος DEMOS, και τον αντικαθιστούμε με ένα κατανεμημένο σύστημα που χειρίζεται την συλλογή ψήφων με ανοχή σε λάθη Βυζαντινού τύπου. Σε αυτή τη διατριβή, παρουσιάζουμε τον σχεδιασμό, ανάλυση ασφάλειας, την ανάπτυξη και αξιολόγηση της πρωτότυπης υλοποίησης αυτού του κατανεμημένου συστατικού συλλογής ψήφων. Παρουσιάζουμε δύο εκδόσεις αυτού του συστατικού: μία πλήρως ασύγχρονη και μία με ελάχιστες υποθέσεις συγχρονισμού αλλά καλύτερη απόδοση. Και οι δύο εκδόσεις παρέχουν άμεση επιβεβαίωση στην ψηφοφόρο ότι η ψήφος της καταχωρήθηκε όπως υποβλήθηκε, χωρίς να απαιτούνται κρυπτογραφικές λειτουργίες από την πλευρά της ψηφοφόρου. Με αυτόν τον τρόπο, η ψηφοφόρος μπορεί να στείλει την ψήφο της χρησιμοποιώντας έναν μη ασφαλή υπολογιστή ή δίκτυο, και να συνεχίσει να είναι εξασφαλισμένη ότι η ψήφος της καταχωρήθηκε σωστά. Για παράδειγμα, μπορεί να ψηφίσει χρησιμοποιώντας έναν δημόσιο υπολογιστή, ή στέλνοντας ένα σύντομο μήνυμα μέσω κινητού τηλεφώνου. Ακόμη και σε αυτές τις περιπτώσεις, η εμπιστευτικότητα της ψήφου διατηρείται στο ακέραιο. Δίνουμε ένα μοντέλο και μια ανάλυση ασφάλειας για τα συστήματα που παρουσιάζουμε. Υλοποιούμε πρωτότυπα από τα πλήρη συστήματα, μετράμε την απόδοσή τους πειραματικά, και επιδεικνύουμε την ικανότητά τους να χειρίζονται εκλογές μεγάλου μεγέθους. Τέλος, παρουσιάζουμε τις διαφορές απόδοσης ανάμεσα στις δύο εκδόσεις του συστήματος. Θεωρούμε ότι τα συστατικά συλλογής ψήφων που παρουσιάζουμε σε αυτή τη διατριβή μπορούν να βρουν εφαρμογή σε οποιοδήποτε σύστημα διαχείρισης εκλογών που στηρίζεται στην τεχνική της εκπροσώπησης των επιλογών στα ψηφοδέλτια με κωδικούς.E-voting systems are a powerful technology for improving democracy by reducing election cost, increasing voter participation, and even allowing voters to directly verify the entire election procedure. Unfortunately, prior internet voting systems have single points of failure, which may result in the compromise of availability, voter secrecy, or integrity of the election results. In this thesis, we consider increasing the fault-tolerance of voting systems by introducing distributed components. This is non-trivial as, besides integrity and availability, voting requires safeguarding confidentiality as well, against a malicious adversary. We focus on the vote collection phase of the voting system, which is a crucial part of the election process. We use the DEMOS state-of-the-art but centralized voting system as the basis for our study. This system uses vote codes to represent voters' choices, an Election Authority to setup the election and handle vote collection and result production, and a Bulletin Board for storing the election transcript for the long-term. We extract the vote collection mechanism from the centralized Election Authority component of the original DEMOS system, and replace it with a distributed system that handles vote collection in a Byzantine fault-tolerant manner. In this thesis, we present the design, security analysis, prototype implementation and experimental evaluation of this vote collection component. We present two versions of this component: one completely asynchronous and one with minimal timing assumptions but better performance. Both versions provide immediate assurance to the voter her vote was recorded as cast, without requiring cryptographic operations on behalf of the voter. This way, a voter may cast her vote using an untrusted computer or network, and still be assured her vote was recorded as cast. For example, she may vote via a public web terminal, or by sending an SMS from a mobile phone. Even in these cases, voter's privacy is still preserved. We provide a model and security analysis of the systems we present. We implement prototypes of the complete systems, we measure their performance experimentally, and we demonstrate their ability to handle large-scale elections. Finally, we demonstrate the performance trade-offs between the two versions of the system. We consider the vote collection components we introduce are applicable to any voting system that uses the code-voting technique

Invariant preservation in geo-replicated data stores

The Internet has enabled people from all around the globe to communicate with each other in a matter of milliseconds. This possibility has a great impact in the way we work, behave and communicate, while the full extent of possibilities are yet to be known. As we become more dependent of Internet services, the more important is to ensure that these systems operate correctly, with low latency and high availability for millions of clients scattered all around the globe. To be able to provide service to a large number of clients, and low access latency for clients in different geographical locations, Internet services typically rely on georeplicated storage systems. Replication comes with costs that may affect service quality. To propagate updates between replicas, systems either choose to lose consistency in favor of better availability and latency (weak consistency), or maintain consistency, but the system might become unavailable during partitioning (strong consistency). In practice, many production systems rely on weak consistency storage systems to enhance user experience, overlooking that applications can become incorrect due to the weaker consistency assumptions. In this thesis, we study how to exploit application’s semantics to build correct applications without affecting the availability and latency of operations. We propose a new consistency model that breaks apart from traditional knowledge that applications consistency is dependent on coordinating the execution of operations across replicas. We show that it is possible to execute most operations with low latency and in an highly available way, while preserving application’s correctness. Our approach consists in specifying the fundamental properties that define the correctness of applications, i.e. the application invariants, and identify and prevent concurrent executions that potentially can make the state of the database inconsistent, i.e. that may violate some invariant. We explore different, complementary, approaches to implement this model. The Indigo approach consists in preventing conflicting operations from executing concurrently, by restricting the operations that each replica can execute at each moment to maintain application’s correctness. The IPA approach does not preclude the execution of any operation, ensuring high availability. To maintain application correctness, operations are modified to prevent invariant violations during replica reconciliation, or, if modifying operations provides an unsatisfactory semantics, it is possible to correct any invariant violations before a client can read an inconsistent state, by executing compensations. Evaluation shows that our approaches can ensure both low latency and high availability for most operations in common Internet application workloads, with small execution overhead in comparison to unmodified weak consistency systems, while enforcing application invariants, as in strong consistency systems

Consistency models in modern distributed systems. An approach to Eventual Consistency

[EN] Eventual consistency has become very famous since the popularity of cloud computing. The presented work analises what this concept is and compares it with other consistency models. Moreover, it explaines a set of techniques to implement it and also a list of scientific papers related to the matter.[ES] La consistencia final ha venido siendo muy famosa desde el estallido del éxito de la computación en la nube o cloud computing. El trabajo analiza en qué consiste dicho concepto y lo compara con los preexistentes, además de aportar una serie de metodologías de implementación de la misma así como un listado de referencias científicas como resultado del estudio en el área.Pascual Miret, L. (2014). Consistency models in modern distributed systems. An approach to Eventual Consistency. http://hdl.handle.net/10251/54786Archivo delegad

An Information-Theoretic Framework for Consistency Maintenance in Distributed Interactive Applications

Distributed Interactive Applications (DIAs) enable geographically dispersed users to interact with each other in a virtual environment. A key factor to the success of a DIA is the maintenance of a consistent view of the shared virtual world for all the participants. However, maintaining consistent states in DIAs is difficult under real networks. State changes communicated by messages over such networks suffer latency leading to inconsistency across the application. Predictive Contract Mechanisms (PCMs) combat this problem through reducing the number of messages transmitted in return for perceptually tolerable inconsistency. This thesis examines the operation of PCMs using concepts and methods derived from information theory. This information theory perspective results in a novel information model of PCMs that quantifies and analyzes the efficiency of such methods in communicating the reduced state information, and a new adaptive multiple-model-based framework for improving consistency in DIAs. The first part of this thesis introduces information measurements of user behavior in DIAs and formalizes the information model for PCM operation. In presenting the information model, the statistical dependence in the entity state, which makes using extrapolation models to predict future user behavior possible, is evaluated. The efficiency of a PCM to exploit such predictability to reduce the amount of network resources required to maintain consistency is also investigated. It is demonstrated that from the information theory perspective, PCMs can be interpreted as a form of information reduction and compression. The second part of this thesis proposes an Information-Based Dynamic Extrapolation Model for dynamically selecting between extrapolation algorithms based on information evaluation and inferred network conditions. This model adapts PCM configurations to both user behavior and network conditions, and makes the most information-efficient use of the available network resources. In doing so, it improves PCM performance and consistency in DIAs

Inverted GUI Development for IoT with Applications in E-Health

In the context of Internet of Things (IoT), the research of this dissertation is concerned with the development of applications for end-user devices, i.e. devices through which the end-user directly interacts with systems. The complexity of such applications is partly due to network intricacies, and partly because GUI (Graphical User Interface) development is generally complicated and time consuming. We employ a middleware framework called PalCom to manage the former, and focus our research on the problems of the latter, by expanding the scope of PalCom to also enable GUI development. In particular, the research goal is a more efficient GUI development approach that does not require program code to be written.To enable end-users with little or no programming experience to participate in the GUI development process, we eliminate the need for programming by introducing a new development approach. We view this approach as “inverted” in that the development focus is on presenting functionality from an application model as graphical components in a GUI, rather than on retroactively attaching functionality to manually added graphical components. The inverted GUI development approach is supported in two steps. First, we design a language for describing GUIs, and implement interpreters that communicate with remotely hosted application models and render GUI descriptions as fully functional GUIs. Second, we implement a graphical editor for developing GUIs in order to make the language more accessible.The presented solution is evaluated by its application in a number of research projects in the domain of e-health. From the GUIs developed in those projects, we conclude that the GUI language is practically viable for building full-blown, professional grade GUIs. Furthermore, the presented graphical editor is evaluated by direct comparison to a market leading product in a controlled experiment. From this, we conclude that the editor is accessible to new users, and that it can be more efficient to use than the commercial alternative

An action research study on the adoption of a Lean Management System in a healthcare organization attempting to transform its culture

Abstract: This dissertation focuses on the adoption of an integrated performance management system (inspired by a Lean philosophy) in a Quebec healthcare institution. Several institutions in the field of healthcare have adopted Lean principles with the objective of improving the quality of care and services. Unfortunately, an emphasis on the adoption of tools and techniques has limited its potential for improvement since one of the main pillars of a lean philosophy, respect for people, has been lost in translation. The belief that people are the greatest asset of an organization, and that investment in their development is essential, is central to the dynamic learning ability that is the heart of a Lean philosophy (Holweg, 2007). Studies of organizational learning from a cultural perspective highlight that learning is the result of the interactions between people and organizational elements, and this learning is embodied in the artifacts of culture. Organizational culture is, therefore, a social, dynamic and cyclical phenomenon that is constantly redefining itself. In this study, an organization is seen as a culture (contrary to the view that an organization 'has' a culture), whereby an organization is defined as a loosely structured and incompletely shared system of symbols and meanings that emerges through dynamic interaction . This interaction leads either to a reinforcement or to a change in symbols and meanings. The objective of this study is to illustrate from a cultural perspective, how a healthcare organization implements an integrated lean management system. The research strategy is a longitudinal study, carried out as action research. It is situated in a postmodern pragmatist paradigm (rooted in the philosophical tradition of Chia (2003)) where the researcher participates directly with members of the organization, introducing reflexive practices that help guide action and generate knowledge. Two iterative cycles were completed, during which research participants supported the organization in changing 'the way things are done', with the goal of improving the safety and delivery of care through efficient processes, judicious use of resources, dedication of employees to their practice, and accessibility of care and services. In other words, to refocus actions on the 'core business' (clinical operations). Qualitative data was collected over a three-year period. The data consisted of the researcher's observation notes, sixty-two semi-structured interviews, and institutional documents. The data collected was analyzed in several stages; a descriptive account of the process was generated first; an analysis of the lean transformation over the period of the study was then conducted; and finally, the implementation process was analyzed from a cultural perspective. The descriptive narrative account of the change process begins with the CHUS in 2014 introducing their integrated performance management system (SPCHUS) and ends with the CIUSSS of Estrie-CHUS (created as a result of the reform adopted in February and implemented in April 2015) and the adoption of their system SGIP. Included in the descriptive narrative are the key learnings of the research participants following each action research cycle, and the adjustments made to the introduction of the performance management system for the next steps in implementation. Of particular note in the learnings and the subsequent adjustments was the behavioral nature of the changes, with little (if any) questioning of the notably classic beliefs of management manifested in individual actions and interactions. A lean philosophy of management inherently relies on very different beliefs, and the discrepancy between the beliefs of the current and desired system should be made explicit in order for changes in the underlying mindset to occur. As Toussaint and Berry (2013) so eloquently state "before we can change we need to understand why we act as we do" (p. 11). The analysis of the lean transformation that occurred over both action research cycles highlighted how the instrumentalism of the implementation, the bureaucratic nature of the organization, and the various understandings coexisting in the organization concerning Lean contributed to the lack of learning in the organization. The present research reinforced previous findings that organizational learning is fundamental for achieving a philosophy driven level of transformation, and that moving from a tool-driven to a system-driven level of transformation is extremely difficult in public healthcare (Mackenzie and Hall, 2015). In addition, the in-depth study of the implementation of a management system rooted in a lean philosophy answers a suggestion for research in this key area that is under investigated in the current literature (D'Andreamatteo, Ianni, Lega and Sargiacomo, 2015). The final level of analysis attempts to partially fill the gap in the scientific literature of analyzing lean implementations from a cultural perspective. This study analyzes the change process using Hatch's (1993) dynamic model of organizational culture with very specific examples allowing the researcher to illustrate that the adoption of an integrated management system, inspired by a Lean philosophy, requires a cultural change. The key to cultural change is the change in meanings that arises through interaction, and as this study has illustrated, this occurs through organizational learning. This requires an approach to implementation that demonstrates a desire to learn. It also requires a change of perspective concerning the role of the leader in culture change as the orchestrater of change, which dominates the literature. Instead, a leader's role becomes one of managing meaning, of inviting input from others, asking probing questions, encouraging multiple points of view and providing opportunities for discussion and reflection. Thus, while leaders are making enormous efforts to re-structure and re-conceptualize public healthcare institutions, this study illustrates the need to pay attention to meanings that are generated through interaction, as cultural evolution emerges precisely from these interactions (Houle and Roberts, 2016).Résumé : Cette thèse porte sur l'adoption d'un système intégré de gestion de la performance (inspiré de la philosophie de gestion Lean) dans un établissement de soins de santé québécois. Dans le but d'améliorer la qualité de leurs soins et services, plusieurs institutions dans le domaine des soins de santé ont adhéré aux principes de gestion Lean, avec des niveaux très variables de succès. L'objectif de la présente étude est d'illustrer d'un point de vue culturel, comment une organisation de soins de santé met en œuvre un système de gestion Lean. La stratégie de recherche est longitudinale et réalisée dans le cadre d'une recherche-action. Des données qualitatives ont été recueillies sur une période de trois ans. Elles sont issues des notes et observations de la chercheuse, de soixante-deux entrevues semi-structurées ainsi que de documents institutionnels secondaires. Les données recueillies ont été analysées à différents stades : une description du processus a d'abord été établie, suivie d'une analyse de la transformation Lean pendant la période de l'étude. Pour conclure, le processus de changement a été analysé dans une perspective culturelle selon le modèle dynamique de culture organisationnelle de Hatch (1993). Le chercheur, exemples très spécifiques à l'appui, illustre que l'adoption d'un système intégré de gestion, inspiré de la philosophie Lean, requiert un changement culturel. La clé du changement culturel se situe dans les interactions amenant à l'apprentissage de deuxième niveau. L'étude approfondie de la mise en place d'un système de gestion inspiré de la philosophie de gestion Lean répond à une suggestion de recherche dans ce domaine clé étudié actuellement (D'Andreamatteo et coll., 2015). Également, la recherche tente de combler des écarts, dans la littérature scientifique, identifiés par plusieurs auteurs qui indiquent qu'une analyse d'une perspective culturelle permettra une compréhension plus profonde des transformations Lean illustrant ainsi pourquoi les transformations s'avèrent une réussite ou un échec, et par le fait même, d'identifier des pistes pour gérer une telle transformation (Davies et Mannion, 2013 ; Synder et coll., 2016 ; Taher et coll., 2016)

Notes on Theory of Distributed Systems

Notes for the Yale course CPSC 465/565 Theory of Distributed Systems

MODELING AND CONTROL OF INTERLINE POWER FLOW CONTROLLER FOR POWER SYSTEM STABILITY ENHANCEMENT

Mitigation of power system oscillations is the problem of concern in the power industry as these oscillations, when exhibiting poor damping; affect the transmission line power transfer capability and power system stability. These oscillations greatly restrict power system operations and, in some cases, can also lead to widespread system disturbances. In this context, the Flexible AC Transmission System (FACTS) device, Interline Power Flow Controller (IPFC) employed to improve the transmission capability can be additionally utilized for damping control of power system oscillations. IPFC based damping controller design for power system stability requires proper and adequate mathematical representation of power system incorporating the FACTS device. This thesis reports the investigation on the development of steady state model, the dynamic nonlinear mathematical model of the power system installed with the IPFC for stability studies and the linearized extended Phillips Heffron model for the design of control techniques to enhance the damping of the lightly damped oscillations modes. In this context, the mathematical models of the single machine infinite bus (SMIB) power system and multi-machine power system incorporated with IPFC are established. The controllers for the IPFC are designed for enhancing the power system stability. The eigenvalue analysis and nonlinear simulation studies of the investigations conducted on the SMIB and Multi-machine power systems installed with IPFC demonstrate that the control designs are effective in damping the power system oscillations. The results presented in this thesis would provide useful information to electric power utilities engaged in scheduling and operating with the FACTS device, IPFC