A framework for dependable adaptation in probabilistic environments

Distributed applications executing in probabilistic environments, like the Internet, often need to make timing assumptions, for instance about the maximum message delay or the round-trip delay. In the case of adaptive systems these bounds should be computed at run-time, using probabilistic or other more or less ad hoc approaches, typically with the objective of improving the application performance. From a dependability perspective, however, the concern is to secure some properties on which the application can rely. Building on previous work where we set the stage for dependable adaptation, we introduce a general framework for implementing improved solutions for dependable adaptation. During its lifetime, a system alternates periods where its temporal behavior is well characterized, with transition periods where a variation of the environment conditions occurs. The proposed framework rests on the detection of these stable and transient phases, and it is generic in the sense that different detection mechanisms can be included. We use a set of simple phase detection mechanisms inspired in well-known probabilistic tests, along with synthetically generated data flows, to illustrate how the framework can be applie

A Probabilistic Framework for Automatic and Dependable Adaptation in Dynamic Environments

Reviewed by Miguel P. CorreiaDistributed protocols executing in uncertain environments, like the Internet, had better adapt dynamically to environment changes in order to preserve QoS. In earlier work, it was shown that QoS adaptation should be dependable, if correctness of protocol properties is to be maintained. More recently, some ideas concerning specific strategies and methodologies for improving QoS adaptation have been proposed. In this paper we describe a complete framework for dependable QoS adaptation. We assume that during its life-time, a system alternates periods where its temporal behavior is well characterized, with transition periods during which a variation of the environment conditions occurs. Our method is based on the following: if the environment is generically characterized in analytical terms, and we can detect the alternation of these stable and transient phases, we can improve the effectiveness and dependability of QoS adaptation. To prove our point we provide detailed evaluation results of the proposed solutions. Our evaluation is based on synthetic data flows generated from probabilistic distributions, as well as on real data traces collected in various Internet-based environments. Our results show that the proposed strategies can indeed be effective, allowing protocols to adapt to the available QoS in a dependable way.EC project IST-FP6-STREP-26979 (HIDENETS), and FCT, through the Multiannual and CMU-Portugal programmes

Support for dependable and adaptive distributed systems and applications

Tese de doutoramento, Informática (Engenharia Informática), Universidade de Lisboa, Faculdade de Ciências, 2011Distributed applications executing in uncertain environments, like the Internet, need to make timing/synchrony assumptions (for instance, about the maximum message transmission delay), in order to make progress. In the case of adaptive systems these temporal bounds should be computed at runtime, using probabilistic or specifically designed ad hoc approaches, typically with the objective of improving the application performance. From a dependability perspective, however, the concern is to secure some properties on which the application can rely. This thesis addresses the problem of supporting adaptive systems and applications in stochastic environments, from a dependability perspective: maintaining the correctness of system properties after adaptation. The idea behind dependable adaptation consists in ensuring that the assumed bounds for fundamental variables (e.g., network delays) are secured with a known and constant probability. Assuming that during its lifetime a system alternates periods where its temporal behavior is well characterized (stable phases), with transition periods where a variation of the network conditions occurs (transient phases), the proposed approach is based on the following: if the environment is generically characterized in analytical terms and it is possible to detect the alternation of these stable and transient phases, then it is possible to effectively and dependably adapt applications. Based on this idea, the thesis introduces Adaptare, a framework for supporting dependable adaptation in stochastic environments. An extensive evaluation of Adaptare is provided, assessing the correctness and effectiveness of the implemented mechanisms. The results indicate that the proposed strategies and methodologies are indeed effective to support dependable adaptation of distributed systems and applications. Finally, the applicability of Adaptare is evaluated in the context of two fundamental problems in distributed systems: consensus and failure detection. The thesis proposes solutions for these problems based on modular architectures in which Adaptare is used as a middleware for dependable adaptation of assumed timeouts.Aplicações distribuídas que executam em ambientes incertos, como a Internet, baseiam-se em pressupostos sobre tempo/sincronia (por exemplo, assumem um tempo máximo para a transmissão de mensagens) a fim de assegurar progresso. No caso de sistemas adaptativos, esses limites temporais devem ser calculados em tempo de execução, usando abordagens probabilísticas ou desenhadas de forma específica e ad hoc, tipicamente visando melhorar o desempenho da aplicação. Sob o ponto de vista da confiabilidade, no entanto, o objetivo é garantir algumas propriedades nas quais a aplicação pode confiar. Esta tese aborda o problema de suportar sistemas adaptativos e aplicações que operam em ambientes estocásticos, numa perspectiva de confiabilidade: mantendo a correção das propriedades do sistema após a adaptação. A ideia da adaptação confiável consiste em garantir que os limites assumidos para variáveis fundamentais (por exemplo, latências de transmissão) são assegurados com uma probabilidade conhecida e constante. Supondo que durante a execução o sistema alterna períodos nos quais o seu comportamento temporal é bem caracterizado (fases estáveis), com períodos de transição durante os quais ocorrem variações das condições da rede (fases transientes), a abordagem proposta baseia-se no seguinte: se o ambiente é genericamente caracterizado em termos analíticos e é possível detetar a alternância entre fases estáveis e transientes, então é possível adaptar as aplicações de forma efetiva e confiável. Com base nesta ideia, a tese apresenta uma plataforma para suportar a adaptação confiável em ambientes estocásticos, denominada Adaptare. A tese contém uma extensa avaliação do Adaptare, que foi realizada para verificar a correção e eficácia dos mecanismos desenvolvidos. Os resultados indicam que as estratégias e metodologias propostas são de facto efetivas para suportar a adaptação confiável de sistemas e aplicações distribuídas. Finalmente, a aplicabilidade do Adaptare é avaliada no contexto de dois problemas fundamentais em sistemas distribuídos: consenso e deteção de falhas. A tese propõe soluções para estes problemas baseadas em arquiteturas modulares nas quais o Adaptare é usado como um middleware para a adaptação confiável de timeouts.Fundação para a Ciência e a Tecnologia (FCT