Optimizing recovery protocols for replicated database systems

En la actualidad, el uso de tecnologías de informacíon y sistemas de cómputo tienen una gran influencia en la vida diaria. Dentro de los sistemas informáticos actualmente en uso, son de gran relevancia los sistemas distribuidos por la capacidad que pueden tener para escalar, proporcionar soporte para la tolerancia a fallos y mejorar el desempeño de aplicaciones y proporcionar alta disponibilidad. Los sistemas replicados son un caso especial de los sistemas distribuidos. Esta tesis está centrada en el área de las bases de datos replicadas debido al uso extendido que en el presente se hace de ellas, requiriendo características como: bajos tiempos de respuesta, alto rendimiento en los procesos, balanceo de carga entre las replicas, consistencia e integridad de datos y tolerancia a fallos. En este contexto, el desarrollo de aplicaciones utilizando bases de datos replicadas presenta dificultades que pueden verse atenuadas mediante el uso de servicios de soporte a mas bajo nivel tales como servicios de comunicacion y pertenencia. El uso de los servicios proporcionados por los sistemas de comunicación de grupos permiten ocultar los detalles de las comunicaciones y facilitan el diseño de protocolos de replicación y recuperación. En esta tesis, se presenta un estudio de las alternativas y estrategias empleadas en los protocolos de replicación y recuperación en las bases de datos replicadas. También se revisan diferentes conceptos sobre los sistemas de comunicación de grupos y sincronia virtual. Se caracterizan y clasifican diferentes tipos de protocolos de replicación con respecto a la interacción o soporte que pudieran dar a la recuperación, sin embargo el enfoque se dirige a los protocolos basados en sistemas de comunicación de grupos. Debido a que los sistemas comerciales actuales permiten a los programadores y administradores de sistemas de bases de datos renunciar en alguna medida a la consistencia con la finalidad de aumentar el rendimiento, es importante determinar el nivel de consistencia necesario. En el caso de las bases de datos replicadas la consistencia está muy relacionada con el nivel de aislamiento establecido entre las transacciones. Una de las propuestas centrales de esta tesis es un protocolo de recuperación para un protocolo de replicación basado en certificación. Los protocolos de replicación de base de datos basados en certificación proveen buenas bases para el desarrollo de sus respectivos protocolos de recuperación cuando se utiliza el nivel de aislamiento snapshot. Para tal nivel de aislamiento no se requiere que los readsets sean transferidos entre las réplicas ni revisados en la fase de cetificación y ya que estos protocolos mantienen un histórico de la lista de writesets que es utilizada para certificar las transacciones, este histórico provee la información necesaria para transferir el estado perdido por la réplica en recuperación. Se hace un estudio del rendimiento del protocolo de recuperación básico y de la versión optimizada en la que se compacta la información a transferir. Se presentan los resultados obtenidos en las pruebas de la implementación del protocolo de recuperación en el middleware de soporte. La segunda propuesta esta basada en aplicar el principio de compactación de la informacion de recuperación en un protocolo de recuperación para los protocolos de replicación basados en votación débil. El objetivo es minimizar el tiempo necesario para transfeir y aplicar la información perdida por la réplica en recuperación obteniendo con esto un protocolo de recuperación mas eficiente. Se ha verificado el buen desempeño de este algoritmo a través de una simulación. Para efectuar la simulación se ha hecho uso del entorno de simulación Omnet++. En los resultados de los experimentos puede apreciarse que este protocolo de recuperación tiene buenos resultados en múltiples escenarios. Finalmente, se presenta la verificación de la corrección de ambos algoritmos de recuperación en el Capítulo 5.Nowadays, information technology and computing systems have a great relevance on our lives. Among current computer systems, distributed systems are one of the most important because of their scalability, fault tolerance, performance improvements and high availability. Replicated systems are a specific case of distributed system. This Ph.D. thesis is centered in the replicated database field due to their extended usage, requiring among other properties: low response times, high throughput, load balancing among replicas, data consistency, data integrity and fault tolerance. In this scope, the development of applications that use replicated databases raises some problems that can be reduced using other fault-tolerant building blocks, as group communication and membership services. Thus, the usage of the services provided by group communication systems (GCS) hides several communication details, simplifying the design of replication and recovery protocols. This Ph.D. thesis surveys the alternatives and strategies being used in the replication and recovery protocols for database replication systems. It also summarizes different concepts about group communication systems and virtual synchrony. As a result, the thesis provides a classification of database replication protocols according to their support to (and interaction with) recovery protocols, always assuming that both kinds of protocol rely on a GCS. Since current commercial DBMSs allow that programmers and database administrators sacrifice consistency with the aim of improving performance, it is important to select the appropriate level of consistency. Regarding (replicated) databases, consistency is strongly related to the isolation levels being assigned to transactions. One of the main proposals of this thesis is a recovery protocol for a replication protocol based on certification. Certification-based database replication protocols provide a good basis for the development of their recovery strategies when a snapshot isolation level is assumed. In that level readsets are not needed in the validation step. As a result, they do not need to be transmitted to other replicas. Additionally, these protocols hold a writeset list that is used in the certification/validation step. That list maintains the set of writesets needed by the recovery protocol. This thesis evaluates the performance of a recovery protocol based on the writeset list tranfer (basic protocol) and of an optimized version that compacts the information to be transferred. The second proposal applies the compaction principle to a recovery protocol designed for weak-voting replication protocols. Its aim is to minimize the time needed for transferring and applying the writesets lost by the recovering replica, obtaining in this way an efficient recovery. The performance of this recovery algorithm has been checked implementing a simulator. To this end, the Omnet++ simulating framework has been used. The simulation results confirm that this recovery protocol provides good results in multiple scenarios. Finally, the correction of both recovery protocols is also justified and presented in Chapter 5.García Muñoz, LH. (2013). Optimizing recovery protocols for replicated database systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/31632TESI

Crash recovery with partial amnesia failure model issues

Replicated systems are a kind of distributed systems whose main goal is to ensure that computer systems are highly available, fault tolerant and provide high performance. One of the last trends in replication techniques managed by replication protocols, make use of Group Communication Sys- tem, and more specifically of the communication primitive atomic broadcast for developing more eficient replication protocols. An important aspect in these systems consists in how they manage the disconnection of nodes {which degrades their service{ and the connec- tion/reconnection of nodes for maintaining their original support. This task is delegated in replicated systems to recovery protocols. How it works de- pends specially on the failure model adopted. A model commonly used for systems managing large state is the crash-recovery with partial amnesia be- cause it implies short recovery periods. But, assuming it implies arising several problems. Most of them have been already solved in the literature: view management, abort of local transactions started in crashed nodes { when referring to transactional environments{ or for example the reinclu- sion of new nodes to the replicated system. Anyway, there is one problem related to the assumption of this second failure model that has not been completely considered: the amnesia phenomenon. Phenomenon that can lead to inconsistencies if it is not correctly managed. This work presents this inconsistency problem due to the amnesia and formalizes it, de ning the properties that must be ful lled for avoiding it and de ning possible solutions. Besides, it also presents and formalizes an inconsistency problem {due to the amnesia{ which appears under a speci c sequence of events allowed by the majority partition progress condition that will imply to stop the system, proposing the properties for overcoming it and proposing di erent solutions. As a consequence it proposes a new majority partition progress condition. In the sequel there is deDe Juan Marín, R. (2008). Crash recovery with partial amnesia failure model issues [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/3302Palanci

Visual Recollection for Non-Declarative Representations

Recollection is a pattern completion process that enables retrieval of arbitrarily associated information following minimal study. These attributes enable recollection to support retrieval of many kinds of mnemonic representations, from highly associative contextual information to very specific low-level representations. However, recollection is typically studied in the context of declarative memory tasks, in which participants exhibit recollection by explicitly reporting on the recollected information. Is it the case that recollection is limited to declarable representations, or is it a more general process that occurs for any representation? Two experiments and a novel analysis technique are presented to answer this question. The results suggest that recollection is not limited to declarable representations. These results argue against theories of recognition memory that restrict the representational input allowed to mnemonic processes; mnemonic processes in general may act on arbitrary representations

Estimation of post-traumatic amnesia in emergency department attendees presenting with head injury

Objectives: To explore whether a semi–structured post-traumatic amnesia (PTA) assessment interview (PTA-I) provides a practicable but equivalent estimation of PTA in patients attending the Emergency Department (ED) with head injury (HI) compared to the established Westmead PTA Scale Revised (R-WPTAS). Procedure: PTA was assessed using the R-WPTAS (includes a visual memory component) and the PTA-I (includes retrospective and verbal memory components), in patients attending an ED with (n=30) or without (n= 30) HI. Outcome measures were the Post-concussion Syndrome Checklist (PCSC) and the Glasgow Coma Scale (GCS). McNemar’s Tests and Chi-square analyses were used to determine the results. Results: The verbal memory component overestimated PTA in the control group by 24 %. Overall, the PTA-I did not discriminate between HI and control participants. However the retrospective PTA assessment embedded within the PTA-I did, with 100 % accuracy. Conclusions: The use of a verbal memory component to assess PTA in the ED is not supported by the results of this study. A retrospective PTA assessment appears to allow more accurate decision making regarding the admission criteria used in the ED and has advantages over the R-WPTAS: fewer test materials and no repeat assessments required to achieve an estimate of PTA duration

The role of nondeclarative memory in the skill for language::Evidence from syntactic priming in patients with amnesia

Contains fulltext : 172885.pdf (publisher's version ) (Closed access)Syntactic priming, the phenomenon in which participants adopt the linguistic behaviour of their partner, is widely used in psycholinguistics to investigate syntactic operations. Although the phenomenon of syntactic priming is well documented, the memory system that supports the retention of this syntactic information long enough to influence future utterances, is not as widely investigated. We aim to shed light on this issue by assessing patients with Korsakoff's amnesia on an active-passive syntactic priming task and compare their performance to controls matched in age, education, and premorbid intelligence. Patients with Korsakoff's syndrome display deficits in all subdomains of declarative memory, yet their nondeclarative memory remains intact, making them an ideal patient group to determine which memory system supports syntactic priming. In line with the hypothesis that syntactic priming relies on nondeclarative memory, the patient group shows strong priming tendencies (12.6% passive structure repetition). Our healthy control group did not show a priming tendency, presumably due to cognitive interference between declarative and nondeclarative memory. We discuss the results in relation to amnesia, aging, and compensatory mechanisms.9 p

Models of probabilistic category learning in Parkinson's disease: Strategy use and the effects of L-dopa

Probabilistic category learning (PCL) has become an increasingly popular paradigm to study the brain bases of learning and memory. It has been argued that PCL relies on procedural habit learning, which is impaired in Parkinson's disease (PD). However, as PD patients were typically tested under medication, it is possible that levodopa (L-dopa) caused impaired performance in PCL. We present formal models of rule-based strategy switching in PCL, to re-analyse the data from [Jahanshahi, M., Wilkinson, L, Gahir, H., Dharminda, A., & Lagnado, D.A., (2009). Medication impairs probabilistic classification learning in Parkinson's disease. Manuscript submitted for publication] comparing PD patients on and off medication (within subjects) to matched controls. Our analysis shows that PD patients followed a similar strategy switch process as controls when off medication, but not when on medication. On medication, PD patients mainly followed a random guessing strategy, with only few switching to the better Single Cue strategies. PD patients on medication and controls made more use of the optimal Multi-Cue strategy. In addition, while controls and PD patients off medication only switched to strategies which did not decrease performance, strategy switches of PD patients on medication were not always directed as such. Finally, results indicated that PD patients on medication responded according to a probability matching strategy indicative of associative learning, while the behaviour of PD patients off medication and controls was consistent with a rule-based hypothesis testing procedure. (C) 2009 Elsevier Inc. All rights reserved

Amnesia for violent crime among young offenders

Amnesia for the perpetration of violent offences is an important issue in medico-legal proceedings. Previous studies of amnesia have mainly relied on selected groups of unconvicted offenders, which raises the question of how reliable the findings are. The purpose of this study was to examine the prevalence and phenomenological qualities of amnesia in violent offenders. In semi-structured interviews with 105 young offenders convicted of serious violence, 20 (19%) reported partial amnesia for their offence and only one (1%) reported complete amnesia. Amnesia was associated with high alcohol intake, emotional ties to the victim, and cognitive processing during the assault. Complete amnesia for violent crime appears to be less frequent than suggested by previous reports using unconvicted samples. The findings have implications for the clinical assessment of claimed amnesia for violent crime and are potentially of medico-legal significance

Circumventing Concept Erasure Methods For Text-to-Image Generative Models

Text-to-image generative models can produce photo-realistic images for an extremely broad range of concepts, and their usage has proliferated widely among the general public. On the flip side, these models have numerous drawbacks, including their potential to generate images featuring sexually explicit content, mirror artistic styles without permission, or even hallucinate (or deepfake) the likenesses of celebrities. Consequently, various methods have been proposed in order to "erase" sensitive concepts from text-to-image models. In this work, we examine five recently proposed concept erasure methods, and show that targeted concepts are not fully excised from any of these methods. Specifically, we leverage the existence of special learned word embeddings that can retrieve "erased" concepts from the sanitized models with no alterations to their weights. Our results highlight the brittleness of post hoc concept erasure methods, and call into question their use in the algorithmic toolkit for AI safety

Occupational performance and information processing in adults with agitation following traumatic brain injury

Agitation following traumatic brain injury (TBI) is characterised by a heightened state of activity with disorganised information processing that interferes with learning and achieving functional goals. This thesis outlines a series of studies across four research phases, investigating how occupational performance of adults with TBI is affected by agitated behaviour and information processing difficulties. Clinicians report the presence of agitation interferes with engagement in therapy and achievement of rehabilitation goals. Research Phase One used a retrospective chart review of 80 adults with severe TBI to identify a high incidence of agitated behaviour during inpatient TBI rehabilitation. Agitated behaviour was associated with lengthier rehabilitation admission, prolonged duration of post-traumatic amnesia (PTA), and poor cognitive functioning at discharge. The association between agitation and poor cognition persisted for at least two years after discharge, highlighting the significant impact of agitated behaviour on people’s ability to relearn cognitive skills for daily function. These initial research findings directed subsequent research phases, in which an information processing model was adopted to examine application of cognitive strategies during occupational performance. An emerging occupational therapy assessment, The Perceive, Recall, Plan and Perform (PRPP) System of Task Analysis, was selected as the primary method for evaluating how application of cognitive strategies during occupational performance is affected in agitated patients. Clinical utility of this measure was established in a case study of an adult demonstrating severely agitated behaviour during inpatient TBI rehabilitation, followed by examination of instrument reliability and validity with ten experienced occupational therapists and five adults with agitated behaviour following brain injury. The PRPP System of Task Analysis emerged as a valid and reliable method for determining strategy application deficits during occupational performance of adults with agitated behaviour, in acute stages of TBI rehabilitation. Consistent patterns of processing deficits were related to the Perceive and Recall Quadrants of the PRPP System. The assessment tool forms part of a dynamic, interactive assessment and intervention system. The PRPP System of Intervention was evaluated in the final research phase, using an experimental single case design with replication across eight adults. The effectiveness of PRPP Intervention was examined in comparison to conventional occupational therapy in an ABAB design. Efficacy of the PRPP Intervention was demonstrated, with patients applying significantly more information processing strategies to occupational performance tasks during PRPP Intervention than during conventional occupational therapy sessions. Agitated behaviour concurrently reduced over the period of the study. Relationships between information processing and agitated behaviour are hypothesised