Middleware-based Database Replication: The Gaps between Theory and Practice

The need for high availability and performance in data management systems has been fueling a long running interest in database replication from both academia and industry. However, academic groups often attack replication problems in isolation, overlooking the need for completeness in their solutions, while commercial teams take a holistic approach that often misses opportunities for fundamental innovation. This has created over time a gap between academic research and industrial practice. This paper aims to characterize the gap along three axes: performance, availability, and administration. We build on our own experience developing and deploying replication systems in commercial and academic settings, as well as on a large body of prior related work. We sift through representative examples from the last decade of open-source, academic, and commercial database replication systems and combine this material with case studies from real systems deployed at Fortune 500 customers. We propose two agendas, one for academic research and one for industrial R&D, which we believe can bridge the gap within 5-10 years. This way, we hope to both motivate and help researchers in making the theory and practice of middleware-based database replication more relevant to each other.Comment: 14 pages. Appears in Proc. ACM SIGMOD International Conference on Management of Data, Vancouver, Canada, June 200

Programming your way out of the past: ISIS and the META Project

The ISIS distributed programming system and the META Project are described. The ISIS programming toolkit is an aid to low-level programming that makes it easy to build fault-tolerant distributed applications that exploit replication and concurrent execution. The META Project is reexamining high-level mechanisms such as the filesystem, shell language, and administration tools in distributed systems

On the cost of database clusters reconfiguration

Database clusters based on share-nothing replication techniques are currently widely accepted as a practical solution to scalability and availability of the data tier. A key issue when planning such systems is the ability to meet service level agreements when load spikes occur or cluster nodes fail. This translates into the ability to provision and deploy additional nodes. Many current research efforts focus on designing autonomic controllers to perform such reconfiguration, tuned to quickly react to system changes and spawn new replicas based on resource usage and performance measurements. In contrast, we are concerned about the inherent impact of deploying an additional node to an online cluster, considering both the time required to finish such an action as well as the impact on resource usage and performance of the cluster as a whole. If noticeable, such impact hinders the practicability of self-management techniques, since it adds an additional dimension that has to be accounted for. Our approach is to systematically benchmark a number of different reconfiguration scenarios to assess the cost of bringing a new replica online. We consider factors such as: workload characteristics, incremental and parallel recovery, flow control and outdatedness of the recovering replica. As a result, we show that research should be refocused from optimizing the capture and transmition of changes to applying them, which in a realistic setting dominates the cost of the recovery operation.Work supported by the Spanish Government under research grant TIN2006-14738-C02-02

Improving DBMS performance through diverse redundancy

Database replication is widely used to improve both fault tolerance and DBMS performance. Non-diverse database replication has a significant limitation - it is effective against crash failures only. Diverse redundancy is an effective mechanism of tolerating a wider range of failures, including many non-crash failures. However it has not been adopted in practice because many see DBMS performance as the main concern. In this paper we show experimental evidence that diverse redundancy (diverse replication) can bring benefits in terms of DBMS performance, too. We report on experimental results with an optimistic architecture built with two diverse DBMSs under a load derived from TPC-C benchmark, which show that a diverse pair performs faster not only than non-diverse pairs but also than the individual copies of the DBMSs used. This result is important because it shows potential for DBMS performance better than anything achievable with the available off-the-shelf servers

Enhancing Fault / Intrusion Tolerance through Design and Configuration Diversity

Fault/intrusion tolerance is usually the only viable way of improving the system dependability and security in the presence of continuously evolving threats. Many of the solutions in the literature concern a specific snapshot in the production or deployment of a fault-tolerant system and no immediate considerations are made about how the system should evolve to deal with novel threats. In this paper we outline and evaluate a set of operating systems’ and applications’ reconfiguration rules which can be used to modify the state of a system replica prior to deployment or in between recoveries, and hence increase the replicas chance of a longer intrusion-free operation

Process membership in asynchronous environments

The development of reliable distributed software is simplified by the ability to assume a fail-stop failure model. The emulation of such a model in an asynchronous distributed environment is discussed. The solution proposed, called Strong-GMP, can be supported through a highly efficient protocol, and was implemented as part of a distributed systems software project at Cornell University. The precise definition of the problem, the protocol, correctness proofs, and an analysis of costs are addressed

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

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

Maintaining consistency in distributed systems

In systems designed as assemblies of independently developed components, concurrent access to data or data structures normally arises within individual programs, and is controlled using mutual exclusion constructs, such as semaphores and monitors. Where data is persistent and/or sets of operation are related to one another, transactions or linearizability may be more appropriate. Systems that incorporate cooperative styles of distributed execution often replicate or distribute data within groups of components. In these cases, group oriented consistency properties must be maintained, and tools based on the virtual synchrony execution model greatly simplify the task confronting an application developer. All three styles of distributed computing are likely to be seen in future systems - often, within the same application. This leads us to propose an integrated approach that permits applications that use virtual synchrony with concurrent objects that respect a linearizability constraint, and vice versa. Transactional subsystems are treated as a special case of linearizability