Exploiting Atomic Broadcast in Replicated Databases (Extended Abstract)

In spite of the fact that many applications require replicated databases either for performance or fault-tolerance, replication has remained a research issue until\ud recently. Today, the demand for practical replication schemes has greatly increased and some simple protocols are being implemented in databases (Oracle and Sybase, for instance) or in application development tools (Lotus Notes).\ud These, however, are ad hoc implementations and the issue of replicated data management is still a source of controversy among database practitioners and researchers. On one hand, traditional synchronous protocols are too expensive in terms of message cost and communication latency, and they are susceptible to deadlocks when compared to non-replicated databases. An alternative approach based on asynchronous updates may result in inconsistencies and an ever increasing number of reconciliation rules are needed 5].\ud The distributed systems and computing communities have in general been interested in the broader problem of fault-tolerance in distributed applications. In particular, several systems such as ISIS 4], Amoeba 8], Trans/Total 10], and Transis 2] provide broadcast communication to support fault-tolerant applications. Broadcast communication primitives typically provide reliability, atomicity, and ordering properties at a single operation (or equivalently message) level.\ud Transactions, on the other hand, require reliability, atomicity, and ordering guarantees not for a single operation but for a group of operations. In order to use\ud broadcast for transaction management in replicated data, this mismatch needs to be addressed 12].\ud In this paper, we propose a series of protocols that bridge the gap between database transactions and broadcast communication in the context of replicated databases. Our goal is twofold. From a database perspective, by using a powerful broadcast communication primitive, we hope to simplify the management of replicated databases and obtain some bene ts. In particular, broadcasts can be used to guarantee the consistency of multiple copies of data and at the same time reduce the probability of deadlocks. In fact, in this paper we show that broadcast primitives hold the promise of eliminating single object deadlocks,\ud and either of localizing deadlock resolution or even completely eliminating deadlocks. Our second goal is to provide an application platform for evaluating some\ud of the broadcast primitives. We show that if broadcasts are powerful enough to provide certain "atomicity" guarantees, the database application can execute transactions very e ciently while almost completely eliminating deadlocks. How-\ud ever, if such atomic properties are weakened due to various system assumptions, a database application that requires strict database consistency must perform its own atomic commitment operations in spite of the properties of broadcast primitives.\u

SUPPORTING MULTIPLE ISOLATION LEVELS IN REPLICATED ENVIRONMENTS

La replicación de bases de datos aporta fiabilidad y escalabilidad aunque hacerlo de forma transparente no es una tarea sencilla. Una base de datos replicada es transparente si puede reemplazar a una base de datos centralizada tradicional sin que sea necesario adaptar el resto de componentes del sistema. La transparencia en bases de datos replicadas puede obtenerse siempre que (a) la gestión de la replicación quede totalmente oculta a dichos componentes y (b) se ofrezca la misma funcionalidad que en una base de datos tradicional. Para mejorar el rendimiento general del sistema, los gestores de bases de datos centralizadas actuales permiten ejecutar de forma concurrente transacciones bajo distintos niveles de aislamiento. Por ejemplo, la especificación del benchmark TPC-C permite la ejecución de algunas transacciones con niveles de aislamiento débiles. No obstante, este soporte todavía no está disponible en los protocolos de replicación. En esta tesis mostramos cómo estos protocolos pueden ser extendidos para permitir la ejecución de transacciones con distintos niveles de aislamiento.Bernabe Gisbert, JM. (2014). SUPPORTING MULTIPLE ISOLATION LEVELS IN REPLICATED ENVIRONMENTS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/36535TESI