Management of concurrency in a reliable object-oriented computing system

PhD ThesisModern computing systems support concurrency as a means of increasing the performance of the system. However, the potential for increased performance is not without its problems. For example, lost updates and inconsistent retrieval are but two of the possible consequences of unconstrained concurrency. Many concurrency control techniques have been designed to combat these problems; this thesis considers the applicability of some of these techniques in the context of a reliable object-oriented system supporting atomic actions. The object-oriented programming paradigm is one approach to handling the inherent complexity of modern computer programs. By modeling entities from the real world as objects which have well-defined interfaces, the interactions in the system can be carefully controlled. By structuring sequences of such interactions as atomic actions, then the consistency of the system is assured. Objects are encapsulated entities such that their internal representation is not externally visible. This thesis postulates that this encapsulation should also include the capability for an object to be responsible for its own concurrency control. Given this latter assumption, this thesis explores the means by which the property of type-inheritance possessed by object-oriented languages can be exploited to allow programmers to explicitly control the level of concurrency an object supports. In particular, a object-oriented concurrency controller based upon the technique of two-phase locking is described and implemented using type-inheritance. The thesis also shows how this inheritance-based approach is highly flexible such that the basic concurrency control capabilities can be adopted unchanged or overridden with more type-specific concurrency control if requiredUK Science and Engineering Research Council, Serc/Alve

Serializable Isolation for Snapshot Databases

Many popular database management systems implement a multiversion concurrency control algorithm called snapshot isolation rather than providing full serializability based on locking. There are well-known anomalies permitted by snapshot isolation that can lead to violations of data consistency by interleaving transactions that would maintain consistency if run serially. Until now, the only way to prevent these anomalies was to modify the applications by introducing explicit locking or artificial update conflicts, following careful analysis of conflicts between all pairs of transactions. This thesis describes a modification to the concurrency control algorithm of a database management system that automatically detects and prevents snapshot isolation anomalies at runtime for arbitrary applications, thus providing serializable isolation. The new algorithm preserves the properties that make snapshot isolation attractive, including that readers do not block writers and vice versa. An implementation of the algorithm in a relational database management system is described, along with a benchmark and performance study, showing that the throughput approaches that of snapshot isolation in most cases

Transaction processing in real-time database systems

Scheduling transactions in a real-time database requires an integrated approach in which the schedule does not only guarantee execution before the deadline, but also maintains data consistency. The problem has been studied under a common framework which considers both concurrency control issues and the real-time constraints in centralized and distributed transaction processing. A real-time transaction processing model has been defined for a centralized system. The proposed protocols use a unified approach to maximize concurrency while meeting real-time constraints at the same time. In order to test the behavior of the model and the proposed protocols, a real-time transaction processing testbed has been developed using discrete event simulation techniques. The results indicate that different protocols work better under different load scenarios and that the overall performance can be significantly enhanced by modifying the underlying system configuration. Among other system and transaction parameters, the effect of data partitioning, buffer management, preemption, disk contention, locking mode and multiprocessing has been studied;For the distributed environment, new concepts of real-time nested transactions and priority propagation have been proposed. Real-time nested transactions incorporate the deadline requirements in the hierarchical structure of nested transactions. Priority propagation addresses the issues related to transaction aborts in real-time nested transaction processing. The notion of priority ceiling has been used to avoid the priority inversion problem. The proposed protocols exhibit freedom from deadlock and have tightly bounded waiting period. Both of these properties make them very suitable for distributed real-time transaction processing environment