Concurrency Control for Perceivedly Instantaneous Transactions in Valid-Time Databases

Although temporal databases have received considerable attention as a topic for research, little work in the area has paid attention to the concurrency control mechanisms that might be employed in temporal databases. This paper describes how the notion of the current time --- also called `now' --- in valid-time databases can cause standard serialisation theory to give what are at least unintuitive results, if not actually incorrect results. The paper then describes two modifications to standard serialisation theory which correct the behaviour to give what we term perceivably instantaneous transactions; transactions where serialising T 1 and T 2 as [T 1 ; T 2 ] always implies that the current time seen by T 1 is less than or equal to the current time seen by T 2 . 1 Introduction Query languages for valid-time temporal database normally contain a notion of "currenttime " [TCG + 93, Sno95], usually represented as the value of a special variable now. While it is agreed that the value of..

A study of two transaction-processing architectures for distributed real-time database systems

Cataloged from PDF version of article.A real-time data base system (RTDBS) is designed to provide timely response to the transactions of data-intensive applications. Processing a transaction in a distributed RTDBS environment presents the design choice of how to provide access to remote data referenced by the transaction. Satisfaction of the timing constraints of transactions should be the primary factor to be considered in scheduling accesses to remote data. In this article, we describe and analyze two different alternative approaches to this fundamental design decision. With the first alternative, transaction operations are executed at the sites where required data pages reside. The other alternative is based on transmitting data pages wherever they are needed. Although the latter approach is characterized by large message volumes carrying data pages, it is shown in our experiments to perform better than the other approach under most of the work loads and system configurations tested. The performance metric used in the evaluations is the fraction of transactions that satisfy their timing constraints. © 1995

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