Concurrency Control In Database Systems Research Articles

Concurrency Control in Database Systems

Concurrency control mechanisms including the wait, time-stamp and rollback mechanisms have been briefly discussed. The concepts of validation in optimistic approach are summarized in a detailed view. Various algorithms have been discussed regarding the degree of concurrency and classes of serializability. Practical questions relating arrival rate of transactions have been presented. Performance evaluation of concurrency control algorithms including degree of concurrency and system behavior have been briefly conceptualized. At last, ideas like multidimensional timestamps, relaxation of two-phase locking, system defined prewrites, flexible transactions and adaptability for increasing concurrency have been summarized.

Concurrency Control in Database Systems

Concurrency Control in Database Systems

Concurrency control in database systems

Ideas that are used in the design, development, and performance of concurrency control mechanisms have been summarized. The locking, time-stamp, optimistic-based mechanisms are included. The ideas of validation in the optimistic approach are presented in some detail. The degree of concurrency and classes of serializability for various algorithms have been presented. Questions that relate arrival rate of transactions with degree of concurrency and performance have been briefly presented. Finally, several useful ideas for increasing concurrency have been summarized. They include flexible transactions, adaptability, prewrites, multidimensional time stamps, and relaxation of two-phase locking.

On relaxing serializability by constraining transaction readsets

Although, concurrency control in database systems is primarily based on serializability, many recent applications have rendered traditional serializability-based criteria inefficient or inappropriate. However, non-serializable executions may violate database consistency. In this paper, we propose a new approach to ensuring the correctness of non-serializable executions. The approach is based on relating transaction views of the database to the integrity constraints of the system. The underlying concepts of view closure and view consistency are defined. Then, drawing upon this approach, we develop a new correctness criterion for multidatabases, which are confederations of pre-existing heterogeneous and autonomous distributed database systems. This criterion, called view-based two-level serializability, relaxes serializability while respecting the autonomy of local database systems and preserving multidatabase consistency. We investigate the application of the criterion to various practical multidatabase scenarios and discuss implementation issues.

Performance modeling of distributed timestamp ordering: Perfect and imperfect clocks

This work presents a model of a distributed database system which provides the framework to study the performance of timestamp ordering concurrency control. Locking and timestamping are two popular approaches to concurrency control in database systems. Timestamp-based algorithms have been proposed to protect distributed databases from inconsistencies during concurrent access. In these algorithms, transactions may reach a particular site in different order than their timestamps, due to unexpected network delays. This causes conflicts which the distributed concurrency control mechanism has to cope with. We exhibit an analytical solution, which has been tested with extensive simulation. The accuracy seems to be very high. We assume perfect and also imperfect clocks for synchronization and quantify the way in which local clock inaccuracies affect the phenomenon of transaction conflicts. In particular, we derive a lot of interesting performance measures such as probability of abort, mean waiting time, throughput, mean queue length and others.

An adaptive strategy integrating locking with optimistic concurrency control

This paper presents an adaptive strategy called K-locking algorithm for concurrency control in database system. The algorithm integrates an optimistic approach with the K-lock mechanism to control the degree of transaction interference. It is shown that the K-locking strategy is adaptive to the changes in transaction parameters and outperforms both an optimistic approach and a pessimistic approach.

Deadlock prevention and avoidance methods using timestamps for locking control in distributed database systems

AbstractLocking is used widely for concurrency control in database systems, and locking methods are categorized as static locking or dynamic locking. For distributed database systems, this paper proposes a new deadlock prevention method for static locking and a new deadlock avoidance method for dynamic locking. The common advantages of these methods are as follows: (1) system performance is improved by using time stamps; (2) since transactions are executed without explicitly using time‐stamp ordering, after a transaction lock is permitted, even when another transaction with a smaller time stamp arrives, the rollback of the transaction is unnecessary; and (3) since time stamping is used only for deadlock resolution, the proposed methods can easily be introduced to existing distributed database systems that use locking control.

Performance analysis of the basic timestamp ordering algorithm via Markov modeling

Locking and timestamping are two popular approaches to concurrency control in database systems. Although more than a dozen analytic performance studies of locking techniques have recently appeared in the literature, analytic performance study of times tamp-based concurrency control algorithms largely remains and unexplored area. Since a large class of concurrency control algorithms used timestamps, there is a strong need of analytic performance study of these algorithms. In this paper, we analyze the performance of the basic timestamp ordering concurrency algorithm for database systems. Due to the complex nature of concurrency control algorithms, it is very difficult to accurately analyze their perfomance mathematically. We show that even after making some assumptions, an exact performance model of the basic timestamp ordering concurrency control algorithm is so complicated that it is pratically impossible to find its closed-form solution. We circumvent this problem in the following way: We analyze a single transaction in isolation rather than analyzing the whole system. (This reduces the complexity of the analysis substantially.) We reflect the presence of other trasactions on the transaction under consideration by the probability that a conflicting access to a data object has been made. We assume that a transaction sees the average state of the system and all the transactions exhibit the average steady-state behavior, which permits us to work with averages rather than with probability distributions. Therefore, the technique used in the analysis is approximate. We illustrate the technique with numerical examples. We plot a set of performance measures for several values of the model parameters. The results of analysis are validated against a simulation study. We also extend the analysis to open database systems where transactions arrive from an external source.

Performance issues in database systems

The performance of transaction processing systems is affected by contention for hardware as well as software resources (data objects). Software contention becomes prominent in database systems because concurrency control mechanism, which is used to insure integrity of the database, restrict concurrent and conflicting access to data objects of a database. Locking is the most popular scheme to achieve concurrency control in database systems. Performance of a database system is greatly determined by its underlying concurrency control algorithm because it determines the performance degradation in presence of software contentions. In this paper, we will examine the issues in the performance analysis of concurrency control algorithms and their affect on the overall database system performance. With many alternatives now available to design a database system, there is a tremendous need to assess the suitability of a design to a particular environment and need. In this paper, we will discuss difficulties in modeling concurrency control algorithms and describe how these difficulties have been overcome.

Conflict management in a source version management system

To support concurrent development a source version management system must support multiple programmers concurrently accessing shared source code, while maintaining the integrity of the source database. This problem is similar to the problem of concurrency control in database systems, but has a few significant differences. This leads us to view the possible solutions as variations on database transaction management and concurrency control. In particular we consider the possibility of letting transactions interact and managing the conflicts which result. 1

Multidimensional Timestamp Protocols for Concurrency Control

We propose multidimensional timestamp protocols for concurrency control in database systems where each transaction is assigned a timestamp vector containing multiple elements. The timestamp vectors for two transactions can be equal if timestamp elements are assigned the same values. The serializability order among the transactions is determined by a topological sort of the corresponding timestamp vectors. The timestamp in our protocols is assigned dynamically and is not just based on the starting/finishing time as in conservative and optimistic timestamp methods. The concurrency control can be enforced based on more precise dependency information derived dynamically from the operations of the transactions. Several classes of logs have been identified based on the degree of concurrency or the number of logs accepted by a concurrency controller. The class recognized by our protocols is within D-serializable (DSR), and is different from all previously known classes such as two phase locking (2PL), strictly serializable (SSR), timestamp ordering (TO), which have been defined in literature. The protocols have been analyzed to study the complexity of recognition of logs. We briefly discuss the implementation of the concurrency control algorithm for the new class, and give a timestamp vector processing mechanism. The extension of the protocols for nested transaction and distributed database models has also been included.

Special Issue: Database Management

Nabil R. Adam is an Associate Professor at the Graduate School of Management, Rutgers University. He received his M.S., M.Phil, and Ph.D. in Industrial Engineering from Columbia University. His papers have appeared in such journals as Management Science and HE Transactions. He edited a special issue on computer simulation of the Communications of the ACM and co-edited a special issue on simulation of Operations Research. His current research interests include physical design and concurrency control in database systems, management information systems, computer simulation, and scheduling. He has served as a consultant in the area of database management to several major organizations.

On optimistic methods for concurrency control

Most current approaches to concurrency control in database systems rely on locking of data objects as a control mechanism. In this paper, two families of nonlocking concurrency controls are presented. The methods used are “optimistic” in the sense that they rely mainly on transaction backup as a control mechanism, “hoping” that conflicts between transactions will not occur. Applications for which these methods should be more efficient than locking are discussed.