Copyright © CC-BY-NC 2019, BJMSR Bangladesh Journal of Multidisciplinary Scientific Research; Vol. 1, No. 2 ISSN 2687-850X E-ISSN 2687-8518 July-September; 2019 Published by Centre for Research on Islamic Banking & Finance and Business 26 Concurrency Control in Database Systems Dardina Tasmere Senior Lecturer Department of Computer Science and Engineering Bangladesh Army University of Engineering & Technology, Natore, Bangladesh E-mail: titly_tonu@yahoo.com Md. Nazmus Salehin B.Sc Student Department of Computer Science and Engineering Bangladesh Army University of Engineering & Technology, Natore, Bangladesh E-mail: mdnazmussalehinnayan@gmail.com Abstract 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. Keywords: Concurrency, Control, Database Systems. 1. Introduction Database systems are important for managing the data efficiently and allowing users to perform multiple tasks on it with the ease. From space station mechanism to credit card transactions, from railway station to telecommunications phonebook at our home – everywhere database is used. A database state which are the values of the database objects representing real-world entity is changed by the execution of a user transaction. In a distributed database system, the concurrency control problem occurs when several users access multiple databases on multiple sites. Correctness of the database is maintained by a scheduler that keeps an eye on the system and control the concurrent accesses. According to (Bhargava, 1983) database integrity and serializability measures the correctness of a database. A database is considered to be correct if a set of constraints (predicates or rules) are satisfied. Serializability ensures that a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order. It assumes that all accesses to the database are done using read and write operations. In this paper, we include some ideas that have been used for designing concurrency control algorithms and evaluated these algorithm’s performance. Finally, we have taken into account some fact for increasing the degree of concurrency. 2. Concurrency Control Approaches and Algorithms Our main point of focus is to process conflicting transactions correctly. Each transaction has a read and write set. Two transactions are said to be in conflictive they are different transactions; they are on the different set (one is read set and another is writing set) and/or they are on the same set where both of them are write sets. This concept is illustrated in Figure-1. Figure 1. Types of conflicts for two transactions.