Speculative Concurrency Control (SCC) was proposed in [Best92b] as a new concurrency control approach especially suited for real-time database applications. SCC relies on the use of redundancy to ensure that serializable schedules are discovered and adopted as early as possible, thus increasing the likelihood of the timely commitment of transactions with strict timing constraints. Using SCC, several shadow transactions execute on behalf of a given uncommitted transaction so as to protect against the hazards of blockages and restarts, which are characteristics of Pessimistic and Optimistic Concurrency Control algorithms, respectively. We present SCC-nS, a generic algorithm that characterizes a family of SCCbased algorithms. Under the SCC-nS algorithm, shadows executing on behalf of a transaction are either optimistic or speculative. Optimistic shadows execute under an assumed serialization order, which requires them to wait for no other conflicting transactions. They execute unhindered...

Real-time systems (RTS) respond to their environment within specified time constraints. RTS are inherently concurrent and typically manage shared data resources, so they require synchronization to ensure both logical and timing correctness. Much research in managing shared data has been carried out in the context of database systems. It is therefore appropriate to ask, "What principles and techniques can RTS borrow from database concurrency control?" Concurrency control techniques can seldom be transferred from database to the real-time domain without change; the performance considerations are too different. We discuss common features and differences between the two domains, paying special attention to the assumptions and goals of different classes of real-time systems. We then survey an array of correctness criteria and concurrency control techniques with regard to their applicability in soft, firm, and hard-deadline RTS. Real-time scheduling techniques and their interaction with conc...

Many real-time applications havevery tight time constraints which couldn't be met by disk resident databases. For those applications, main memory database where entire database is stored in main memory is the proper choice. It has been shown that coarse-granule locking is better than fine-granule locking for main-memory databases. Coarse-granule locking makes it easy to extract data access patterns correctly from canned transactions of main memory real-time database systems. In this paper, we propose two real-time transaction scheduling algorithms -- CCA-ALF (Cost Conscious Approach with Average Load Factor) and EDF-CR-ALF (Earliest Deadline First-Conditional Restart with ALF) -- which use both static (e.g., deadline) and dynamic information (e.g., system load) for main memory databases by utilizing data access patterns of transactions. We compare the performance of those algorithms with CCA and EDF-HP which do not use system load information at all. Our simulations on main memory databases indicate that: i) CCA-ALF is better than EDF-HP, CCA, and EDF-CR-ALF in terms of miss percent and mean lateness, and ii) CCA-ALF adapts well to the changes in the system load.

AN OBJECT-ORIENTED REAL-TIME DATABASE SYSTEM FOR MULTIPROCESSORS. by Victor Bradley Lortz Chair: Kang G. Shin Complex real-time systems need databases to support concurrent data access and provide well-defined interfaces between software modules. However, conventional database systems and prior real-time database systems do not provide the performance or predictability needed by high-speed, hard real-time applications. To address this need, we have designed, implemented, and evaluated an object-oriented software system called MDARTS (Multiprocessor Database Architecture for Real-Time Systems). MDARTS avoids the client-server overhead of most prior real-time database systems and object-oriented real-time systems by moving transaction execution into application tasks. By eliminating these sources of overhead and focusing on basic data management services for hard real-time systems (data sharing, serializable transactions, and multiprocessor support), our MDARTS prototype provides guarant...

Database systems for real-time applications must satisfy timing constraints associated with transactions. Typically, a timing constraint is expressed in the form of a deadline and is represented as a priority to be used by schedulers. Recently, security has become another important issue in many real-time applications. In many systems, sensitive information is shared by multiple users with different levels of security clearance. As more advanced database systems are being used in applications that need to support timeliness while managing sensitive information, there is an urgent need to develop protocols for concurrency control in transaction management that satisfy both timing and security requirements. In this paper, we propose a new multiversion concurrency control protocol that ensures that both security and real-time requirements are met. The proposed protocol is primarily based on locking.

this paper, we present a priority inversion disk scheduling algorithm where both priority and seek optimization are considered. The algorithm is evaluated through detailed simulation and its performance is compared with traditional scheduling algorithms. We then identified the conditions when our algorithm performs better. The simulation results show that under these conditions, we achieve close to 50% reduction in total system response time. Based on this, an adaptive disk scheduling algorithm was constructed which consistenly outperforms the traditional approach. 1 Introduction

Concurrency control protocols based on multiversions have been used in some commercial transaction processing systems in order to provide the serializable executions of transactions. In the existing protocols, transactions are allowed to read only the most recent version of each data item in order to ensure the correct execution of transactions. However, this feature is not desirable in some advanced database systems which have more requirements such as timing or security constraints besides serializability. In this paper, we propose a new correctness criteria, called F-serializability, for multiversion concurrency control protocols. It is the extended definition of `1-serial' and relaxes the condition so that a protocol provides not only the most recent version but also the previous one to transactions, if necessary. We prove that whenever a multiversion schedule is F-serializable, the schedule is also one-copy serializable. This is the first contribution of our paper. Next, we propos...

By definition, firm real-time transactions are discarded when they miss their deadlines, as there is no value to completing them after they miss their deadlines. Several approaches that exploit the semantics of firm deadlines to improve the performance of firm real-time database systems (RTDBS) have been proposed in the literature. In this paper, we develop a novel policy termed stop/resume deferred restart policy, and a concurrency control algorithm based on the policy (termed the Alternative Version Concurrency Control or AVCC). When conflicts (due to priority) occur, AVCC maintains the immediately restarted version as well as the stop/resume deferred restarted version of a transaction in order to use one of the two to meet the firm deadline. We show that our policy generates serializable schedules and is deadlock free. Our simulation results show that AVCC performs substantially better than the traditionally used 2PL-HP for wide ranges of the system load for firm deadline transactio...

Speculative Concurrency Control (SCC) [Best92a] is a new concurrency control approach especially suited for real-time database applications.

This qualifying dissertation is intended to review the state-of-the-art of Real-Time Database Systems under a uniprocessor and centralized environments. Due to the heterogeneity of the issues, the large amounts of information, and space limitation, we limit our presentation to the most important issues to the overall design, construction, and advancement of Real-Time Database Systems. Such topics are believed to include Transaction Scheduling, Admission Control, Memory Management, and Disk Scheduling. Furthermore, Transaction Scheduling consists of Concurrency Control Protocols, Conflict Resolution Protocols, and Deadlocks. Out of these issues, the most emphasis is placed on Concurrency Control and Conflict Resolution protocols due to their severe role on the overall systems performance. Other important issues that were not included in our presentation include Fault Tolerance and Failure Recovery, Predictability, and most important of all, Minimizing Transaction Support; i.e., Relaxin...