Value-cognizant Admission Control Strategies for Real-Time DBMS

We propose and evaluate an admission control paradigm for RTDBS, in which a transaction is submitted to the system as a pair of processes: a primary task, and a recovery block. The execution requirements of the primary task are not known a priori, whereas those of the recovery block are known a priori. Upon the submission of a transaction, an Admission Control Mechanism is employed to decide whether to admit or reject that transaction. Once admitted, a transaction is guaranteed to nish executing before its deadline. A transaction is considered to have nished executing if exactly one of two things occur: Either its primary task is completed (successful commitment), or its recovery block is completed (safe termination). Committed transactions bring a proot to the system, whereas a terminated transaction brings no proot. The goal of the admission control and scheduling protocols (e.g., concurrency control, I/O scheduling, memory management) employed in the system is to maximize system proot. We describe a number of admission control strategies and contrast (through simulations) their relative performance. 1 Introduction The main challenge involved in scheduling transactions in a Real-Time DataBase Management System (RTDBMS) is that the resources needed to execute a transaction are not known a priori. For example, the set of objects to be read (written) by a transaction may be dependent on user input (e.g., in a stock market application) or dependent on sensory inputs (e.g., in a process control application). Therefore , the a priori reservation of resources (e.g., read/write locks on data objects) to guarantee a particular Worst Case Execution Time (WCET) becomes impossible|and the non-deterministic delays associated with the on-they acquisition of such resources pose the real challenge of integrating scheduling and concurrency control techniques. Current real-time concurrency control mechanisms resolve the above challenge by relaxing the deadline semantics (thus suggesting best-eeort mechanisms for concurrency control in the presence of soft and rm, but not hard deadlines), or by restricting the set of acceptable transactions to a nite set of transactions with execution requirements that