We propose and evaluate admission control mechanisms for ACCORD, an Admission Control and Capacity Overload management Real-time Database framework|an architecture and a transaction model|for hard deadline RTDB systems. The system architecture consists of admission control and scheduling components which provide early noti cation of failure to submitted transactions that are deemed not valuable or incapable of completing on time. In particular, our Concurrency Admission Control Manager (CACM) ensures that transactions which are admitted do not overburden the system by requiring a level of concurrency that is not sustainable. The transaction model consists of two components: a primary task and a compensating task. The execution requirements of the primary task are not known a priori, whereas those of the compensating task are known a priori. Upon the submission of a transaction, the Admission Control Mechanisms are 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 compensating task is completed (safe termination). Committed transactions bring a pro t to the system, whereas a terminated transaction brings no pro t. 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 pro t. In that respect, we describe a number of concurrency admission control strategies and contrast (through simulations) their relative performance. This work has been partially supported by NSF (grant CCR-9706685). This work was conducted as part of the author's Ph.D. thesis at Boston University.
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