ADMISSION CONTROL AND SCHEDULING STRATEGIES FOR REAL-TIME DATABASE SYSTEMS
--------------------------------------------------------------------------

The proliferation of Real-Time DataBase (RTDB) systems as repositories of
information used by time-critical applications has been tremendous during
the last decade.  Many such systems continue to admit transactions to the
point of overload which results in degraded performance.  By the
appropriate use of admission control and overload management techniques,
the performance of such systems may be enhanced.  Moreover, for some
safety-critical applications (such as command and control systems), safety
constraints require the early notification of transaction failure.  Failure
to do so results in wasting precious system resources, which could have
been used by other admitted transactions, not to mention wasting precious
time which could have been used to attempt alternative options for the
failing transaction.

In this dissertation, we propose 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
notification of failure to submitted transactions that are deemed not
valuable or incapable of completing on time.  The transaction model
consists of two components: a primary task and a compensating task.
Transactions which are admitted to the system are guaranteed, by the
deadline of the transaction, one of two outcomes: either the primary task
will successfully commit or the compensating task will safely terminate.
Our admission control mechanisms permit transactions to fail at the
earliest possible point in time (i.e. at submission time) rather than at a
later time.  Also as a system becomes overloaded, our admission control
techniques allow for the utilization of system resources in the most
profitable way.

The contributions of this thesis are: (1) the novel ACCORD framework for
RTDB systems including a system architecture and a transaction model, (2)
value-cognizant admission control mechanisms based upon workload, (3)
value-cognizant admission control mechanisms based upon the level of
concurrency conflicts, and (4) new scheduling algorithms suitable for
ACCORD.  These contributions are validated by an extensive experimental
evaluation of ACCORD, through simulation, which confirms the performance
benefits of admission control, overload management, and early failure
notification.