Towards Predictable Transaction Executions in Real-Time Database Systems

Even though considerable research has been done on concurrency control protocols that take the timing constraints of transactions into account, most of these protocols do not predict { before a transaction begins execution { whether the transaction will meet its deadline. Hence the protocols do not directly tackle the problems introduced by the various sources of unpredictability encountered in typical database systems, including data dependence of transaction execution, data and resource connicts, dynamic paging in virtual memory systems, disk I/O, and transaction aborts with the resulting rollbacks and restarts. On the other hand, the approach described in this paper has the potential to provide predictable transaction executions. This approach exploits the access invariance property that many transactions possess or can be designed to possess. The execution path of a transaction with this property is unlikely to change as a result of data modiications by other concurrent transactions. In this approach, a transaction goes through two phases. In the rst phase, called the prefetch phase, a transaction is run once, bringing the necessary data into main memory that is not in memory already. No writes are performed in this phase and data connicts with other transactions are not considered. The overall computational demands of the transaction are also determined during this phase. At the end of the prefetch phase, the system attempts to guarantee that the transaction will complete by its deadline. This is done by planning the execution of the transaction { taking into account data and resource connicts with the transactions already guaranteed { such that the transaction meets its deadline. If such a plan can be constructed, the transaction's execution phase begins and if access invariance holds then the transaction is committed at the end of this phase. A number of variations on this approach (such as using optimistic concurrency control in the prefetch phase or handling occasional failures of access invariance) are also investigated. The beneets of the approach are also discussed.