Semantics-based concurrency control: beyond commutativity

Abstract

The concurrency of transactions executing on atomic data types can be enhanced through the use of semantic information about operations defined on these types. Hitherto, commutativity of operations has been exploited to provide enchanced concurrency while avoiding cascading aborts. We have identified a property known as recoverability which can be used to decrease the delay involved in processing noncommuting operations while still avoiding cascading aborts. When an invoked operation is recoverable with respect to an uncommitted operation, the invoked operation can be executed by forcing a commit dependency between the invoked operation and the uncommitted operation; the transaction invoking the operation will not have to wait for the uncommitted operation to abort or commit. Further, this commit dependency only affects the order in which the operations should commit, if both commit; if either operation aborts, the other can still commit thus avoiding cascading aborts. To ensure the serializability of transactions, we force the recoverability relationship between transactions to be acyclic. Simulation studies, based on the model presented by Agrawal et al. [1], indicate that using recoverability, the turnaround time of transactions can be reduced. Further, our studies show enchancement in concurrency even when resource constraints are taken into consideration. The magnitude of enchancement is dependent on the resource contention; the lower the resource contention, the higher the improvement.