Bounded Inconsistency for Type-Specific Concurrency Control

Abstract

The traditional correctness criterion of serializability in databases is considered too restrictive especially when databases are used to model advanced applications. In general, two approaches are adopted to address this problem. The first approach considers placing more structure on data objects to exploit type specific properties while keeping serializability as the correctness criterion. The other approach uses explicit semantics of transactions and databases to permit interleaved executions of transactions that are non-serializable. In this paper, we attempt to bridge the gap between the two approaches by using the notion of serializability with bounded inconsistency. Users are free to specify the maximum level of inconsistency that can be allowed in the executions of operations dynamically. In particular, if no inconsistency is allowed in the execution of any operation, the protocol will be reduced to a standard strict two phase locking protocol based on type-specific semantics of data objects. On the other hand, if inconsistency is not bounded, the execution of transactions is unrestricted in the proposed model. The proposed protocols have been implemented and the paper includes a performance comparison with the two-phase locking protocol. The results demonstrate that the associated overhead in the proposed protocol is not overwhelming and the gains in transition throughput can be significant for object based systems. Bounded inconsistency can be applied to many areas which do not require exact values of the data such as for gathering information for statistical purpose, for decision support systems, and for reasoning in expert systems which can tolerate uncertainty in input data.