Ensuring consistency in multidatabases by preserving two-level serializability
ACM Transactions on Database Systems (TODS)
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Serializability is the traditionally accepted notion of correctness in most database systems. However, in a multidatabase system (MDBS) environment consisting of pre-existing and autonomous database systems, requiring schedules to be serializable could severely hurt performance. Besides, in a number of instances, the semantics of transactions can be exploited in order to permit certain non-serializable executions. In this paper, we propose a powerful, yet simple mechanism for specifying, in an MDBS environment, the set of non-serializable executions that are unacceptable. The undesirable interleavings among transactions are specified using regular expressions over transaction types. The mechanism facilitates the development of efficient graph-based schemes for ensuring that the concurrent execution of transactions in the MDBS environment meet the specifications. We examine the trade-off between the complexities and the degree of concurrency permitted by the various schemes. Finally, we show that the problem of scheduling operations for execution so as to permit optimal concurrency is NP-complete.