Exploiting Transact ion Semantics in Mult idat abase Systems

Serializability is the traditionally accepted notion of correctness in most database systems. However, in a multidatabase system (MDBS) environment, where a number of pre-existing and autonomous database systems are integrated, requiring serializability could adversely affect the performance of the system. To enhance performance, one of the options is to relax the serializability requirement, and permit certain nonserializable executions. In this paper, we propose a powerful, yet simple mechanism, for specifying the set of non-serializable executions that are unacceptable in an MDBS environment. The undesirable interleavings among transactions are specified using regular expressions over transaction types. The mechanism facilitates the development of eficient graph-based schemes for ensuring that the concurrent execution of transactions meet the specifications. We analyze the complexities of the developed schemes and show that they are easily implementable in an MDBS environment.