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A multidatabase system (MDBS), consists of a number of sites, each of which runs a distinct commercial database management system (DBMS). The goal of an MDBS is to integrate the various DBMSs to allow applications to access data residing in several DBMSs, without requiring modifications to the individual DBMSs. This implies that each site is allowed a high degree of local autonomy. This autonomy requirement makes the task of ensuring both the atomicity and isolation properties of transactions, in the presence of failures, difficult. In this paper, we develop a semantically rich transaction model for MDBS applications. We relax the atomicity requirement on transactions and propose a new suitable correctness criterion. We also develop net «commit and concurrency control protocols that ensure correctness and do not violate the local autonomy of the various sites.