Concepts such as transaction and a serializable history were introduced when only centralized database systems existed. As distributed systems developed, these concepts were extended to distributed systems. However, an important difference between a distributed system and a centralized system are the relatively large communication delays between the sites and the autonomy of local systems. As a result, the traditional concepts of the transaction and the serializable history, though well suited for traditional centralized systems, may not be suitable in distributed systems. Moreover, as the database concepts have been applied to new areas such as computer-aided design, office information systems and cooperative work, refined and generalized transaction models are needed.
In this research area at our department new transaction models together with protocols for transaction management in heterogeneous distributed database systems have been developed. One approach exploits the information of distributed consistency constraints. This approach has resulted in the introduction of the notions of ``partial serializability'' and ``combined transactions''. The transaction research at our department is partly performed in cooperation with INRIA (France) and Purdue University in the United States. This research has resulted in the notions of ``value dates'' and ``fragmented composite objects''. Researchers of the group are Ph.Lic. Juha Puustjärvi and M.Sc. Henry Tirri.