An Execution Model for Distributed Database Transactions
                     and Its Implementation in VPL

          Eva Kühn and Franz Puntigam and Ahmed K. Elmagarmid
    University of Technology Vienna, Institute of Computer Languages
           Argentinierstr. 8, A-1040 Vienna, Austria, Europe
                 {eva,franz}@mips.complang.tuwien.ac.at

We present an execution model for distributed transactions that can be
employed for multidatabase systems. We use the Flex Transaction model
that has been proposed as a highly general and flexible tool for the
specification of distributed transactions and extend it by allowing
nested and possibly recursive transaction specifications. We show how
a given transaction specification and its execution model can be mapped
into a representation in a new concurrent Prolog language, the VPL (Vienna
Parallel Logic) language. The representation in VPL can be considered as an 
executable specification. We show some optimizations concerning this 
mapping and define a significant subclass of Flex Transactions with a 
declarative representation in VPL that can be modeled by AND/OR 
structures. We argue that it is more advantageous to use VPL directly 
for the transaction specification because it provides more flexibility and
more control aspects than the Flex Transaction model.