Coordinated backward between client processes and data servers

The authors discuss backward error recovery for complex software systems, where different subsystems may belong to essentially different application areas. Such heterogeneous subsystems are naturally built according to different design ‘models’, namely the ‘object-action’ model (where the long-term state of the computation is encapsulated in data objects, and active processes invoke operations on these objects), and the ‘process-conversation’ model (where the state is contained in the processes, communicating via messages). To allow backward error recovery in these two ‘models’ of computation, two different schemes are most appropriate: atomic transactions for the object-action model, and conversations for the process-conversation model. Assuming that each of these two kinds of subsystem already has functioning mechanisms for backward error recovery, the authors describe the additional provisions needed for co-ordination between these heterogeneous subsystems. The solution involves altering the virtual machine on which the programs run, and programming conventions which seem rather natural and can be automatically enforced. The approach is demonstrated by a simple example.