Delta-4: an open architecture for dependable systems

The Delta-4 project, carried out within the framework of the European Strategic Programme for Research in Information Technology (ESPRIT), has defined an open, fault-tolerant distributed computing architecture for use in application areas such as computer-integrated manufacturing, process control and office automation. The project has demonstrated its architecture via Unix-based implementations running demonstration applications at Credit Agricole (a credit card authorisation system) and Renault (a process control application within an integrated manufacturing cell). The Delta-4 approach to fault-tolerance is based upon the replication of software components on distinct host computers using a range of different replication strategies. The author presents an overview of the Delta-4 architecture, and of the facilities which it provides for supporting the requirements of real-time systems with respect to throughput and response. The problems of replica divergence are discussed, and a solution based on message selection and pre-emption synchronisation messages is proposed