Abstract: A fault tolerant computer system employing primary tasks and corresponding backup tasks. The system operates to provide fault tolerant operation even where uncontrolled external events may occur whose time of occurrence may affect task performance. For this purpose, external event data is stored for each external event occurring during performance of a primary task which indicates the event type and the relationship between the occurrence of the external event and the occurrence of a predetermined primary task event, such as a memory access operation. This external event data is sent to each respective backup task along with messages transmitted to the respective primary task. In the event a primary task fails, the backup task will replay the failed primary task by processing these transmitted messages while using the transmitted external event data to redeliver each external signal to the backup task at an appropriate time which will assure that the backup task properly recovers the primary task.

Abstract: A fault tolerant computer system which operates to automatically provide fault tolerant operation in a manner which reduces the frequency of checkpointing, while requiring neither simultaneity nor atomicity with respect to information transmitted to backups. These advantage are accomplished by providing operation such that message transmissions from primary tasks running on a data processor to external backups may be delayed until an action is performed which will persist after failure of the data processor. Although such operation may result in the backups hot having received all of the messages transmitted to their respective primary tasks when failure occurs, recovery is nevertheless obtained using the backups, since transmission to backups is controlled so that all backups agree on the state of the data processor at the time of failure, which need not be the actual state of the data processor at the time of failure.