Abstract: A software fault tolerant computer system includes a primary virtual machine and a secondary virtual machine. The secondary virtual machine is operable to replicate the primary virtual machine by replicating operations performed on the primary virtual machine. The primary and the secondary virtual machines are further operable to test for equivalent operation of the primary and secondary virtual machines at predetermined stages of operation. This provides software fault tolerance wherein both a unit of replication and a component that implements the fault tolerance mechanisms is a virtual machine (VM). Since a VM as used by the invention has full knowledge of the semantics of application-level code, fault tolerance mechanisms can be provided by the VMs without requiring any increase in application complexity. Co-ordination of replicated states and computations is achieved with characteristics of both active and passive replication.

Abstract: A file server system tolerant to hardware and software failures is located over a plurality of hardware nodes. The nodes of the system act as hosts for software components of the system. Several of the software components can be replicated. The replicable software components include the system file store, a checker and a logger. The replicated components have one primary copy and one or more back-up copies. Replica copies of a given replicated component are each located at different nodes. Location and handling of replica copies of a given replicable component is under the control of a replication manager which is a (non-replicable) software component of the system. The replication manager is distributed meaning it can have one of its instances running on each node of the system. These instances inter-communicate to maintain coherence. The failure detector is also distributed, its instances running on each of the nodes, and contributing to an early detection of hardware and software failures.