06625751 is referenced by 152 patents.
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.