Fault-tolerance for off-the-shelf applications and hardware

The concept of middleware provides a transparent way to augment and change the characteristics of a service provider as seen from a client. Fault tolerant policies are ideal candidates for middleware implementation. We have defined and implemented operating system based middleware support that provides the power and flexibility needed by diverse fault tolerant policies. This mechanism, called the sentry, has been built into the UNIX 4.3 BSD operating system server running on a Mach 3.0 kernel. To demonstrate the effectiveness of the mechanism several policies have been implemented using sentries including checkpointing and journaling. The implementation shows that complex fault tolerant policies can be efficiently and transparently implemented as middleware. Performance overhead of input journaling is less than 5% and application suspension during the checkpoint is typically under 10 seconds in length. A standard hard disk is used to store journal and checkpoint information with dedicated storage requirements of less than 20 MB.>