Replication

One way of making a system fault-tollerant is to replicate its state. This could also mean that we could increase performance by accession the replicas concrrently.  The problem is of course how to achieve a transparent interface, we should preserve the view of one image (or not?). What are the formal requirements that we must place on the service.

What if we want to have a highly available service or increase performance, could we improve the sytem at the expence of consistency?