QUESTION - Fault Tolerance

(a) Three computers together provide a replicated service. The manufacturers claim that each computer has a mean time between failure of five days; a failure typically takes four hours to fix. What is the availability of the replicated service?

(b) In the gossip architecture, why does a replica manager need to keep both a ‘replica' timestamp and a ‘value' timestamp?

(c) Explain the difference between linearizability and sequential consistency, and why the latter is more practical to implement, in general.

(d) Explain why allowing backups to process read operations leads to sequentially consistent rather than linearizable executions in a passive replication system.