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Nibble 2 for Operating Systems and Architecture

Q: How do clustered systems differ from multiprocessor systems? 
What is required for 2 machines belonging to a cluster to cooperate to provide a highly available service?

A: Clustered systems are typically constructed by combining multiple computers into a single system to perform a computational task distributed across the cluster.
Multiprocessor systems on the other hand could be a single entity comprising of multiple CPUs.
A clustered system is less tightly coupled than a multiprocessor system. 
Clustered systems communicate using messages, while processors in a multiprocessor system communicate using shared memory. 
In order for 2 machines to provide a highly available service, the state on the two machines should be replicated and should be consistently updated. When one of the machines fails, the other could then take over the functionality of the failed machine.

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