In every case, a detailed balance between performance and availability needs should be
done. If replicas are installed within a cluster, performance is guaranteed, since ordered
multicasts within a fast local area network exhibit very reasonable numbers, mainly if just
a few replicas are installed (two to four). If performance of the whole system tolerates placing
replicas at different Internet sites, an important gain in availability would be achieved.
Performance would basically be constrained by ordered multicasts made for write operations.
However, it should be clearly highlighted that most times, having just two or three
Internet-wide replicas enhances availability up to extraordinary levels, and that most times,
ordered multicasts can be efficient enough (Anker, Dolev, Greenman, & Shnayderman,
2003) for write operations.
One of the most important problems to analyze when planning an Internet-wide setting is
network partitioning. If partitions within a cluster can be avoided, in wide area networks they
are real threats. Therefore, if strict consistency needs to be guaranteed, only those partitions
with the majority of replicas should be allowed to make progress. This means, for instance,
that if we install three replicas for the service, and one of them finds itself disconnected
from the rest, it should stop accepting client requests.
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