Continuous availability; Tolerance to network partitions
Data replication is a technique to provide high availability and scalability by introducing redundancy. The data remains available as long as some replicas are accessible and, as the load can be distributed across replicas, adding more replicas potentially allows for increased throughput. Challenges arise when the data has to be updated as the replicas must be kept consistent. The most intuitive approach is to always execute all write operations at all replicas. Then, all replicas always have the same state and a read operation can read any replica. The main problem with this Read-One-Write-All (ROWA) approach is that as soon as one replica is no more available write operations cannot be performed anymore. A further problem is that executing all updates always on all replicas makes write operations very expensive.
Quorum systems address both these issues. They allow write operations to succeed if they execute...
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