NoSQL databases

CAP theorem

• In distributed databases, the partition tolerance is necessary (DB cannot be down when problems on network or DB servers occur), so the coice is between A and C
• Possible service unavailabilities: total, for writes, for subset of records
• Possible reasons of unavailabilities: dead nodes, subset of cluster is invisible to clients or the rest of cluster
• MySQL Cluster: CP system when single cluster (synchronous replications), AP for two asynchronously replicated clusters
• article about CAP and MySQL Cluster