Learning Apache Cassandra - Second Edition

Fault-tolerant immediate consistency with QUORUM

If we consider a request successful once it's been fulfilled by a single replica, we're very resilient to node failure, but we have to be willing to accept eventual consistency. If we require that every replica responds to a request before that request is complete, we have immediate consistency at the cost of making every replica a single point of failure. But we can achieve both immediate consistency and failure tolerance by defining a successful request as one that's fulfilled by just enough replicas.

This is the QUORUM consistency level, and just enough is precisely defined as the strict majority of replicas. For our replication factor of three, QUORUM requires that two nodes fulfill the ...