Seven Databases in Seven Weeks, 2nd Edition

CAP in the Wild

The databases in this book largely occupy one corner or another of the CAP trade-off triangle. Redis, PostgreSQL, and Neo4J are consistent and available (CA); they don’t distribute data and so partitioning is not an issue (though arguably, CAP doesn’t make much sense in non-distributed systems). MongoDB and HBase are generally consistent and partition tolerant (CP). In the event of a network partition, they can become unable to respond to certain types of queries (for example, in a Mongo replica set you flag slaveok to false for reads). In practice, hardware failure is handled gracefully—other still-networked nodes can cover for the downed server—but strictly speaking, in the CAP theorem sense, they are unavailable. Finally, ...