NoSQL 4 main types:
- Key/value: redis.
- Wide column store: Cassandra.
- Document: MongoDB, CouchDB. [http://blog.scottlogic.com/2014/08/04/mongodb-vs-couchdb.html](MongoDB vs CouchDB).
MongoDB replica sets provide strict consistency, thus CP. Master-Slave Replication ONLY.
CouchDB uses a replication model called Eventual Consistency, where clients can write data to one node of the database without waiting for other nodes to come into agreement, thus AP, see [http://docs.couchdb.org/en/latest/intro/consistency.html](CouchDB.org: consistency). Master-Master Replication.
- Graph: neo4j.