Cassandra isn't just multi-master, it's symmetric and masterless (peer-to-peer). It also isn't the only system with that property: other Dynamo "implementations

Cassandra isn't just multi-master, it's symmetric and masterless (peer-to-peer). It also isn't the only system with that property: other Dynamo "implementations" (Dynomite, Riak, Voldemort) are the same way. There are also multi-master systems that employ 3-PC like fault tolerant distributed commit protocols (e.g., Paxos with Leases, ZAB) and leader elections, allowing transactions to go through even if one of the coordinators fails.