> Does anyone know how cassandra chooses the nodes for its other replicant > copies?
This keeps coming up so I added a FAQ entry: http://wiki.apache.org/cassandra/FAQ#replicaplacement I don't quite like the phrasing but I couldn't come up anything that was sufficiently clear and complete right now. > The first node gets the first copy because its token is assigned for that > key. But what about the other copies of the data? > Do the replicant nodes stay the same based on the token range? Or are the > other copies send to any random node based on its load and availability? > I think this is important in order to understand because it affects how to > plan for situations where a significant number of nodes are suddenly > unavailable, such as the loss of a data center. I hope the above is answered by the FAQ. If it's unclear please say so and we can clarify. > If the replicants are copied just based on random availability, then quorum > writes could survive on the remaining nodes. But if the replicant nodes are > somehow pre-determined, those replicants may node be available and writes > will fail. I'm not really following this though. Why would you ever want data to be placed based on "random availability"? If you are writing at QUORUM, a quorum of nodes in the replicate set must have ack:ed the write in order for the read to be considered successful (similar for reads). If a sufficient amount of nodes are up, you're fine. If not, then no - fundamentally that would violate the requirement of quorum. For example, if you're at RF=3, at least two nodes (in the replica set for a given key) must be responding to your request in order for them to succeed at QUORUM. -- / Peter Schuller
