When the down data center comes back up, the Quorum reads will result in a read-repair, so you will get valid data. Besides that, hinted handoff will take care of getting data replicated to a previously down node.
You're example is a little unrealistic because you could theoretically have a DC with only one node. So CL.ONE would work every time. But if you have more than 1 node, you have to decide if your application can tolerate getting "NULL" for a read if the write hasn't propagated from the responsible node to the replica. disclaimer: I'm a cassandra novice. On Apr 12, 2011, at 5:12 PM, Raj N wrote: > Hi experts, > We are planning to deploy Cassandra in 2 datacenters. Let assume there > are 3 nodes, RF=3, 2 nodes in 1 DC and 1 node in 2nd DC. Under normal > operations, we would read and write at QUORUM. What we want to do though is > if we lose a datacenter which has 2 nodes, DC1 in this case, we want to > downgrade our consistency to ONE. Basically I am saying that whenever there > is a partition, then prefer availability over consistency. In order to do > this we plan to catch UnavailableException and take corrective action. So try > QUORUM under normal circumstances, if unavailable try ONE. My questions - > Do you guys see any flaws with this approach? > What happens when DC1 comes back up and we start reading/writing at QUORUM > again? Will we read stale data in this case? > > Thanks > -Raj
