[norkakn]

Yeah, it's worse than it sounds though. If you write to 2/3 nodes, and the third gets elected, you can roll back majority confirmed writes. I think it did even worse things when near capacity, but those are harder to pin down.

[pmontra]

So the "majority" write concern is not safe. However it seems that the number of nodes that must confirm a write can be configured http://docs.mongodb.org/manual/core/replica-set-write-concer... From what I read there it seems that if you want to get a confirmation from all the nodes and one gets offline you'll be waiting forever (but there is wtimeout). I'm afraid that any db cluster with any technology has similar problems.