So by default when the SDK returns 'true' for a write, yes it's just been written to RAM on the first (master) node. However you can if you desire 'observe' the write, which will return true when the write has hit disk and/or the RAM of the replica nodes (depending on what options you specify).
Ultimately it's a performance tradeoff - if you want sub microsecond writes for your workload then something has to give - certainly for many workloads (session stores, ad tracking, messaging queues) the tradeoff that you may loose the last few writes if you loose a node and it's replicas simultaneously is an acceptable one.
Ultimately it's a performance tradeoff - if you want sub microsecond writes for your workload then something has to give - certainly for many workloads (session stores, ad tracking, messaging queues) the tradeoff that you may loose the last few writes if you loose a node and it's replicas simultaneously is an acceptable one.
(Full disclosure: I work for Couchbase).