Hi, There is a JIRA for decoupling the size of the group size used for consensus with level of data redundancy. https://issues.apache.org/jira/browse/CASSANDRA-13442
It's been discussed quite a bit offline and I did a presentation on it at NGCC. Hopefully we will see some movement on it soon. Ariel On Wed, Mar 14, 2018, at 5:40 PM, Carl Mueller wrote: > Currently there is little use for RF4. You're getting the requirements of > QUORUM-3 but only one extra backup. > > I'd like to propose something that would make RF4 a sort of more heavily > backed up RF3. > > A lot of this is probably achievable with strictly driver-level logic, so > perhaps it would belong more there. > > Basically the idea is to have four replicas of the data, but only have to > practically do QUORUM with three nodes. We consider the first three > replicas the "primary replicas". On an ongoing basis for QUORUM reads and > writes, we would rely on only those three replicas to satisfy > two-out-of-three QUORUM. Writes are persisted to the fourth replica in the > normal manner of cassandra, it just doesn't count towards the QUORUM write. > > On reads, with token and node health awareness by the driver, if the > primaries are all healthy, two-of-three QUORUM is calculated from those. > > If however one of the three primaries is down, read QUORUM is a bit > different: > 1) if the first two replies come from the two remaining primaries and > agree, the is returned > 2) if the first two replies are a primary and the "hot spare" and those > agree, that is returned > 3) if the primary and hot spare disagree, wait for the next primary to > return, and then take the agreement (hopefully) that results > > Then once the previous primary comes back online, the read quorum goes back > to preferring that set, with the assuming hinted handoff and repair will > get it back up to snuff. > > There could also be some mechanism examining the hinted handoff status of > the four to determine when to reactivate the primary that was down. > > For mutations, one could prefer a "QUORUM plus" that was a quorum of the > primaries plus the hot spare. > > Of course one could do multiple hot spares, so RF5 could still be treated > as RF3 + hot spares. > > The goal here is more data resiliency but not having to rely on as many > nodes for resiliency. > > Since the data is ring-distributed, the fact there are primary owners of > ranges should still be evenly distributed and no hot nodes should result --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@cassandra.apache.org For additional commands, e-mail: dev-h...@cassandra.apache.org
