And we don't send read request to all of the three replicas (R1, R2, R3) if CL=QUOROM; just 2 of them depending on proximity
On Wed, Oct 24, 2012 at 10:20 PM, Hiller, Dean <dean.hil...@nrel.gov> wrote: > The user will meet the required consistency unless you encounter some kind > of bug in cassandra. You will either get the older value or the newer > value. If you read quorum, and maybe a write CL=1 just happened, you may > get the older or new value depending on if the node that received the > write was involved. If you read quorum and your wrote CL=QUOROM, then you > may get the newer value or the older value depending on who gets their > first so to speak. > > In your scenario, if the read repair read from R2 just before the write is > applied, you get the old value. If it read from R2 just after the write > was applied, it gets the new value. BOTH of these met the consistency > constraint. A better example to clear this up may be the following... If > you read a value at CL=QUOROM, and you have a write 20ms later, you get > the old value, right? And it met the consistency level, right? NOW, what > about if the write is 1ms later? What if it the right is .00001ms later? > It still met the consistency level, right? If it is .00001ms before, you > get the new value as it repairs first with the new node. > > It is just when programming, your read may get the newer value or older > value and generally if you write the code in a way that works, this > concept works out great in most cases(in some cases, you need to think a > bit differently and solve it other ways). > > I hope that clears it up > > Later, > Dean > > On 10/24/12 8:02 AM, "shankarpnsn" <shankarp...@gmail.com> wrote: > > >Hiller, Dean wrote > >> in general it is okay to get the older or newer value. If you are > >>reading > >> 2 rows however instead of one, that may change. > > > >This is certainly interesting, as it could mean that the user could see a > >value that never met the required consistency. For instance with 3 > >replicas > ><R1,R2,R3> and a quorum consistency, assume that R1 is initiating a read > >(becomes the coordinator) - notices a conflict with R2 (assume R1 has a > >more > >recent value) and initiates a read repair with its value. Meanwhile R2 and > >R3 have seen two different writes with newer values than what was computed > >by the read repair. If R1 were to respond back to the user with the value > >that was computed at the time of read repair, wouldn't it be a value that > >never met the consistency constraint? I was thinking if this should > >trigger > >another round of repair that tries to reach the consistency constraint > >with > >a newer value or time-out, which is the expected case when you don't meet > >the required consistency. Please let me know if I'm missing something > >here. > > > > > > > >-- > >View this message in context: > > > http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/What-does > >-ReadRepair-exactly-do-tp7583261p7583366.html > >Sent from the cassandra-u...@incubator.apache.org mailing list archive at > >Nabble.com. > >
