Doesn't CL=LOCAL_QUORUM solve your problem? On Thu, Mar 24, 2011 at 9:33 AM, <jonathan.co...@gmail.com> wrote:
> Hi Nate - > > That sounds really promising and I'm looking forward to trying that out. > > My original question came up while thinking how to achieve quorum (with > rf=3) with a loss of 1 of 2 data centers. My logic was that if you had 2 > replicas in the same data center where the client originally written to, > then that client is guaranteed to be able to satisfy quorum, even if the > other data center is unreachable. > > But I think there is no way to guarantee where the first write is written > to. That would be based on the token range, which could very well be in any > data center. > > Jon > > > > > On Mar 24, 2011 3:05pm, Nate McCall <n...@datastax.com> wrote: > > We have a load balancing policy which selects the host best on latency > > > > and uses a Phi convict algorithm in a method similar to DynamicSnitch. > > > > Using this policy, you would inherently get the closest replica > > > > whenever possible as that would most likely be the best performing. > > > > > > > > This policy is still in trunk and 0.7.0 tip. We should have a new > > > > release out containing the above in the next few days. > > > > > > > > On Thu, Mar 24, 2011 at 8:46 AM, Jonathan Colby > > > > jonathan.co...@gmail.com> wrote: > > > > > Indeed I found the big flaw in my own logic. Even writing to the > "local" cassandra nodes does not guarantee where the replicas will end up. > The decision where to write the first replica is based on the token ring, > which is spread out on all nodes regardless of datacenter. right ? > > > > > > > > > > On Mar 24, 2011, at 2:02 PM, Jonathan Colby wrote: > > > > > > > > > >> Hi - > > > > >> > > > > >> Our cluster is spread between 2 datacenters. We have a > straight-forward IP assignment so that OldNetworkTopology (rackinferring > snitch) works well. We have cassandra clients written in Hector in each > of those data centers. The Hector clients all have a list of all cassandra > nodes across both data centers. RF=3. > > > > >> > > > > >> Is there an order as to which data center gets the first write? In > other words, would (or can) the Hector client do its first write to the > cassandra nodes in its own data center? > > > > >> > > > > >> It would be ideal it Hector chose the "local" cassandra nodes. That > way, if one data center is unreachable, the Quorum of replicas in cassandra > is still reached (because it was written to the working data center first). > > > > >> > > > > >> Otherwise, if the cassandra writes are really random from the Hector > client point-of-view, a data center outage would result in a read failure > for any data that has 2 replicas in the lost data center. > > > > >> > > > > >> Is anyone doing this? Is there a flaw in my logic? > > > > >> > > > > >> > > > > > > > > > > > > >
