On Fri, Jul 13, 2012 at 4:04 AM, aaron morton <aa...@thelastpickle.com> wrote: > The logic is here > https://github.com/apache/cassandra/blob/cassandra-1.1/src/java/org/apache/cassandra/locator/NetworkTopologyStrategy.java#L78
Thanks Aaron for pointing to the code. > > a. n>r : I am assuming, have 1 replica in each rack. > > You have 1 replica in the first n racks. > > b. n<r : ?? I am assuming, try to equally distribute replicas across > in each racks. > > int(n/r) racks will have the same number of replicas. n % r will have more. Did you mean r%n ( since r>n) ? Shouldn't the logic be : all racks will have at least int(r/n) and r%n will have 1 additional replica ? Sample use case ( r = 8, n = 3 ) n1 : 3 ( 2+1 ) n2: 3 ( 2+1 ) n3: 2 Is the above understanding correct ? -Thanks, Prasenjit > > This is why multi rack replication can be tricky. > > Hope that helps. > > > ----------------- > Aaron Morton > Freelance Developer > @aaronmorton > http://www.thelastpickle.com > > On 12/07/2012, at 8:05 PM, prasenjit mukherjee wrote: > > Thanks. Some follow up questions : > > 1. How do the reads use strategy/snitch information ? I am assuming > the reads can go to any of the replicas. WIll it also use the > snitch/strategy info to find next 'R' replicas 'closest' to > coordinator-node ? > > 2. In a single DC ( with n racks and r replicas ) what algorithm > cassandra uses to write its replicas in following scenarios : > a. n>r : I am assuming, have 1 replica in each rack. > b. n<r : ?? I am assuming, try to equally distribute replicas across > in each racks. > > -Thanks, > Prasenjit > > On Thu, Jul 12, 2012 at 11:24 AM, Tyler Hobbs <ty...@datastax.com> wrote: > > I highly recommend specifying the same rack for all nodes (using > > cassandra-topology.properties) unless you really have a good reason not too > > (and you probably don't). The way that replicas are chosen when multiple > > racks are in play can be fairly confusing and lead to a data imbalance if > > you don't catch it. > > > > On Wed, Jul 11, 2012 at 10:53 PM, prasenjit mukherjee <prasen....@gmail.com> > > wrote: > > > As far as I know there isn't any way to use the rack name in the > > strategy_options for a keyspace. You > > might want to look at the code to dig into that, perhaps. > > > Aha, I was wondering if I could do that as well ( specify rack options ) > > :) > > > Thanks for the pointer, I will dig into the code. > > > -Thanks, > > Prasenjit > > > On Thu, Jul 12, 2012 at 5:33 AM, Richard Lowe <richard.l...@arkivum.com> > > wrote: > > If you then specify the parameters for the keyspace to use these, you > > can control exactly which set of nodes replicas end up on. > > > For example, in cassandra-cli: > > > create keyspace ks1 with placement_strategy = > > 'org.apache.cassandra.locator.NetworkTopologyStrategy' and strategy_options > > = { DC1_realtime: 2, DC1_analytics: 1, DC2_realtime: 1 }; > > > As far as I know there isn't any way to use the rack name in the > > strategy_options for a keyspace. You might want to look at the code to dig > > into that, perhaps. > > > Whichever snitch you use, the nodes are sorted in order of proximity to > > the client node. How this is determined depends on the snitch that's used > > but most (the ones that ship with Cassandra) will use the default ordering > > of same-node < same-rack < same-datacenter < different-datacenter. Each > > snitch has methods to tell Cassandra which rack and DC a node is in, so it > > always knows which node is closest. Used with the Bloom filters this can > > tell us where the nearest replica is. > > > > > -----Original Message----- > > From: prasenjit mukherjee [mailto:prasen....@gmail.com] > > Sent: 11 July 2012 06:33 > > To: user > > Subject: How to come up with a predefined topology > > > Quoting from > > http://www.datastax.com/docs/0.8/cluster_architecture/replication#networktopologystrategy > > : > > > "Asymmetrical replication groupings are also possible depending on your > > use case. For example, you may want to have three replicas per data center > > to serve real-time application requests, and then have a single replica in a > > separate data center designated to running analytics." > > > Have 2 questions : > > 1. Any example how to configure a topology with 3 replicas in one DC ( > > with 2 in 1 rack + 1 in another rack ) and one replica in another DC ? > > The default networktopologystrategy with rackinferringsnitch will only > > give me equal distribution ( 2+2 ) > > > 2. I am assuming the reads can go to any of the replicas. Is there a > > client which will send query to a node ( in cassandra ring ) which is > > closest to the client ? > > > -Thanks, > > Prasenjit > > > > > > > > -- > > Tyler Hobbs > > DataStax > > >
