Hi, If you want the full immediate consistency of a traditional relational > database, then go with CL=ALL, otherwise, take your pick from the many > degrees of immediacy that Cassandra offers:
My understanding is using RF 3 and LOCAL_QUORUM for both reads and writes will provide a strong consistency and a high availability. One node can go down and also without lowering the consistency. Or RF = 5, Quorum = 3, allowing 2 nodes node if you need more availability / redundancy (Never saw that in use so far). I prefer this to ALL, that will produce the cluster to be partially unavailable any time a node is down (crash or just a node restart). I also never saw anyone using CL = ALL so far, for this exact reason I believe. Writing to 2 nodes out of 3 and reading from 2 nodes out of 3 --> using RF = 3 / R&W CL = LOCAL_QUORUM, will give a strong and (almost) immediate consistency. Remember there is no lock though, so immediate = as soon as write succeed on enough nodes, 2 in our case). If you need a strong consistency and are reading the same data in the 2 DCs, consider using EACH_QUORUM as Jack mentioned earlier. Also, when using 2 datacenter, make sure to pin clients to one DC. Else you might perform LOCAL_QUORUM operations on any of the 2 DCs, which would remove the strong consistency. More information about this last point on step 4 there: https://docs.datastax.com/en/cassandra/2.1/cassandra/operations/ops_add_dc_to_cluster_t.html About consistency https://wiki.apache.org/cassandra/ArchitectureOverview#Consistency. C*heers, ----------------------- Alain Rodriguez - al...@thelastpickle.com France The Last Pickle - Apache Cassandra Consulting http://www.thelastpickle.com 2016-03-28 5:44 GMT+02:00 Jack Krupansky <jack.krupan...@gmail.com>: > The third choice is EACH_QUORUM which assures QUORUM in each data center > (all data centers.) > > There is no "immediate consistency" per se in Cassandra. Cassandra offers > "eventual consistency" and "tunable consistency" or the degree of immediate > consistency, which is the CL that you specify - you specify the degree of > immediate consistency that you require. If you want the full immediate > consistency of a traditional relational database, then go with CL=ALL, > otherwise, take your pick from the many degrees of immediacy that Cassandra > offers: > > http://docs.datastax.com/en/cassandra/3.x/cassandra/dml/dmlConfigConsistency.html > . > > In short, Cassandra does indeed guarantee the degree of immediate > consistency that you specify (and presumably want.) > > > -- Jack Krupansky > > On Sun, Mar 27, 2016 at 6:36 PM, Harikrishnan A <hari...@yahoo.com> wrote: > >> Hello, >> >> I have a question regarding consistency Level settings in a multi Data >> Center Environment. What is the preferred CL settings in this scenario for >> an immediate consistency , QUORUM or LOCAL_QUORUM ? >> >> If the replication Factor is set to 3 each ( 2 Data Centers) , the >> QUORUM ( writes/read) waits for 2 consistent response from nodes across the >> cluster ( out of total 6 replications) , where as LOCAL_QUORUM expects 2 >> consistent response from 3 replicas within that LOCAL data Center. If my >> above understanding is correct, does that mean QUORUM doesn't guarantee >> Immediate consistency ( in this multi Data Center Scenario) where as >> LOCAL_QUORUM does guarantee the immediate consistency. >> >> >> Thanks & Regards, >> Hari >> >> >> >
