At Quorum - if 2 of 3 nodes are down, a read should not be returned, right ?
But yes - if single node READs are opted for, it will go through. The original question was - "Why is Cassandra called eventually consistent data store?" Because at write time, there is not a guarantee that all replicas are consistent. But they eventually will be! At Quorum write and Read - you will not get inconsistent results and your read will force consistency, if such a state has not yet been arrived at for the particular piece of data. But you have the option of or writing and reading at a lower standard, which could result in inconsistencies. HTH, -JA On Fri, Feb 18, 2011 at 12:00 AM, Stu Hood <stuh...@gmail.com> wrote: > But, the reason that it isn't safe to say that we are a strongly consistent > store is that if 2 of your 3 replicas were to die and come back with no > data, QUORUM might return the wrong result. > > A requirement of a strongly consistent store is that replicas cannot begin > answering queries until they are consistent: this is not a requirement in > Cassandra, althought arguably should be an option at some point in the > distant future. > > > On Thu, Feb 17, 2011 at 5:26 PM, Aaron Morton <aa...@thelastpickle.com>wrote: > >> For background... >> >> http://wiki.apache.org/cassandra/ArchitectureOverview >> (There is a section on consistency in there) >> >> For deep background... >> http://www.allthingsdistributed.com/2008/12/eventually_consistent.html >> >> http://s3.amazonaws.com/AllThingsDistributed/sosp/amazon-dynamo-sosp2007.pdf >> >> In short, yes (for all your questions) if you read and write at Quorum you >> have consistency behavior for your operations. Even though some nodes >> may have an inconsistent view of the data, e.g. one node is partitioned by >> a broken network or is overloaded and does not respond. >> >> Aaron >> >> On 18 Feb, 2011,at 02:11 PM, mcasandra <mohitanch...@gmail.com> wrote: >> >> >> Why is Cassandra called eventually consistent data store? Wouldn't it be >> consistent if QUORAM is used? >> >> Another question is when I specify replication factor of 3 and write with >> factor of 2 and read with factor of 2 then what happens? >> >> 1. When write occurs cassandra will return to the client only when the >> writes go to commit log on 2 nodes successfully? >> >> 2. When read happens cassandra will return only when it is able to read >> from >> 2 nodes and determine that it has consistent copy? >> -- >> View this message in context: >> http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/Understand-eventually-consistent-tp6038330p6038330.html >> Sent from the cassandra-u...@incubator.apache.org mailing list archive at >> Nabble.com. >> >> >
