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From Anthony John <chirayit...@gmail.com>
Subject Re: Understand eventually consistent
Date Fri, 18 Feb 2011 13:04:22 GMT
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 <stuhood@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 <aaron@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 <mohitanchlia@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-user@incubator.apache.org mailing list archive at
>> Nabble.com.
>>
>>
>

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