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Date: Thu, 18 Oct 2012 15:42:57 -0700
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Subject: Re: hadoop consistency level
From: Andrey Ilinykh <ailinykh@gmail.com>
To: user@cassandra.apache.org
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On Thu, Oct 18, 2012 at 2:31 PM, Jeremy Hanna
<jeremy.hanna1234@gmail.com> wrote:
>
> On Oct 18, 2012, at 3:52 PM, Andrey Ilinykh <ailinykh@gmail.com> wrote:
>
>> On Thu, Oct 18, 2012 at 1:34 PM, Michael Kjellman
>> <mkjellman@barracuda.com> wrote:
>>> Not sure I understand your question (if there is one..)
>>>
>>> You are more than welcome to do CL ONE and assuming you have hadoop nod=
es
>>> in the right places on your ring things could work out very nicely. If =
you
>>> need to guarantee that you have all the data in your job then you'll ne=
ed
>>> to use QUORUM.
>>>
>>> If you don't specify a CL in your job config it will default to ONE (at
>>> least that's what my read of the ConfigHelper source for 1.1.6 shows)
>>>
>> I have two questions.
>> 1. I can benefit from data locality (and Hadoop) only with CL ONE. Is
>> it correct?
>
> Yes and at QUORUM it's quasi local.  The job tracker finds out where a ra=
nge is and sends a task to a replica with the data (local).  In the case of=
 CL.QUORUM (see the Read Path section of http://wiki.apache.org/cassandra/A=
rchitectureInternals), it will do an actual read of the data on the node cl=
osest (local).  Then it will get a digest from other nodes to verify that t=
hey have the same data.  So in the case of RF=3D3 and QUORUM, it will read =
the data on the local node where the task is running and will check the nex=
t closest replica for a digest to verify that it is consistent.  Informatio=
n is sent across the wire and there is the latency of that, but it's not th=
e data that's sent.
>
>> 2. With CL QUORUM cassandra reads data from all replicas. In this case
>> Hadoop doesn't give me any  benefits. Application running outside the
>> cluster has the same performance. Is it correct?
>
> CL QUORUM does not read data from all replicas.  Applications running out=
side the cluster have to copy the data from the cluster, a much more copy/n=
etwork intensive operation than using CL.QUORUM with the built-in Hadoop su=
pport.
>

Thank you very much, guys! I have a much clearer picture now.

Andrey