Mailing-List: contact user-help@cassandra.apache.org; run by ezmlm
Precedence: bulk
Reply-To: user@cassandra.apache.org
Received-SPF: pass (nike.apache.org: domain of jeremy.hanna1234@gmail.com
 designates 209.85.214.172 as permitted sender)
Content-Type: text/plain; charset=iso-8859-1
Mime-Version: 1.0 (Mac OS X Mail 6.2 \(1499\))
Subject: Re: hadoop consistency level
From: Jeremy Hanna <jeremy.hanna1234@gmail.com>
In-Reply-To: 
 <CAK0tFt6NuV1rs3b0DfXKJJyWtAwviFynH1gW2oEvHeK3s5D12g@mail.gmail.com>
Date: Thu, 18 Oct 2012 16:31:15 -0500
Content-Transfer-Encoding: quoted-printable
Message-Id: <326A0EF1-1584-4355-92F4-D3C94CAE5AEA@gmail.com>
References: 
 <CAK0tFt4cpCOugCv5QtuzAW_OJp6svfJSQ74HQwp6+dxm6Uk7SQ@mail.gmail.com>
 <CCA5B541.1721D%mkjellman@barracuda.com>
 <CAK0tFt6NuV1rs3b0DfXKJJyWtAwviFynH1gW2oEvHeK3s5D12g@mail.gmail.com>
To: user@cassandra.apache.org


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..)
>>=20
>> You are more than welcome to do CL ONE and assuming you have hadoop =
nodes
>> 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 =
need
>> to use QUORUM.
>>=20
>> 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)
>>=20
> 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 =
range 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/ArchitectureInternals), it will do an =
actual read of the data on the node closest (local).  Then it will get a =
digest from other nodes to verify that they 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 next closest replica for a =
digest to verify that it is consistent.  Information is sent across the =
wire and there is the latency of that, but it's not the 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 =
outside the cluster have to copy the data from the cluster, a much more =
copy/network intensive operation than using CL.QUORUM with the built-in =
Hadoop support.

>=20
> Thank you,
>  Andrey