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From anuja jain <anujaja...@gmail.com>
Subject Re: Is replication possible with already existing data?
Date Fri, 09 Oct 2015 07:08:56 GMT
Hi Ajay,


On Fri, Oct 9, 2015 at 9:00 AM, Ajay Garg <ajaygargnsit@gmail.com> wrote:

> On Thu, Oct 8, 2015 at 9:47 AM, Ajay Garg <ajaygargnsit@gmail.com> wrote:
> > Thanks Eric for the reply.
> >
> >
> > On Thu, Oct 8, 2015 at 1:44 AM, Eric Stevens <mightye@gmail.com> wrote:
> >> If you're at 1 node (N=1) and RF=1 now, and you want to go N=3 RF=3, you
> >> ought to be able to increase RF to 3 before bootstrapping your new
> nodes,
> >> with no downtime and no loss of data (even temporary).  Effective RF is
> >> min-bounded by N, so temporarily having RF > N ought to behave as RF =
> N.
> >>
> >> If you're starting at N > RF and you want to increase RF, things get
> >> harrier
> >> if you can't afford temporary consistency issues.
> >>
> >
> > We are ok with temporary consistency issues.
> >
> > Also, I was going through the following articles
> >
> https://10kloc.wordpress.com/2012/12/27/cassandra-chapter-5-data-replication-strategies/
> >
> > and following doubts came up in my mind ::
> >
> >
> > a)
> > Let's say at site-1, Application-Server (APP1) uses the two
> > Cassandra-instances (CAS11 and CAS12), and APP1 generally uses CAS11 for
> all
> > its needs (of course, whatever happens on CAS11, the same is replicated
> to
> > CAS12 at Cassandra-level).
> >
> > Now, if CAS11 goes down, will it be the responsibility of APP1 to
> "detect"
> > this and pick up CAS12 for its needs?
> > Or some automatic Cassandra-magic will happen?
> >
>
In this case, it will be the responsibility of APP1 to start connection to
 CAS12. On the other hand if your APP1 is connecting to cassandra using
Java driver, you can add multiple contact points(CAS11 and CAS12 here) so
that if CAS11 is down it will directly connect to CAS12.

> b)
> > In the same above scenario, let's say before CAS11 goes down, the amount
> of
> > data in both CAS11 and CAS12 was "x".
> >
> > After CAS11 goes down, the data is being put in CAS12 only.
> > After some time, CAS11 comes back up.
> >
> > Now, data in CAS11 is still "x", while data in CAS12 is "y" (obviously,
> "y"
> >> "x").
> >
> > Now, will the additional ("y" - "x") data be automatically
> > put/replicated/whatever back in CAS11 through Cassandra?
> > Or it has to be done manually?
> >
>
> In such a case, CAS12 will store hints for the data to be stored on CAS11
(the tokens of which lies within the range of tokens CAS11 holds)  and
whenever CAS11 is up again, the hints will be transferred to it and the
data will be distributed evenly.


> >
> > If there are easy recommended solutions to above, I am beginning to think
> > that a 2*2 (2 nodes each at 2 data-centres) will be the ideal setup
> > (allowing failures of entire site, or a few nodes on the same site).
> >
> > I am sorry for asking such newbie questions, and I will be grateful if
> these
> > silly questions could be answered by the experts :)
> >
> >
> > Thanks and Regards,
> > Ajay
>
>
>
> --
> Regards,
> Ajay
>

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