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From Robert Coli <rc...@eventbrite.com>
Subject Re: Query a datacenter
Date Sat, 26 Oct 2013 00:34:21 GMT
On Fri, Oct 25, 2013 at 2:47 PM, srmore <comomore@gmail.com> wrote:

> I don't know whether this is possible but was just curious, can you query
> for the data in the remote datacenter with a CL.ONE ?
>

A coordinator at CL.ONE picks which replica(s) to query based in large part
on the dynamic snitch. If your remote data center has a lower badness score
from the perspective of the dynamic snitch, a CL.ONE request might go there.

1.2.11 adds [1] a LOCAL_ONE consistencylevel which does the opposite of
what you are asking, restricting CL.ONE from going cross-DC.


> There could be a case where one might not have a QUORUM and would like to
> read the most recent  data which includes the data from the other
> datacenter. AFAIK to reliably read the data from other datacenter we only
> have CL.EACH_QUORUM.
>

Using CL.QUORUM requires a QUORUM number of responses, it does not care
from which data center those responses come.


> Also, is there a way one can control how frequently the data is replicated
> across the datacenters ?
>

Data centers don't really exist in this context [2], so your question is
"can one control how frequently data is replicated between replicas" and
the answer is "no." All replication always goes to every replica.

=Rob
[1] https://issues.apache.org/jira/browse/CASSANDRA-6202
[2] this is slightly glib/reductive/inaccurate, but accurate enough for the
purposes of this response.

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