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Date: Fri, 10 Oct 2014 11:16:33 +0000 (UTC)
From: "mck (JIRA)" <jira@apache.org>
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Subject: [jira] [Created] (CASSANDRA-8098) Allow CqlInputFormat to be
 restricted to more than one data-center
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mck created CASSANDRA-8098:
------------------------------

             Summary: Allow CqlInputFormat to be restricted to more than one data-center
                 Key: CASSANDRA-8098
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8098
             Project: Cassandra
          Issue Type: Improvement
          Components: Hadoop
            Reporter: mck
            Assignee: mck


Today, using CqlInputFormat, it's only possible to 
 - enforce data-locality to one specific data-center, or
 - disable it by changing CL from LOCAL_ONE to ONE.

We need a way to enforce data-locality to specific *data-centers*, and would like to contribute a solution.

Suggested ideas
 - CqlInputFormat (gently) calls describeLocalRing against all the listed connection addresses and merge the results into one masterRangeNodes list, or 
 - changing the signature of describeLocalRing(..) to describeRings(String keyspace, String[] dc) and having the job specify which DCs it will be running within.


*Long description*
A lot has changed since CASSANDRA-2388 that has made life a lot easier with integrating c* and hadoop, for example: CqlInputFormat, CL.LOCAL_ONE, LimitedLocalNodeFirstLocalBalancingPolicy, vnodes, and describe_local_ring.

When using CqlInputFormat, if you don't want to be stuck within datacenter-locality you can for example change the consistency level from LOCAL_ONE to ONE. That's great, but describe_local_ring + CL.LOCAL_ONE in its current implementation isn't enough for us. We have multiple datacenters for offline, multiple for online, because we still want the availability advantages that come from aligning virtual datacenters to physical datacenters for the offline stuff too. 

Some of our jobs have their own InputFormat implementation that uses describe_ring, LOCAL_ONE, and data with only replica in the offline datacenters. Works very well, except the last point kinda sucks because we have online clients that want to read this data and have to then do so through nodes in the offline datacenters. Underlying performance improvements: eg cross_node_timeout and speculative requests; have helped but there's still the need to separate online and offline. If we wanted to push replica out on to the online nodes, i think the best approach is for us is to have to filter out those splits/locations in getRangeMap(..)

Back to this issue we also have jobs using CqlInputFormat. Specifying multiple client input addresses doesn't help take advantage of the multiple offline datacenters because the Cassandra.Client only makes one call to describe_local_ring, and StorageService.describeLocalRing(..) only checks against its own address. It would work to have either a) CqlInputFormat call describeLocalRing against all the listed connection addresses and merge the results into one masterRangeNodes list, or b) something along the lines of changing the signature of describeLocalRing(..) to describeRings(String keyspace, String[] dc) and having the job specify which DCs it will be running within.


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