[jira] [Created] (CASSANDRA-6218) Reduce WAN traffic while doing repairs

["sankalp kohli (JIRA)" <ji...@apache.org>]

sankalp kohli created CASSANDRA-6218:
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             Summary: Reduce WAN traffic while doing repairs
                 Key: CASSANDRA-6218
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6218
             Project: Cassandra
          Issue Type: Improvement
          Components: Core
            Reporter: sankalp kohli
            Priority: Minor


The way we send out data that does not match over WAN can be improved. 
Example: Say there are four nodes(A,B,C,D) which are replica of a range we are repairing. A, B is in DC1 and C,D is in DC2. If A does not have the data which other replicas have, then we will have following streams
1) A to B and back
2) A to C and back(Goes over WAN)
3) A to D and back(Goes over WAN)
One of the ways of doing it to reduce WAN traffic is this.
1) Repair A and B only with each other and C and D with each other starting at same time t. 
2) Once these repairs have finished, A,B and C,D are in sync with respect to time t. 
3) Now run a repair between A and C, the streams which are exchanged as a result of the diff will also be streamed to B and D via A and C(C and D behaves like a proxy to the streams).

For a replication of DC1:2,DC2:2, the WAN traffic will get reduced by 50% and even more for higher replication factors. 
 



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