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From "sankalp kohli (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-6218) Reduce WAN traffic while doing repairs
Date Fri, 18 Oct 2013 20:46:51 GMT
sankalp kohli created CASSANDRA-6218:

             Summary: Reduce WAN traffic while doing repairs
                 Key: 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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