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.
Another easy way to do this is to have repair command take nodes with which you want to repair with. Then we can do something like this.
1) Run repair between (A and B) and (C and D)
2) Run repair between (A and C)
3) Run repair between (A and B) and (C and D)
But this will increase the traffic inside the DC as we wont be doing proxy.