Hi Nicu,

By default (with those lines commented), your application will only read
data from the local datacenter.

If you get different results when those lines are uncommented it means that
the data is not consistent between your datacenters. You'll need to run a
full repair to ensure that the data is consistent across both datacenters.

For your migration you should join the new DC, run a full repair, then
switch your applications over to the new DC (i.e. change withLocalDc()).
You can then safely decommission the nodes in your original DC.

I would recommend doing the repair one node at a time (sub-range repair
would be even better), to allow you to better control the process and limit
the potential performance impact on your applications.


On Tue, 6 Mar 2018 at 23:18 Marasoiu, Nicu <nicu.maras...@metrosystems.net>

> Hi,
> We normally use a single data center, but when switching from one to
> another we will temporarily have two. The applications works with the local
> DC part of the cluster.
> Normally the configuration of the driver would contain (the commented
> lines absent):
> .withLoadBalancingPolicy(DCAwareRoundRobinPolicy.builder()
>         .withLocalDc(localDataCenter)
> //          .withUsedHostsPerRemoteDc(3)
> //          .allowRemoteDCsForLocalConsistencyLevel()
>         .build())
> However, the queries returned empty. Uncommenting the lines, the queries
> returned data. Multiple runs on multiple apps in multiple locations/dcs,
> gave data with lines commented/absent.
> All these apps connect to a cassandra cluster within a single data center
> currently. We are not clear why the empty results happened. Do you have any
> idea what might have caused this?
> Having the commented lines uncommented is dangerous in the context of how
> the data center migration is planned, since the queries would be able to
> reach the remote nodes which are not having all the data initially.
> Please advise,
> Thanks,
> Nicu Marasoiu
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