In my experience,
I'd use two methods to make sure that you are covering your ass.
1. "old school" methodology would be to do the SStable load from old to new
cluster -- if you do incremental snapshots, then you could technically
minimize downtime and just load the latest increments with a little
Either:
double-write at the driver level from one of the apps and perform an
initial and a subsequent sstable loads (or whatever ETL method you want to
use) to merge the data with good assurances.
use a trigger to replicate the writes, with some sstable loads / ETL.
use change data capture with
Maybe others will have a different or better solution but, in my experience to
accomplish HA we simply y write from our application to the new cluster. You
then export the data from the old cluster using cql2json or any method you
choose, to the new cluster. That will cover all live(now) data vi
