Cassandra can do it since it has cell level timestamps, so you can mirror online writes and clone existing data to the new database, and there's no danger of newer mutations being overwritten by the bulk restored data.
Doing an active no-downtime database migration basically involves having a coherent row-level merge policy (assuming you AT LEAST have a per-row last updated column), or other tricks. Or maybe you temporarily write cell-level timestamps and then drop it later.
Or if you have data that expires on a window, you just do double-writes for that period and then switch over.
Doing an active no-downtime database migration basically involves having a coherent row-level merge policy (assuming you AT LEAST have a per-row last updated column), or other tricks. Or maybe you temporarily write cell-level timestamps and then drop it later.
Or if you have data that expires on a window, you just do double-writes for that period and then switch over.