Similar to DSE's option: allocate_tokens_for_local_replication_factor
Currently the ReplicationAwareTokenAllocator requires a defined keyspace and a replica factor specified in the current datacenter.
This is problematic in a number of ways. The real keyspace can not be used when adding new datacenters as, in practice, all its nodes need to be up and running before it has the capacity to replicate data into it. New datacenters (or lift-and-shifting a cluster via datacenter migration) therefore has to be done using a dummy keyspace that duplicates the replication strategy+factor of the real keyspace. This gets even more difficult come version 4.0, as the replica factor can not even be defined in new datacenters before those datacenters are up and running.
These issues are removed by avoiding the keyspace definition and lookup, and presuming the replica strategy is by datacenter, ie NTS. This can be done with the use of an allocate_tokens_for_dc_rf option.
It may also be of value considering whether allocate_tokens_for_dc_rf=3 becomes the default? as this is the replication factor for the vast majority of datacenters in production. I suspect this would be a good improvement over the existing randomly generated tokens algorithm.
Initial patch is available in https://github.com/thelastpickle/cassandra/commit/fc4865b0399570e58f11215565ba17dc4a53da97
The patch does not remove the existing allocate_tokens_for_keyspace option, as that provides the codebase for handling different replication strategies.