Description
The problem/use-case that the feature addresses
When running regular Valkey it's pretty easy to spin another process and make it replicaof primary server. That can be very useful if you want to replicate from one server to another (say prod to dev environment).
However when running CLUSTER MODE replicas automatically become part of cluster topology. While it is possible to filter our some of the replicas via a module there is still a question of gossip.
Description of the feature
Ability to add 1 or more nodes to each shards that act purely as replicas for that shard. They are excluded from cluster topology.
Together these nodes will form a separate read-only cluster.
Similar to https://redis.io/docs/latest/operate/rs/databases/import-export/replica-of/create/#different-cluster
Alternatives you've considered
Related to https://github.com/valkey-io/valkey/issues/2767 which would enable daisy-chain replication.
It is also possible to make nodes replicaof specific shards and data will replicate however then those nodes are just r/o replicas w/o cluster mode enabled.
Additional information
As I said, some of this can be hacked via a module by filtering out specific replicas so they will be hidden from cluster topology.