Function managed_cluster_replica_name 

pub fn managed_cluster_replica_name(index: u32) -> String

The canonical name of the index-th (zero-based) replica of a managed cluster.

A managed cluster’s replicas are derived from its replication_factor: for a factor of N they are named r1 through rN.

ALTER CLUSTER computes the replicas it creates and drops by this rule, and so does the catalog-open reconciler that materializes builtin replicas. The two have to share it, or ALTER cannot find the replicas it means to change. The cluster controller deliberately does not: its ReplicaNameGen picks names that avoid the observed set, which is why ALTER against a controller-owned cluster drops by observed id rather than by derived name.