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In make_rf_change_plan, load balancer schedules necessary migrations, considering the load of nodes and other pending tablet transitions. Requests from ongoing_rf_changes are processed concurrently, independently from one another. In each request racks are processed concurrently. No tablet replica will be removed until all required replicas are added. While adding replicas to each rack we always start with base tables and won't proceed with views until they are done (while removing - the other way around). Node availability is checked at two levels for extending actions: 1) In prepare_per_rack_rf_change_plan: the entire RF change request is aborted if any node in the target dc+rack is down, or if there are no live (non-excluded) nodes at all. Shrinking is never aborted. 2) In make_rf_change_plan: extending is skipped for a given round if any normal, non-excluded node in the target dc+rack is missing from the balanced node set. Shrinking always proceeds regardless. The resulting behavior per node state combination (extending only): - all up -> proceed - some excluded + some up -> proceed (excluded nodes are skipped) - any down node -> abort - all excluded (no live) -> abort When the last step is finished: - in system_schema.keyspaces: - next_replication is cleared; - new keyspace properties are saved (if request succeeded); - request is removed from ongoing_rf_changes; - the request is marked as done in system.topology_requests.