If there’s a lot of rooms, I’d be more worried about the impact of sequentially messaging every single room process (which is what map + get_state does) than about the impact of duplicating the registration.

Have you considered using a duplicate registry, and registering directly in your room’s init rather than using :via? Then you could register with {id, title} as the value and get all the id/titles quickly without adding load to your processes.

Registry.lookup/2 would return all entries under a given key for a duplicate registry.

They’re synchronous, but if you call Room.start_link from multiple processes around the same time for the same id, they may all see Registry.match return [] and enter GenServer.start_link, rather than return {:error, {:already_registered, pid}}. There is a window between Registry.match and Registry.register. Unique registry does not have this problem since register also checks if it’s already registered in one atomic operation.