-spec node_reachable(node()) -> boolean().

Is Node usable for a synchronous message right now — the local node, or a currently-connected one?

This is the predicate the registry uses everywhere it must decide whether a cast or call to a peer will actually be delivered (as opposed to silently triggering an automatic distribution reconnect, which would heal a partition before it can be observed — see dgen_registry_member:cast_to_member/2 and dgen_registry_elector:call_to_member/2). The local node is trivially reachable; a remote node is reachable iff it is in nodes().