private void validateInput(Node node, NodeState ns) {
            ns.verifyValidInSystemState(node.getType());
            if (node.getType() != type) {
                throw new IllegalArgumentException("NodeType '" + node.getType() + "' differs from '" + type + "'");
            }
        }
Maintains a bitset where all non-down nodes have a bit set. All nodes that differ from defaultUp
and defaultDown are stored explicitly in a hash map.
vdslib/src/main/java/com/yahoo/vdslib/state/ClusterState.java