private void updateStartTimestamps(ClusterState state) {
        for(int i=0; i<2; ++i) {
            NodeType nodeType = (i == 0 ? NodeType.DISTRIBUTOR : NodeType.STORAGE);
            for (int j=0, n=state.getNodeCount(nodeType); j<n; ++j) {
                Node node = new Node(nodeType, j);
                NodeState ns = state.getNodeState(node);
                if (ns.getStartTimestamp() != 0) {
                    Long oldValue = highestStartTimestamps.get(node);
                    if (oldValue != null && oldValue > ns.getStartTimestamp()) {
                        throw new Error("Somehow start timestamp of node " + node + " has gone down");
                    }
                    highestStartTimestamps.put(node, ns.getStartTimestamp());
                }
            }
        }
    }
Returns the latest system state version received, or empty if none are received yet.
clustercontroller-core/src/test/java/com/yahoo/vespa/clustercontroller/core/DummyVdsNode.java