private Optional<Integer> getLatestSystemStateVersion() {
        synchronized(timer) {
            if (clusterStateBundles.isEmpty()) {
                return Optional.empty();
            }
            return Optional.of(clusterStateBundles.get(0).getVersion());
        }
    }
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