public void restart() {
        synchronized (nextMasterData) {
            masterData = new TreeMap<>();
            nextMasterData.clear();
            session.getChildren(paths.indexesRoot(), changeWatcher, childListener, null);
        }
    }
Calling restart, ignores what we currently know and starts another cycle. Typically called after reconnecting to ZooKeeperServer.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/database/MasterDataGatherer.java